The Field Effect TransistorIn the Bipolar Junction Transistor, we saw that the outputCollector current of the transistor is proportional to inputcurrent flowing into the Base terminal of the device,thereby making the bipolar transistor a "CURRENT" operateddevice (Beta model). The Field Effect Transistor, or simplyFET however, uses the voltage that is applied to their inputterminal, called the Gate to control the current flowingthrough them resulting in the output current beingproportional to the input voltage. As their operation relieson an electric field (hence the name field effect) generatedby the input Gate voltage, this then makes the Field EffectTransistor a "VOLTAGE" operated device.

Typical FieldEffect Transistor

The Field Effect Transistor is a three terminal unipolarsemiconductor device that has very similar characteristicsto those of their Bipolar Transistor counterparts ie, highefficiency, instant operation, robust and cheap and can beused in most electronic circuit applications to replacetheir equivalent bipolar junction transistors (BJT) cousins.

Field effect transistors can be made much smaller than anequivalent BJT transistor and along with their low powerconsumption and power dissipation makes them ideal for usein integrated circuits such as the CMOS range of digitallogic chips.

We remember from the previous tutorials that there are twobasic types of Bipolar Transistor construction, NPN and PNP,which basically describes the physical arrangement of the P-type and N-type semiconductor materials from which they aremade. This is also true of FET's as there are also two basicclassifications of Field Effect Transistor, called the N-channel FET and the P-channel FET.

The field effect transistor is a three terminal device thatis constructed with no PN-junctions within the main currentcarrying path between the Drain and the Source terminals,which correspond in function to the Collector and theEmitter respectively of the bipolar transistor. The currentpath between these two terminals is called the "channel"which may be made of either a P-type or an N-typesemiconductor material. The control of current flowing inthis channel is achieved by varying the voltage applied tothe Gate. As their name implies, Bipolar Transistors are"Bipolar" devices because they operate with both types ofcharge carriers, Holes and Electrons. The Field EffectTransistor on the other hand is a "Unipolar" device thatdepends only on the conduction of electrons (N-channel) orholes (P-channel).

The Field Effect Transistor has one major advantage over itsstandard bipolar transistor cousins, in that their inputimpedance, ( Rin ) is very high, (thousands of Ohms), whilethe BJT is comparatively low. This very high input impedancemakes them very sensitive to input voltage signals, but theprice of this high sensitivity also means that they can beeasily damaged by static electricity. There are two maintypes of field effect transistor, the Junction Field EffectTransistor or JFET and the Insulated-gate Field EffectTransistor or IGFET), which is more commonly known as thestandard Metal Oxide Semiconductor Field Effect Transistoror MOSFET for short.

The Junction Field Effect Transistor

We saw previously that a bipolar junction transistor isconstructed using two PN-junctions in the main currentcarrying path between the Emitter and the Collectorterminals. The Junction Field Effect Transistor (JUGFET orJFET) has no PN-junctions but instead has a narrow piece ofhigh-resistivity semiconductor material forming a "Channel"of either N-type or P-type silicon for the majority carriersto flow through with two ohmic electrical connections ateither end commonly called the Drain and the Sourcerespectively.

There are two basic configurations of junction field effecttransistor, the N-channel JFET and the P-channel JFET. TheN-channel JFET's channel is doped with donor impuritiesmeaning that the flow of current through the channel isnegative (hence the term N-channel) in the form ofelectrons. Likewise, the P-channel JFET's channel is dopedwith acceptor impurities meaning that the flow of currentthrough the channel is positive (hence the term P-channel)in the form of holes. N-channel JFET's have a greaterchannel conductivity (lower resistance) than theirequivalent P-channel types, since electrons have a highermobility through a conductor compared to holes. This makesthe N-channel JFET's a more efficient conductor compared totheir P-channel counterparts.

We have said previously that there are two ohmic electricalconnections at either end of the channel called the Drainand the Source. But within this channel there is a thirdelectrical connection which is called the Gate terminal andthis can also be a P-type or N-type material forming a PN-junction with the main channel. The relationship between theconnections of a junction field effect transistor and abipolar junction transistor are compared below.

Comparison of connections between a JFET and a BJT

BipolarTransistor

Field EffectTransistor

Emitter - (E)     >>     Source- (S)Base - (B)     >>     Gate - (G)Collector - (C)     >>     Drain- (D)     

The symbols and basic construction for both configurations ofJFETs are shown below.

The semiconductor "channel" of the Junction Field EffectTransistor is a resistive path through which a voltage VDS

causes a current ID to flow. The JFET can conduct currentequally well in either direction. A voltage gradient is thusformed down the length of the channel with this voltagebecoming less positive as we go from the Drain terminal tothe Source terminal. The PN-junction therefore has a high

reverse bias at the Drain terminal and a lower reverse biasat the Source terminal. This bias causes a "depletion layer"to be formed within the channel and whose width increaseswith the bias.

The magnitude of the current flowing through the channelbetween the Drain and the Source terminals is controlled bya voltage applied to the Gate terminal, which is a reverse-biased. In an N-channel JFET this Gate voltage is negativewhile for a P-channel JFET the Gate voltage is positive. Themain difference between the JFET and a BJT device is thatwhen the JFET junction is reverse-biased the Gate current ispractically zero, whereas the Base current of the BJT isalways some value greater than zero.

Bias arrangement for an N-channel JFET and correspondingcircuit symbols.

The cross sectional diagram above shows an N-typesemiconductor channel with a P-type region called the Gatediffused into the N-type channel forming a reverse biasedPN-junction and it is this junction which forms thedepletion region around the Gate area when no externalvoltages are applied. JFETs are therefore known as depletionmode devices. This depletion region produces a potentialgradient which is of varying thickness around the PN-

junction and restrict the current flow through the channelby reducing its effective width and thus increasing theoverall resistance of the channel itself. The most-depletedportion of the depletion region is in between the Gate andthe Drain, while the least-depleted area is between the Gateand the Source. Then the JFET's channel conducts with zerobias voltage applied (i.e. the depletion region has nearzero width).

With no external Gate voltage ( VG = 0 ), and a smallvoltage ( VDS ) applied between the Drain and the Source,maximum saturation current ( IDSS ) will flow through thechannel from the Drain to the Source restricted only by thesmall depletion region around the junctions.

If a small negative voltage ( -VGS ) is now applied to theGate the size of the depletion region begins to increasereducing the overall effective area of the channel and thusreducing the current flowing through it, a sort of"squeezing" effect takes place. So by applying a reversebias voltage increases the width of the depletion regionwhich in turn reduces the conduction of the channel. Sincethe PN-junction is reverse biased, little current will flowinto the gate connection. As the Gate voltage ( -VGS ) ismade more negative, the width of the channel decreases untilno more current flows between the Drain and the Source andthe FET is said to be "pinched-off" (similar to the cut-offregion for a BJT). The voltage at which the channel closesis called the "pinch-off voltage", ( VP ).

JFET Channel Pinched-off

In this pinch-off region the Gate voltage, VGS controls thechannel current and VDS has little or no effect.

JFET ModelThe result is that the FET acts more like a voltagecontrolled resistor which has zero resistance when VGS = 0and maximum "ON" resistance ( RDS ) when the Gate voltage isvery negative. Under normal operating conditions, the JFETgate is always negatively biased relative to the source.

It is essential that the Gate voltage is never positivesince if it is all the channel current will flow to the Gateand not to the Source, the result is damage to the JFET.Then to close the channel:

No Gate voltage ( VGS ) and VDS is increased from zero. No VDS and Gate control is decreased negatively from

zero. VDS and VGS varying.

The P-channel Junction Field Effect Transistor operates thesame as the N-channel above, with the following exceptions:

1). Channel current is positive due to holes, 2). Thepolarity of the biasing voltage needs to be reversed.

The output characteristics of an N-channel JFET with thegate short-circuited to the source is given as

Output characteristic V-I curves of a typicaljunction FET.

The voltage VGS applied to the Gate controls the currentflowing between the Drain and the Source terminals. VGS

refers to the voltage applied between the Gate and theSource while VDS refers to the voltage applied between theDrain and the Source. Because a Junction Field EffectTransistor is a voltage controlled device, "NO current flowsinto the gate!" then the Source current ( IS ) flowing outof the device equals the Drain current flowing into it andtherefore ( ID = IS ).

The characteristics curves example shown above, shows thefour different regions of operation for a JFET and these aregiven as:

Ohmic Region - When VGS = 0 the depletion layer of thechannel is very small and the JFET acts like a voltagecontrolled resistor.

Cut-off Region - This is also known as the pinch-offregion were the Gate voltage, VGS is sufficient tocause the JFET to act as an open circuit as the channelresistance is at maximum. 

Saturation or Active Region - The JFET becomes a goodconductor and is controlled by the Gate-Source voltage,( VGS ) while the Drain-Source voltage, ( VDS ) haslittle or no effect. 

Breakdown Region - The voltage between the Drain andthe Source, ( VDS ) is high enough to causes the JFET'sresistive channel to break down and pass uncontrolledmaximum current.

The characteristics curves for a P-channel junction fieldeffect transistor are the same as those above, except thatthe Drain current ID decreases with an increasing positiveGate-Source voltage, VGS.

The Drain current is zero when VGS = VP. For normaloperation, VGS is biased to be somewhere between VP and 0.

Then we can calculate the Drain current, ID for any givenbias point in the saturation or active region as follows:

Drain current in the active region.

Note that the value of the Drain current will be betweenzero (pinch-off) and IDSS (maximum current). By knowing theDrain current ID and the Drain-Source voltage VDS theresistance of the channel ( ID ) is given as:

Drain-Source channel resistance.

Where: gm is the "transconductance gain" since the JFET is avoltage controlled device and which represents the rate ofchange of the Drain current with respect to the change inGate-Source voltage.

JFET AmplifierJust like the bipolar junction transistor, JFET's can beused to make single stage class A amplifier circuits withthe JFET common source amplifier and characteristics beingvery similar to the BJT common emitter circuit. The mainadvantage JFET amplifiers have over BJT amplifiers is their

high input impedance which is controlled by the Gate biasingresistive network formed by R1 and R2 as shown.

Biasing of JFET Amplifier

This common source (CS) amplifier circuit is biased in classA mode by the voltage divider network formed by R1 and R2.The voltage across the Source resistor RS is generally setat one fourth VDD, ( VDD /4 ). The required Gate voltage canthen be calculated using this RS value. Since the Gatecurrent is zero, ( IG = 0 ) we can set the required DCquiescent voltage by the proper selection of resistors R1and R2.

The control of the Drain current by a negative Gatepotential makes the Junction Field Effect Transistor usefulas a switch and it is essential that the Gate voltage isnever positive for an N-channel JFET as the channel currentwill flow to the Gate and not the Drain resulting in damageto the JFET. The principals of operation for a P-channel

JFET are the same as for the N-channel JFET, except that thepolarity of the voltages need to be reversed.

The Common Source JFET AmplifierSo far we have looked at the bipolar type transistoramplifier and especially the common emitter amplifier, butsmall signal amplifiers can also be made using Field EffectTransistors or FET's for short. These devices have theadvantage over bipolar transistors of having an extremelyhigh input impedance along with a low noise output makingthem ideal for use in amplifier circuits that have verysmall input signals. The design of an amplifier circuitbased around a junction field effect transistor or "JFET",(n-channel FET for this tutorial) or even a metal oxidesilicon FET or "MOSFET" is exactly the same principle asthat for the bipolar transistor circuit used for a Class Aamplifier circuit we looked at in the previous tutorial.Firstly, a suitable quiescent point or "Q-point" needs to befound for the correct biasing of the JFET amplifier circuitwith single amplifier configurations of Common-source (CS),Common-drain (CD) or Source-follower (SF) and the Common-gate (CG) available for most FET devices. These three JFETamplifier configurations correspond to the common-emitter,emitter-follower and the common-base configurations usingbipolar transistors. In this tutorial we will look at theCommon Source JFET Amplifier as this is the most widely usedJFET amplifier design. Then consider the common source JFETamplifier circuit below.

Common Source JFET Amplifier

The amplifier circuit consists of an N-channel JFET, but thedevice could also be an equivalent N-channel depletion-modeMOSFET as the circuit diagram would be the same just achange in the FET, connected in a common sourceconfiguration. The JFET gate voltage Vg is biased throughthe potential divider network set up by resistors R1 and R2and is biased to operate within its saturation region whichis equivalent to the active region of the bipolar junctiontransistor. Unlike a bipolar transistor circuit, thejunction FET takes virtually no input gate current allowingthe gate to be treated as an open circuit. Then no inputcharacteristics curves are required. We can compare the JFETto the bipolar junction transistor (BJT) in the followingtable.

JFET to BJT Comparison

JFET BJTGate, (G) Base, (B)Drain, (D) Collector, (C)Source, (S) Emitter, (E)Gate Supply,(VG)

Base Supply,(VB)

Drain Supply,(VDD)

CollectorSupply, (VCC)

Drain Current,(iD)

CollectorCurrent, (iC)

Since the N-Channel JFET is a depletion mode device and isnormally "ON", a negative gate voltage with respect to thesource is required to modulate or control the drain current.This negative voltage can be provided by biasing from aseparate power supply voltage or by a self biasingarrangement as long as a steady current flows through theJFET even when there is no input signal present and Vgmaintains a reverse bias of the gate-source pn junction. Inthis example the biasing is provided from a potentialdivider network allowing the input signal to produce avoltage fall at the gate as well as voltage rise at the gatewith a sinusoidal signal. Any suitable pair of resistorvalues in the correct proportions would produce the correctbiasing voltage so the DC gate biasing voltage Vg is givenas:

Note that this equation only determines the ratio of theresistors R1 and R2, but in order to take advantage of thevery high input impedance of the JFET as well as reducingthe power dissipation within the circuit, we need to makethese resistor values as high as possible, with values inthe order of 1 to 10MΩ being common.

The input signal, (Vin) of the common source JFET amplifieris applied between the Gate terminal and the zero voltsrail, (0v). With a constant value of gate voltage Vg appliedthe JFET operates within its "Ohmic region" acting like alinear resistive device. The drain circuit contains the loadresistor, Rd. The output voltage, Vout is developed acrossthis load resistance. The efficiency of the common sourceJFET amplifier can be improved by the addition of a

resistor, Rs included in the source lead with the same draincurrent flowing through this resistor. Resistor, Rs is alsoused to set the JFET amplifiers "Q-point".

When the JFET is switched fully "ON" a voltage drop equal toRs x Id is developed across this resistor raising thepotential of the source terminal above 0v or ground level.This voltage drop across Rs due to the drain currentprovides the necessary reverse biasing condition across thegate resistor, R2 effectively generating negative feedback.In order to keep the gate-source junction reverse biased,the source voltage, Vs needs to be higher than the gatevoltage, Vg. This source voltage is therefore given as:

Then the Drain current, Id is also equal to the Sourcecurrent, Is as "No Current" enters the Gate terminal andthis can be given as:

This potential divider biasing circuit improves thestability of the common source JFET amplifier circuit whenbeing fed from a single DC supply compared to that of afixed voltage biasing circuit. Both resistor, Rs and thesource by-pass capacitor, Cs serve basically the samefunction as the emitter resistor and capacitor in the commonemitter bipolar transistor amplifier circuit, namely toprovide good stability and prevent a reduction in the lossof the voltage gain. However, the price paid for astabilized quiescent gate voltage is that more of the supplyvoltage is dropped across Rs.

The the value in farads of the source by-pass capacitor isgenerally fairly high above 100uF and will be polarized.This gives the capacitor an impedance value much smaller,less than 10% of the transconductance, gm (the transfer

coefficient representing gain) value of the device. At highfrequencies the by-pass capacitor acts essentially as ashort-circuit and the source will be effectively connecteddirectly to ground.

The basic circuit and characteristics of a Common SourceJFET Amplifier are very similar to that of the commonemitter amplifier. A DC load line is constructed by joiningthe two points relating to the drain current, Id and thesupply voltage, Vdd remembering that when Id = 0: ( Vdd =Vds ) and when Vds = 0: ( Id = Vdd/RL ). The load line istherefore the intersection of the curves at the Q-point asfollows.

Common Source JFET Amplifier Characteristics Curves

As with the common emitter bipolar circuit, the DC load linefor the common source JFET amplifier produces a straightline equation whose gradient is given as: -1/(Rd + Rs) andthat it crosses the vertical Id axis at point A equal toVdd/(Rd + Rs). The other end of the load line crosses thehorizontal axis at point B which is equal to the supplyvoltage, Vdd. The actual position of the Q-point on the DCload line is generally positioned at the mid centre point ofthe load line (for class-A operation) and is determined bythe mean value of Vg which is biased negatively as the JFETis a depletion-mode device. Like the bipolar common emitteramplifier the output of the Common Source JFET Amplifier is180o out of phase with the input signal.

One of the main disadvantages of using Depletion-mode JFETis that they need to be negatively biased. Should this biasfail for any reason the gate-source voltage may rise andbecome positive causing an increase in drain currentresulting in failure of the drain voltage, Vd. Also the highchannel resistance, Rds(on) of the junction FET, coupledwith high quiescent steady state drain current makes thesedevices run hot so additional heatsink is required. However,most of the problems associated with using JFET's can begreatly reduced by using enhancement-mode MOSFET devicesinstead.

MOSFETs or Metal Oxide Semiconductor FET's have much higherinput impedances and low channel resistances compared to theequivalent JFET. Also the biasing arrangements for MOSFETsare different and unless we bias them positively for N-channel devices and negatively for P-channel devices nodrain current will flow, then we have in effect a fail safetransistor.

JFET Amplifier Current and Power GainsWe said previously that the input current, Ig of a commonsource JFET amplifier is very small because of the extremelyhigh gate impedance, Rg. A common source JFET amplifiertherefore has a very good ratio between its input and outputimpedances and for any amount of output current, Io the JFETamplifier will have very high current gain Ai. Because ofthis common source JFET amplifiers are extremely valuable asimpedance matching circuits or are used as voltageamplifiers. Likewise, because power = current × voltage, andoutput voltages are usually several millivolts or evenvolts, the power gain, Ap is also very high.

The Metal Oxide FET - MOSFETAs well as the Junction Field Effect Transistor (JFET),there is another type of Field Effect Transistor availablewhose Gate input is electrically insulated from the main

current carrying channel and is therefore called anInsulated Gate Field Effect Transistor or IGFET. The mostcommon type of insulated gate FET which is used in manydifferent types of electronic circuits is called the MetalOxide Semiconductor Field Effect Transistor or MOSFET forshort.

The IGFET or MOSFET is a voltage controlled field effecttransistor that differs from a JFET in that it has a "MetalOxide" Gate electrode which is electrically insulated fromthe main semiconductor N-channel or P-channel by a thinlayer of insulating material usually silicon dioxide(commonly known as glass). This insulated metal gateelectrode can be thought of as one plate of a capacitor. Theisolation of the controlling Gate makes the input resistanceof the MOSFET extremely high in the Mega-ohms ( MΩ ) regionthereby making it almost infinite.

As the Gate terminal is isolated from the main currentcarrying channel "NO current flows into the gate" and justlike the JFET, the MOSFET also acts like a voltagecontrolled resistor were the current flowing through themain channel between the Drain and Source is proportional tothe input voltage. Also like the JFET, this very high inputresistance can easily accumulate large amounts of staticcharge resulting in the MOSFET becoming easily damagedunless carefully handled or protected.

Like the previous JFET tutorial, MOSFETs are three terminaldevices with a Gate, Drain and Source and both P-channel(PMOS) and N-channel (NMOS) MOSFETs are available. The maindifference this time is that MOSFETs are available in twobasic forms:

1. Depletion Type   -   the transistor requires theGate-Source voltage, ( VGS ) to switch the device "OFF".The depletion mode MOSFET is equivalent to a "NormallyClosed" switch.

2. Enhancement Type   -   the transistor requires a Gate-Source voltage, ( VGS ) to switch the device "ON". Theenhancement mode MOSFET is equivalent to a "NormallyOpen" switch.

The symbols and basic construction for both configurations ofMOSFETs are shown below.

The four MOSFET symbols above show an additional terminalcalled the Substrate and is not normally used as either aninput or an output connection but instead it is used forgrounding the substrate. It connects to the mainsemiconductive channel through a diode junction to the body

or metal tab of the MOSFET. In discrete type MOSFETs, thissubstrate lead is connected internally to the sourceterminal. When this is the case, as in enhancement types itis omitted from the symbol. The line between the drain andsource connections represents the semiconductive channel. Ifthis is a solid unbroken line then this represents a"Depletion" (normally closed) type MOSFET and if the channelline is shown dotted or broken it is an "Enhancement"(normally open) type MOSFET. The direction of the arrowindicates either a P-channel or an N-channel device.

Basic MOSFET Structure and Symbol

The construction of the Metal Oxide Semiconductor FET isvery different to that of the Junction FET. Both theDepletion and Enhancement type MOSFETs use an electricalfield produced by a gate voltage to alter the flow of chargecarriers, electrons for N-channel or holes for P-channel,through the semiconductive drain-source channel. The gateelectrode is placed on top of a very thin insulating layerand there are a pair of small N-type regions just under thedrain and source electrodes.

We saw in the previous tutorial, that the gate of a JFETmust be biased in such a way as to forward-bias the PN-

junction but with a insulated gate MOSFET device no suchlimitations apply so it is possible to bias the gate of aMOSFET in either polarity, +ve or -ve. This makes MOSFETsespecially valuable as electronic switches or to make logicgates because with no bias they are normally non-conductingand this high gate input resistance means that very littleor no control current is needed as MOSFETs are voltagecontrolled devices. Both the P-channel and the N-channelMOSFETs are available in two basic forms, the Enhancementtype and the Depletion type.

Depletion-mode MOSFETThe Depletion-mode MOSFET, which is less common than theenhancement types is normally switched "ON" without theapplication of a gate bias voltage making it a "normally-closed" device. However, a gate to source voltage ( VGS )will switch the device "OFF". Similar to the JFET types. Foran N-channel MOSFET, a "positive" gate voltage widens thechannel, increasing the flow of the drain current anddecreasing the drain current as the gate voltage goes morenegative. In other words, for an N-channel depletion modeMOSFET: +VGS means more electrons and more current. While a-VGS means less electrons and less current. The opposite isalso true for the P-channel types. Then the depletion modeMOSFET is equivalent to a "normally-closed" switch.

Depletion-mode N-Channel MOSFET and circuit Symbols

Thedepletion-modeMOSFET is

constructed in a similar way to their JFET transistorcounterparts were the drain-source channel is inherentlyconductive with the electrons and holes already presentwithin the N-type or P-type channel. This doping of thechannel produces a conducting path of low resistance betweenthe Drain and Source with zero Gate bias.

Enhancement-mode MOSFETThe more common Enhancement-mode MOSFET is the reverse ofthe depletion-mode type. Here the conducting channel islightly doped or even undoped making it non-conductive. Thisresults in the device being normally "OFF" when the gatebias voltage is equal to zero.

A drain current will only flow when a gate voltage ( VGS )is applied to the gate terminal greater than the thresholdvoltage ( VTH ) level in which conductance takes placemaking it a transconductance device. This positive +ve gatevoltage pushes away the holes within the channel attractingelectrons towards the oxide layer and thereby increasing thethickness of the channel allowing current to flow. This iswhy this kind of transistor is called an enhancement modedevice as the gate voltage enhances the channel.

Increasing this positive gate voltage will cause the channelresistance to decrease further causing an increase in thedrain current, ID through the channel. In other words, foran N-channel enhancement mode MOSFET: +VGS turns thetransistor "ON", while a zero or -VGS turns the transistor"OFF". Then, the enhancement-mode MOSFET is equivalent to a"normally-open" switch.

Enhancement-mode N-Channel MOSFET and circuit Symbols

Enhancement-mode MOSFETs make excellent electronics switchesdue to their low "ON" resistance and extremely high "OFF"resistance as well as their infinitely high gate resistance.Enhancement-mode MOSFETs are used in integrated circuits toproduce CMOS type Logic Gates and power switching circuitsin the form of as PMOS (P-channel) and NMOS (N-channel)gates. CMOS actually stands for Complementary MOS meaningthat the logic device has both PMOS and NMOS within itsdesign.

The MOSFET AmplifierJust like the previous Junction Field Effect transistor,MOSFETs can be used to make single stage class A amplifiercircuits with the Enhancement mode N-channel MOSFET commonsource amplifier being the most popular circuit. Thedepletion mode MOSFET amplifiers are very similar to theJFET amplifiers, except that the MOSFET has a much higherinput impedance. This high input impedance is controlled bythe gate biasing resistive network formed by R1 and R2.Also, the output signal for the enhancement mode commonsource MOSFET amplifier is inverted because when VG is lowthe transistor is switched "OFF" and VD (Vout) is high. When

VG is high the transistor is switched "ON" and VD (Vout) islow as shown.

Enhancement-mode N-Channel MOSFET Amplifier

The DC biasing of this common source (CS) MOSFET amplifiercircuit is virtually identical to the JFET amplifier. TheMOSFET circuit is biased in class A mode by the voltagedivider network formed by resistors R1 and R2. The AC inputresistance is given as RIN = RG = 1MΩ.

Metal Oxide Semiconductor Field Effect Transistors are threeterminal active devices made from different semiconductormaterials that can act as either an insulator or a conductorby the application of a small signal voltage. The MOSFETsability to change between these two states enables it tohave two basic functions: "switching" (digital electronics)or "amplification" (analogue electronics). Then MOSFETs havethe ability to operate within three different regions:

1. Cut-off Region   -  with VGS < Vthreshold   the gate-source voltage is lower than the threshold voltage so

the transistor is switched "fully-OFF" and IDS = 0, thetransistor acts as an open circuit

 

2. Linear (Ohmic) Region   -  with VGS > Vthreshold   andVDS > VGS the transistor is in its constant resistanceregion and acts like a variable resistor whose value isdetermined by the gate voltage, VGS 

3. Saturation Region   -  with VGS > Vthreshold thetransistor is in its constant current region and isswitched "fully-ON". The current IDS = maximum as thetransistor acts as a closed circuit

The MOSFET as a SwitchThe N-channel, Enhancement-mode MOSFET operates using apositive input voltage and has an extremely high inputresistance (almost infinite) making it possible to interfacewith nearly any logic gate or driver capable of producing apositive output. Also, due to this very high input (Gate)resistance we can parallel together many different MOSFETsuntil we achieve the current handling limit required. Whileconnecting together various MOSFETs may enable us to switchhigh currents or high voltage loads, doing so becomesexpensive and impractical in both components and circuitboard space. To overcome this problem Power Field EffectTransistors or Power FET's were developed.

We now know that there are two main differences betweenfield effect transistors, depletion-mode only for JFET's andboth enhancement-mode and depletion-mode for MOSFETs. Inthis tutorial we will look at using the Enhancement-modeMOSFET as a Switch as these transistors require a positivegate voltage to turn "ON" and a zero voltage to turn "OFF"

making them easily understood as switches and also easy tointerface with logic gates.

The operation of the enhancement-mode MOSFET can best bedescribed using its I-V characteristics curves shown below.When the input voltage, ( VIN ) to the gate of thetransistor is zero, the MOSFET conducts virtually no currentand the output voltage, ( VOUT ) is equal to the supplyvoltage VDD. So the MOSFET is "fully-OFF" and in its "cut-off" region.

MOSFET Characteristics Curves

The minimum ON-state gate voltage required to ensure thatthe MOSFET remains fully-ON when carrying the selected draincurrent can be determined from the V-I transfer curvesabove. When VIN is HIGH or equal to VDD, the MOSFET Q-pointmoves to point A along the load line. The drain current ID

increases to its maximum value due to a reduction in thechannel resistance. ID becomes a constant value independentof VDD, and is dependent only on VGS. Therefore, thetransistor behaves like a closed switch but the channel ON-

resistance does not reduce fully to zero due to its RDS(on)

value, but gets very small.

Likewise, when VIN is LOW or reduced to zero, the MOSFET Q-point moves from point A to point B along the load line. Thechannel resistance is very high so the transistor acts likean open circuit and no current flows through the channel. Soif the gate voltage of the MOSFET toggles between twovalues, HIGH and LOW the MOSFET will behave as a "single-pole single-throw" (SPST) solid state switch and this actionis defined as:

1. Cut-off RegionHere the operating conditions of the transistor are zeroinput gate voltage ( VIN ), zero drain current ID and outputvoltage VDS = VDD Therefore the MOSFET is switched "Fully-OFF".

Cut-off Characteristics

The input and Gateare grounded (0v)

Gate-source voltageless than thresholdvoltage VGS < VTH

MOSFET is "fully-OFF"(Cut-off region)

No Drain currentflows ( ID = 0 )

VOUT = VDS = VDD = "1" MOSFET operates as an

"open switch"

Then we can define the "cut-off region" or "OFF mode" of aMOSFET switch as being, gate voltage, VGS < VTH and ID = 0.For a P-channel MOSFET, the gate potential must be negative.

2. Saturation RegionIn the saturation or linear region, the transistor will bebiased so that the maximum amount of gate voltage is appliedto the device which results in the channel resistance RDS(on)

being as small as possible with maximum drain currentflowing through the MOSFET switch. Therefore the MOSFET isswitched "Fully-ON".

Saturation Characteristics

The input and Gateare connected to VDD

Gate-source voltageis much greater thanthreshold voltageVGS > VTH

MOSFET is "fully-ON"(saturation region)

Max Drain currentflows ( ID = VDD /RL )

VDS = 0V (idealsaturation)

Min channelresistanceRDS(on) < 0.1Ω

VOUT = VDS = 0.2V(RDS.ID)

MOSFET operates as a"closed switch"

Then we can define the "saturation region" or "ON mode" of aMOSFET switch as gate-source voltage, VGS > VTH andID = Maximum. For a P-channel MOSFET, the gate potentialmust be positive.

By applying a suitable drive voltage to the gate of an FET,the resistance of the drain-source channel, RDS(on) can bevaried from an "OFF-resistance" of many hundreds of kΩ's,effectively an open circuit, to an "ON-resistance" of lessthan 1Ω, effectively a short circuit. We can also drive theMOSFET to turn "ON" faster or slower, or pass high or lowcurrents. This ability to turn the power MOSFET "ON" and"OFF" allows the device to be used as a very efficientswitch with switching speeds much faster than standardbipolar junction transistors.

An example of using the MOSFET as a switch

In this circuit arrangementan Enhancement-mode N-channel MOSFET is beingused to switch a simplelamp "ON" and "OFF" (couldalso be an LED). The gateinput voltage VGS is takento an appropriate positivevoltage level to turn thedevice and therefore thelamp either fully "ON",( VGS = +ve ) or at a zerovoltage level that turnsthe device fully "OFF",( VGS = 0 ).

If the resistive load ofthe lamp was to be replacedby an inductive load suchas a coil, solenoid orrelay a "flywheel diode"would be required inparallel with the load toprotect the MOSFET from anyself generated back-emf.

Above shows a very simple circuit for switching a resistiveload such as a lamp or LED. But when using power MOSFETs toswitch either inductive or capacitive loads some form ofprotection is required to prevent the MOSFET device frombecoming damaged. Driving an inductive load has the oppositeeffect from driving a capacitive load. For example, acapacitor without an electrical charge is a short circuit,resulting in a high "inrush" of current and when we removethe voltage from an inductive load we have a large reversevoltage build up as the magnetic field collapses, resultingin an induced back-emf in the windings of the inductor.

For the power MOSFET to operate as an analogue switchingdevice, it needs to be switched between its "Cut-off Region"where VGS = 0 and its "Saturation Region" were VGS(on) = +ve.The power dissipated in the MOSFET ( PD ) depends upon thecurrent flowing through the channel ID at saturation andalso the "ON-resistance" of the channel given as RDS(on). Forexample.

Example:Lets assume that the lamp is rated at 6v, 24W and is fully"ON", the standard MOSFET has a channel "ON-resistance"( RDS(on) ) value of 0.1ohms. Calculate the power dissipatedin the MOSFET switch.

The current flowing through the lamp is calculated as:

Then the power dissipated in the MOSFET will be given as:

You may think, well so what!, but when using the MOSFET as aswitch to control DC motors or high inrush current devicesthe "ON" channel resistance ( RDS(on) ) is very important. Forexample, MOSFETs that control DC motors, are subjected to ahigh in-rush current as the motor first begins to rotate asthe starting current is only limited by the resistance ofthe motors windings. Then a high RDS(on) channel resistancevalue would simply result in large amounts of power beingdissipated and wasted within the MOSFET itself resulting inan excessive temperature rise, and which in turn couldresult in the MOSFET becoming very hot and damaged due to athermal overload.

A lower value RDS(on) on the other hand, is also a desirableparameter as it helps to reduce the channels effectivesaturation voltage ( VDS(sat) = ID x RDS(on) ) across the MOSFET.Power MOSFETs generally have a RDS(on) value of less than0.01Ω.

One of the main limitation of a MOSFET is the maximumcurrent it can handle. So the RDS(on) parameter is animportant guide to the switching efficiency of the MOSFETand is simply the ratio of VDS / ID when the transistor isturned "ON". When using a MOSFET or any type of field effecttransistor for that matter as a solid-state switching device

it is always advisable to select ones that have a very lowRDS(on) value or at least mount them onto a suitable heatsinkto help reduce any thermal runaway and damage. Power MOSFETsused as a switch generally have surge-current protectionbuilt into their design, but for high-current applicationsthe bipolar junction transistor is a better choice.

Power MOSFET Motor ControlBecause of the extremely high input or gate resistance thatthe MOSFET has, its very fast switching speeds and the easeat which they can be driven makes them ideal to interfacewith op-amps or standard logic gates. However, care must betaken to ensure that the gate-source input voltage iscorrectly chosen because when using the MOSFET as a switchthe device must obtain a low RDS(on) channel resistance inproportion to this input gate voltage. Low threshold typeMOSFETs may not switch "ON" until a least 3V or 4V has beenapplied to its gate and if the output from the logic gate isonly +5V logic it may be insufficient to fully drive theMOSFET into saturation. Using lower threshold MOSFETsdesigned for interfacing with TTL and CMOS logic gates thathave thresholds as low as 1.5V to 2.0V are available.

Power MOSFETs can be used to control the movement of DCmotors or brushless stepper motors directly from computerlogic or by using pulse-width modulation (PWM) typecontrollers. As a DC motor offers high starting torque andwhich is also proportional to the armature current, MOSFETswitches along with a PWM can be used as a very good speedcontroller that would provide smooth and quiet motoroperation.

Simple Power MOSFET Motor Controller

As the motor load is inductive, a simple flywheel diode isconnected across the inductive load to dissipate any backemf generated by the motor when the MOSFET turns it "OFF". Aclamping network formed by a zener diode in series with thediode can also be used to allow for faster switching andbetter control of the peak reverse voltage and drop-outtime. An additional silicon or zener diode D1 can also beplaced across the channel of a MOSFET switch when usinginductive loads, such as motors, solenoids, etc, forsuppressing overvoltage switching transients and noisegiving extra protection to the MOSFET switch if required.Resistor R2 is used as a pull-down resistor to help pull theTTL output voltage down to 0V when the MOSFET is switched"OFF".

P-channel MOSFET Switch

Thus far we have looked at the N-channel MOSFET as a switchwere the MOSFET is placed between

P-channel MOSFETSwitchthe load and the ground. This also allows the gate drive orswitching signal to be referenced to ground (low-sideswitching). But in some applications we require the use ofP-channel enhancement-mode MOSFET were the load is connecteddirectly to ground and the MOSFET switch is connectedbetween the load and the positive supply rail (high-sideswitching) as we do with PNP transistors.

In a P-channel device the conventional flow of drain currentis in the negative direction so a negative gate-sourcevoltage is applied to switch the transistor "ON". This isachieved because the P-channel MOSFET is "upside down" withits source terminal tied to the positive supply +VDD. Thenwhen the switch goes LOW, the MOSFET turns "ON" and when theswitch goes HIGH the MOSFET turns "OFF".

This upside down connection of a P-channel enhancement modeMOSFET switch allows us to connect it in series with a N-channel enhancement mode MOSFET to produce a complementaryor CMOS switching device as shown across a dual supply.

Complementary MOSFET Motor Controller

The two MOSFETs are configured to produce a bi-directionalswitch from a dual supply with the motor connected betweenthe common drain connection and ground reference. When theinput is LOW the P-channel MOSFET is switched-ON as itsgate-source junction is negatively biased so the motorrotates in one direction. Only the positive +VDD supply railis used to drive the motor.

When the input is HIGH, the P-channel device switches-OFFand the N-channel device switches-ON as its gate-sourcejunction is positively biased. The motor now rotates in theopposite direction because the motors terminal voltage hasbeen reversed as it is now supplied by the negative -VDD

supply rail. Then the P-channel MOSFET is used to switch thepositive supply to the motor for forward direction (high-side switching) while the N-channel MOSFET is used to switchthe negative supply to the motor for reverse direction (low-side switching).

There are a variety of configurations for driving the twoMOSFETs with many different applications. Both the P-channeland the N-channel devices can be driven by a single gatedrive IC as shown. However, to avoid cross conduction withboth MOSFETs conducting at the same time across the twopolarities of the dual supply, fast switching devices arerequired to provide some time difference between them

turning "OFF" and the other turning "ON". One way toovercome this problem is to drive both MOSFETs gatesseparately. This then produces a third option of "STOP" tothe motor when both MOSFETs are "OFF".

MOSFET 1 MOSFET 2 Motor Function

OFF OFF Motor Stopped(OFF)

ON OFF Motor RotatesForward

OFF ON Motor RotatesReverse

ON ON NOT ALLOWED

Introduction to SCR-Silicon ControlledRectifier

SCR-Schematic-Symbol

As the terminology indicates, the SCR is a controlledrectifier constructed of a silicon semiconductor materialwith a third terminal for control purposes. Silicon waschosen because of its high temperature and powercapabilities. The basic operation of the SCR is differentfrom that of an ordinary two-layer semiconductor diode inthat a third terminal called a gate, determines when therectifier switches from the open-circuit to short-circuitstate. It is not enough simply to forward-bias the anode-to-cathode region of the device. In the conduction state thedynamic resistance of the SCR is typically 0.01 to 0.1 ohmand reverse resistance is typically 100 kilo ohm or more. Itis widely used as a switching device in power controlapplications. It can control loads by switching on and offupto many thousand times a second. It can switch on for avariable lengths of time duration, thereby delivering de-sired amount of power to the load. Thus, it possesses theadvantage of a rheostat as well as a switch with none oftheir drawback. A schematic diagram and symbolicrepresentation of an SCR are shown in figures a & brespectively. As illustrated in fig-a, SCR is a three-terminal four-layer semiconductor device, the layers being

alternately of P-type and N-type. The junctions are markedJj, J2 and J3

Construction of an SCR

SCR - construction types

From fig a it is clear that SCR is essentially an ordinaryrectifier (PN) and a junction transistor (N-P-N) combined inone unit to form PNPN device. Three terminals are taken: onefrom the outer P-type material, known as anode, second fromthe outer N-type material, known as cathode and the thirdfrom the base of transistor section known as the gate.

The basic material used for fabrication of an SCR is N-typesilicon. It has a specific resistance of about 6 ohm-mm.Silicon is the natural choice as base material because ofthe following advantages

(i) ability to withstand high junction temperature of theorder of 150° C

(ii) high thermal conductivity;

(iii) less variations in characteristics with temperature;and

(iv) less leakage current in P-N junction.

It consists, essentially, of a four layer pellet of P and Ntype silicon semiconductor materials. The junctions arediffused or alloyed. The material which may be used for Pdiffusion is aluminium and for N diffusion is phosphorous.The contact with anode can be made with an aluminium foiland through cathode and gate by metal sheet. Diffusion mustbe carried out at a proper temperature and for necessaryduration to provide correct concentration because thisdecides the properties of the device. Low power SCRs employthe planar construction shown in fig a. Planar constructionis useful for making a number of units from a silicon wafer.Here, all the junctions are diffused. The other technique isthe mesa construction shown in fig.b. This technique is usedfor high power SCRs. In this technique, the inner junctionJ2 is obtained by diffusion, and then the outer two layersare alloyed to it. The PNPN pellet is properly braced withtungsten or molybdenum plates to provide greater mechanicalstrength and make it capable of handling large currents. Oneof these plates is hard soldered to a copper or an aluminiumstud, which is threaded for attachment to a heat sink. Thisprovides an efficient thermal path for conducting theinternal losses to the surrounding medium. The uses of hardsolder between the pellet and back-up plates minimisesthermal fatigue, when the SCRs are subjected to temperatureinduced stresses. For medium and low power SCRs, the pelletis mounted directly on the copper stud or casing, using asoft solder which absorbs the thermal stresses set up bydifferential expansion and provides a good thermal path forheat transfer. For a larger cooling arrangement, which isrequired for high power SCRs, the press-pack or hockey-puckconstruction is employed, which provides for double-sidedair for cooling.

The salient features to be considered, while designing anSCR, are the diameter and thickness of wafer, composition ofthe base material, type and amount of the material to bediffused into the wafer, shape, position and contact area ofthe gate, shape and size of the SCR, type of heat sink etc.

Fabrication technology determines various properties of thedevice. The voltage rating of a device can be increased bylightly doping the inner two layers and increasing theirthickness. But due to this increased resistance, forwardvoltage drop increases and large triggering currents arerequired causing greater power dissipation accompanied bysmaller current ratings. The heat dissipation of siliconfalls from 1.5 W/cm2 at 25° C to 1.25 W/ cm2 at 125° C. Ahigh voltage power device can seldom be used beyond 125° C.

The current carrying capacity and voltage rating of thedevice can be increased by irradiating silicon withneutrons. The current rating of the device can also beincreased by reducing the current density at the junctionbut this result in a bulky device with large turn-on time.

SCR Characteristics

As already mentioned, the SCR is a four-layer device withthree terminals, namely, the anode, the cathode and thegate. When the anode is made positive with respect to thecathode, junctions J1 and J3 are forward biased and junctionJ2 is reverse-biased and only the leakage current will flowthrough the device. The SCR is then said to be in the

forward blocking state or in the forward mode or off state.But when the cathode is made positive with respect to theanode, junctions J1 and J3 are reverse-biased, a smallreverse leakage current will flow through the SCR and theSGR is said to be in the reverse blocking state or inreverse mode.

When the anode is positive with respect to cathode i.e. whenthe SCR is in forward mode, the SCR does not conduct unlessthe forward voltage exceeds certain value, called theforward breakover voltage, VFB0. In non-conducting state, thecurrent through the SCR is the leakage current which is verysmall and is negligible. If a positive gate current issupplied, the SCR can become conducting at a voltage muchlesser than forward break-over voltage. The larger the gatecurrent, lower the break-over voltage. With sufficientlylarge gate current, the SCR behaves identical to PNrectifier. Once the SCR is switched on, the forward voltagedrop across it is suddenly reduced to very small value, sayabout 1 volt. In the conducting or on-state, the currentthrough the SCR is limited by the external impedance.

When the anode is negative with respect to cathode, that iswhen the SCR is in reverse mode or in blocking state nocurrent flows through the SCR except very small leakagecurrent of the order of few micro-amperes. But if thereverse voltage is increased beyond a certain value, calledthe reverse break-over voltage, VRB0 avalanche break downtakes place. Forward break-over voltage VFB0 is usuallyhigher than reverse breakover voltage,VRBO.

From the foregoing discussion, it can be seen that the SCRhas two stable and reversible operating states. The changeover from off-state to on-state, called turn-on, can beachieved by increasing the forward voltage beyond VFB0. Amore convenient and useful method of turn-on the deviceemploys the gate drive. If the forward voltage is less thanthe forward break-over voltage, VFB0, it can be turned-on byapplying a positive voltage between the gate and thecathode. This method is called the gate control. Another

very important feature of the gate is that once the SCR istriggered to on-state the gate loses its control.

The switching action of gate takes place only when

(i)                 SCR is forward biased i.e. anode ispositive with respect to cathode, and

(ii)                 Suitable positive voltage is appliedbetween the gate and the cathode.

Once the SCR has been switched on, it has no control on theamount of current flowing through it. The current throughthe SCR is entirely controlled by the external impedanceconnected in the circuit and the applied voltage. There is,however, a very small, about 1 V, potential drop across theSCR. The forward current through the SCR can be reduced byreducing the applied voltage or by increasing the circuitimpedance. There is, however, a minimum forward current thatmust be maintained to keep the SCR in conducting state. Thisis called the holding current rating of SCR. If the currentthrough the SCR is reduced below the level of holdingcurrent, the device returns to off-state or blocking state.

The SCR can be switched off by reducing the forward currentbelow the level of holding current which may be done eitherby reducing the applied voltage or by increasing the circuitimpedance.The gate can only trigger or switch-on the SCR, itcannot switch off.

Alternatively the SCR can be switched off by applyingnegative voltage to the anode (reverse mode), the SCRnaturally will be switched off.

Here one point is worth mentioning, the SCR takes certaintime to switch off. The time, called the turn-off time, mustbe allowed before forward voltage may be applied againotherwise the device will switch-on with forward voltagewithout any gate pulse. The turn-off time is about 15 micro-seconds, which is immaterial when dealing with power

frequency, but this becomes important in the invertercircuits, which are to operate at high frequency.

Applications of SCRSix applications of SCR like power control, switching, zero-voltage switching, over-voltage protection, pulse circuitsand battery charging regulator.

1. Power Control.

SCR Power Control Circuit

Because of the bistable characteristics of semiconductordevices, whereby they can be switched on and off, and theefficiency of gate control to trigger such devices, the SCRsare ideally suited for many industrial applications. SCRshave got specific advantages over saturable core reactorsand gas tubes owing to their compactness, reliability, lowlosses, and speedy turn-on and turn-off.

The bistable states (conducting and non-conducting) of theSCR and the property that enables fast transition from onestate to the other are made use of in the control of powerin both ac and dc circuits.

SCR Phase Control

In ac circuits the SCR can be turned-on by the gate at anyangle α with respect to applied voltage. This angle α iscalled the firing angle and power control is obtained byvarying the firing angle. This is known as phase control. Asimple half-wave circuit is shown in figure a. forillustrating the principle of phase control for an inductiveload. The load current, load voltage and supply voltagewaveforms are shown in figure b. The SCR will turn-off bynatural commutation when the current becomes zero. Angle βis known as the conduction angle. By varying the firingangle a, the rms value of the load voltage can be varied.The power consumed by the load decreases with the increasein firing angle a. The reactive power input from the supplyincreases with the increase in firing angle. The loadcurrent wave-form can be improved by connecting a free-wheeling diode D1, as shown by the dotted line in fig-a.With this diode, SCR will be turned-off as soon as the inputvoltage polarity reverses. After that, the load current willfree wheel through the diode and a reverse voltage willappear across the SCR. The main advantage of phase controlis that the load current passes through a natural zero pointduring every half cycle. So, the device turns-off by itselfat the end of every conducting period and no othercommutating circuit is required.

Power control in dc circuits is obtained by varying theduration of on-time and off-time of the device and such amode of operation is called on-off control or choppercontrol. Another important application of SCRs is ininverters, used for converting dc into ac. The inputfrequency is related to the triggering frequency of SCRs inthe inverters. Thus, variable frequency supply can be easilyobtained and used for speed control of ac motors, induction

heating, electrolytic cleaning, fluorescent lighting andseveral other applications. Because of the large power-handling capacity of the SCRs, the SCR controlled inverterhas more or less replaced motor-generator sets and magneticfrequency multipliers for generating high frequency at largepower ratings.

Operation of Power Control in SCR

A commonly used circuit for controlling power in load RL

using two SCRs is shown in figure. Potentiometer R controlsthe angle of conduction of the two SCRs. The greater theresistance of the pot, lesser will be the voltage acrosscapacitors C1 and C2 and hence smaller will be the timeduration of conduction of SCR1 and SCR2 during a cycle.

During positive half cycle capacitor C2 gets chargedthrough diode D1, pot R, and diode D4. When the capacitorgets fully charged, (charge on the capacitor depending uponthe value of R) it discharges through Zener diode Z. Thisgives a pulse to the primary and thereby secondary of thetransformer T2. Thus SCR2, which is forward biased, isturned on and conducts through load RL. During negative halfcycle similar action takes place due to charging ofcapacitor C1 and SCR1 is triggered. Thus power to a load iscontrolled by using SCRs.

2. Switching.

Thyristor, being bistable device is widely used forswitching of power signals owing to their long life, highoperation speed and freedom from other defects associatedwith mechanical and electro-mechanical switches.

AC Circuit Breaker using SCR

Figure shows a circuit in which two SCRs are used for makingand breaking an ac circuit. The input voltage is alternatingand the trigger pulses are applied to the gates of SCRsthrough the control switch S. Resistance R is provided inthe gate circuit to limit the gate current while resistorsR1 and R2 are to protect the diodes D1 and D2 respectively.

For starting the circuit, when switch S is closed, SCR1 willfire at the beginning of the positive half-cycle (the gatetrigger current is assumed to be very small) because duringpositive half cycle SCR1 is forward biased. It will turn-offwhen the current goes through the zero value. As soon asSCR1 is turned-off, SCR2 will fire since the voltagepolarity is already reversed and it gets the proper gatecurrent. The circuit can be broken by opening the switch S.Opening of gate circuit poses no problem, as current throughthis switch is small. As no further gate signal will beapplied to the SCRs when switch S is open, the SCRs will notbe triggered and the load current will be zero. The maximumtime delay for breaking the circuit is one half-cycle.

Thus several hundred amperes of load current can be switchedon/off simply by handling gate current of few mA by anordinary switch. The above circuit is also called the staticcontactor because it does not have any moving part.

DC circuit breaker

SCR Application-DC Circuit Breaker

As shown in figure, Capacitor C provides the requiredcommutation of the main SCR since the current does not havea natural zero value in a dc circuit. When the SCR1 is inconducting state, the load voltage will be equal to thesupply voltage and the capacitor C will be charged throughresistor R. The circuit is broken by turning-off SCR1. Thisis done by firing SCR2, called the auxiliary SCR. CapacitorC discharges through SCR2 and SCR1. This discharge currentis in opposite direction to that flowing through SCR1 andwhen the two become equal SCR2 turns-off. Now capacitor Cgets charged through the load and when the capacitor C getsfully charged, the SCR2 tums-off. Thus the circuit acts as adc circuit breaker. The resistor R is taken of such a valuethat current through R is lower than that of holdingcurrent.

3. Zero Voltage Switching.

SCR Switching Application

In some ac circuits it is necessary to apply the voltage tothe load when the instantaneous value of this voltage isgoing through the zero value. This is to avoid a high rateof increase of current in case of purely resistive loadssuch as lighting and furnace loads, and thereby reduce thegeneration of radio noise and hot-spot temperatures in thedevice carrying the load current. The circuit to achievethis is shown in figure. Only half-wave control is usedhere. The portion of the circuit shown by the dotted linesrelates to the negative half cycle. Whatever may be theinstant of time when switch S is opened (either during thepositive or the negative half cycle), only at the beginningof the following positive half-cycle of the applied voltageSCR1 will be triggered. Similarly, when switch S is closed,SCR1 will stop conducting at the end of the present orprevious positive half-cycle and will not get triggeredagain. Resistors R3 and R4 are designed on the basis ofminimum base and gate currents required for transistor Q1

and SCR1. Resistors Rl and R2 govern rates of the chargingand discharging of capacitor C1 Resistor R5 is used forpreventing large discharge currents when switch S is closed.

4. Over-Voltage Protection.

Over Voltage Circuit Protection

SCRs can be employed for protecting other equipment fromover-voltages owing to their fast switching action. The SCRemployed for protection is connected in parallel with theload. Whenever the voltage exceeds a specified limit, thegate of the SCR will get energized and trigger the SCR. Alarge current will be drawn from the supply mains andvoltage across the load will be reduced. Two SCRs are used—one for the positive half-cycle and the other for negativehalf-cycle, as shown in figure. Resistor R1 limits theshort-circuit current when the SCRs are fired. Zener diodeD5 in series with resistors Rx and R2 constitutes a voltage-sensing circuit.

5. Pulse Circuits.

SCR-Pulse Circuit

SCRs are used for producing high voltage/current pulses ofdesired waveform and duration. The capacitor C is chargedduring the positive half cycle of the input supply and theSCR is triggered during the negative half-cycle. Thecapacitor will discharge through the output circuit, andwhen the SCR forward current becomes zero, it will turn-off.The output circuit is designed to have discharge current ofless than a milli-second duration. The capacitor will againget charged in the following positive half-cycle and the SCRwill be triggered again in the negative half-cycle. Thus thefrequency of the output pulse will be equal to the frequencyof the input supply. For limiting the charging currentresistor R is used. High voltage/current pulses can be usedin spot welding, electronic ignition in automobiles,generation of large magnetic fields of short duration, andin testing of insulation.

6. Battery Charging Regulator.

Battery Charging Regulator

The basic components of the circuits are shown in figure.Diodes D1 and D2 are to establish a full-wave rectified sig-nal across SCR1 and the 12 V battery to be charged. When thebattery is in discharged condition, SCR2 is in the off-stateas will be clear   after discussion. When the full-wave

rectified input is large enough to give the required turn-ongate current (controlled by resistor R1), SCR1 will turn onand the charging of the battery will commence. At thecommencement of charging of battery, voltage VR determinedby the simple voltage-divider circuit is too small to cause11.0 V zener conduction. In the off-state Zener diode iseffectively an open-circuit maintaining SCR2 in the off-state because of zero gate current. The capacitor C isincluded in the circuit to prevent any voltage transients inthe circuit from accidentally turning on of the SCR2. Ascharging continues, the battery voltage increases to a pointwhen VR is large enough to both turn on the 11.0 V Zenerdiode and fire SCR2. Once SCR2 has fired, the short circuitrepresentation for SCR2 will result in a voltage-dividercircuit determined by R1 and R2 that will maintain V2 at alevel too small to turn SCR1 on. When this occurs, thebattery is fully charged and the open-circuit state of SCR1

will cut off the charging current. Thus the regulatorcharges the battery whenever the voltage drops and preventsovercharging when fully charged. There are many moreapplications of SCRs such as in soft start circuits, logicand digital circuits

Introduction to Triac-Its constructionand OperationThe triac is another three-terminal ac switch that istriggered into conduction when a low-energy signal isapplied to its gate terminal. Unlike the SCR, the triacconducts in either direction when turned on. The triac alsodiffers from the SCR in that either a positive or negativegate signal triggers it into conduction. Thus the triac is athree terminal, four layer bidirectional semiconductordevice that controls ac power whereas an SCR controls dcpower or forward biased half cycles of ac in a load. Becauseof its bidirectional conduction property, the triac is

widely used in the field of power electronics for controlpurposes. Triacs of 16 kW rating are readily available inthe market.

“Triac” is an abbreviation for three terminal ac switch.‘Tri’-indicates that the device has three terminals and ‘ac’indicates that the device controls alternating current orcan conduct in either direction.

Triac Circuit Symbol

Construction of a Triac

As mentioned above, triac is a three terminal, four layerbilateral semiconductor device. It incorporates two SCRsconnected in inverse parallel with a common gate terminal ina single chip device. The arrangement of the triac is shownin figure. As seen, it has six doped regions. The gateterminal G makes ohmic contacts with both the N and Pmaterials. This permits trigger pulse of either polarity tostart conduction. Electrical equivalent circuit andschematic symbol are shown in figure.b and figure.crespectively. Since the triac is a bilateral device, theterm “anode” and ”cathode” has no meaning, and therefore,terminals are designated as main terminal 1. (MT1), mainterminal 2 (MT2) and gate G. To avoid confusion, it has

become common practice to specify all voltages and currentsusing MT1 as the reference.

Triac Basic Structure

Operation and Working of a Triac

Though the triac can be turned on without any gate currentprovided the supply voltage becomes equal to the breakovervoltage of the triac but the normal way to turn on the triacis by applying a proper gate current. As in case of SCR,here too, the larger the gate current, the smaller thesupply voltage at which the triac is turned on. Triac canconduct current irrespective of the voltage polarity ofterminals MT1 and MT2 with respect to each other and that ofgate and terminal MT2. Consequently four differentpossibilities of operation of triac exists. They are:

1. Terminal MT2 and gate are positive with respect toterminal MT1

When terminal MT2 is positive with respect to terminal MT1

current flows through path P1-N1-P2-N2. The two junctions P1-N1

and P2-N2 are forward biased whereas junction N1 P2 isblocked. The triac is now said to be positively biased.

A positive gate with respect to terminal MT1 forward biasesthe junction P2-N2 and the breakdown occurs as in a normalSCR.

2. Terminal MT2 is positive but gate is negative withrespect to terminal MT1

Though the flow path of current remains the same as in mode1 but now junction P2-N3 is forward biased and currentcarriers injected into P2 turn on the triac.

3.Terminal MT2 and gate are negative with respect toterminal MT1

When terminal MT2 is negative with respect to terminal MT1,the current flow path is P2-N1-P1-N4. The two junctions P2-N1

and P1 - N4 are forward biased whereas junction N1-P1 isblocked. The triac is now said to be negatively biased.

A negative gate with respect to terminal MT1 injects currentcarriers by forward biasing junction P2-N3 and thusinitiates the conduction.

4. Terminal MT2 is negative but gate is positive withrespect to terminal MT1

Though the flow path of current remains the same as in mode3 but now junction P2-N2 is forward biased, current carriersare injected and therefore, the triac is turned on.

Generally, trigger mode 4 should be avoided especially incircuits where high di/dt  may occur. The sensitivity oftriggering modes 2 and 3 is high and in case of marginaltriggering capability negative gate pulses should be used.Though the triggering mode 1 is more sensitive compared tomodes 2 and 3, it requires a positive gate trigger. However,for bidirectional control and uniform gate trigger modes 2and 3 are preferred.

Characteristics of  Triac

TRIAC Characteristics

Typical V-I characteristics of a triac are shown in figure.The triac has on and off state characteristics similar toSCR but now the char acteristic is applicable to bothpositive and negative voltages. This is expected becausetriac consists of two SCRs connected in parallel butopposite in direc tions.

MT2 is positive with respect to MTX in the first quadrantand it is negative in the third quad rant. As already saidin previous blog posts, the gate triggering may occur in anyof the following four modes.

Quadrant I operation     :     VMT2, positive; VG1 positive

Quadrant II operation    :     VMT21 positive;  VGl negative

Quadrant III operation  :      VMT21 negative; VGl negative

Quadrant IV operation   :     VMT21 negative; VG1 positive

where VMT21 and VGl are the voltages of terminal MT2 and gatewith respect to terminal MT1.

The device, when starts conduction permits a very heavyamount of current to flow through it. This large inrush ofcurrent must be restricted by employing external resistance, otherwise the device may get damaged.

The gate is the control terminal of the device. By applyingproper signal to the gate, the firing angle of the devicecan be controlled. The circuits used in the gate fortriggering the device are called the gate-triggeringcircuits. The gate-triggering circuits for the triac arealmost same like those used for SCRs. These triggeringcircuits usually generate trigger pulses for firing thedevice. The trigger pulse should be of sufficient magnitudeand duration so that firing of the device is assured.Usually, a duration of 35 us is sufficient for sustainingthe firing of the device.

A typical triac has the following voltage/current values:

Instantaneous on-state voltage – 1.5 Volts On-state current – 25 Amperes

Holding current, IH - 75 Milli Amperes

Average triggering current, IG – 5 Milli Amperes

Applications of TriacNext to SCR, the triac is the most widely used member of thethyristor family. In fact, in many of control applications,it has replaced SCR by virtue of its bidirectionalconductivity. Motor speed regulation, temperature control,illumination control, liquid level control, phase controlcircuits, power switches etc. are some of its mainapplications.

However, the triac is less versatile than the SCR when turn-off is considered. Because the triac can conduct in either

direction, forced commutation by reverse-biasing cannotbe employed. So turn-off is either by current starvation,which is usually impracticable, or else by ac linecommutation. There are two limitations enforced on the useof triac at present state of commercially available devices(200 A and 1,000 PRV). The first is the frequency handlingcapability produced by the limiting dv/dt at which the triacremains blocking when no gate signal is applied. This dv/dtvalue is about 20 Vmicros-1 compared with a general figureof 200 Vmicro s-1 for the SCR, so that the limitation offrequency is at the power level of 50 Hz. The same dv/dtlimitation means the load to be controlled is preferably aresistive one. When high frequencies and high dv/dt areinvolved then the back-to-back SCRs cannot be replaced bythe triac.

Triac Applications

1. High Power Lamp Switching.

Use of the triac as an ac on/off switch is shown in figure.When the switch S is in position 1, the triac is cut-off andso the lamp-is’dark. When the switch is put in position 2, asmall gate current flowing through the gate turns the triacon and so the lamp is switched on to give rated output.

Triac Application

2.  AC Power Control.

A triac control circuit is shown in figure. Here it iscontrolling ac power to load by switching on and off duringthe positive and negative half cycles of the inputsinusoidal signal.

During the positive half cycle of the input voltage, diodeD1 is forward biased, D2 is reverse-biased, and the gateterminal is positive with respect to A1 During the negativehalf cycle, the diode D2 is forward biased and diode D1 isreverse-biased, so that the gate becomes positive withrespect to terminal A2- The point of commencement ofconduction is controlled by adjusting the resistance R2.

Unijunction transistor (abbreviated as UJT)

Unijunction transistor (abbreviated as UJT), also called thedouble-base diode is a 2-layer, 3-terminal solid-state(silicon) switching device. The device has-a uniquecharacteristic that when it is triggered, its emittercurrent increases re generatively (due to negativeresistance characteristic) until it is restricted by emitterpower supply. The low cost per unit, combined with itsunique characteristic, have warranted its use in a widevariety of applications. A few include oscillators, pulsegenerators, saw-tooth generators, triggering circuits, phasecontrol, timing circuits, and voltage-or current-regulatedsupplies. The device is in general, a low-power-absorbingdevice under normal operating conditions and provides

tremendous aid in the continual effort to design relativelyefficient systems!

Construction of a UJT

The basic structure of a unijunction transistor is shown infigure. It essentially consists of a lightly-doped N-typesilicon bar with a small piece of heavily doped P-typematerial alloyed to its one side to produce single P-Njunction. The single P-N junction accounts for theterminology unijunction. The silicon bar, at its ends, hastwo ohmic contacts designated as base-1 (B1) and base-2(B2), as shown and the P-type region is termed the emitter(E). The emitter junction is usually located closer to base-2 (B2) than base-1 (B1) so that the device is notsymmetrical, because symmetrical unit does not provideoptimum electrical characteristics for most of theapplications.

UJT Symbol and Construction

The symbol for unijunction transistor is shown in figure.The emitter leg is drawn at an angle to the vertical linerepresenting the N-type material slab and the arrowheadpoints in the direction of conventional current when thedevice is forward-biased, active or in the conducting state.The basic arrangement for the UJT is shown in figure.

A complementary UJT is formed by diffusing an N-type emitterterminal on a P-type base. Except for the polarities ofvoltage and current, the characteristics of a complementaryUJT are exactly the same as those of a conventional UJT.

The worth noting points about UJT are given below:

The device has only one junction, so it is called theunijunction device.

The device, because of one P-N junction, is quitesimilar to a diode but it differs from an ordinarydiode as it has three terminals.

The structure of a UJT is quite similar to that of anN-channel JFET. The main difference is that P-type(gate) material surrounds the N-type (channel) materialin case of JFET and the gate surface of the JFET ismuch larger than emitter junction of UJT.

In a unijunction transistor the emitter is heavilydoped while the N-region is lightly doped, so theresistance between the base terminals is relativelyhigh, typically 4 to 10 kilo Ohm when the emitter isopen.

The N-type silicon bar has a high resistance and theresistance between emitter and base-1 is larger thanthat between emitter and base-2. It is because emitteris closer to base-2 than base-1.

UJT is operated with emitter junction forward- biasedwhile the JFET is normally operated with the gatejunction reverse-biased.

UJT does not have ability to amplify but it has theability to control a large ac power with a smallsignal. It exhibits a negative resistancecharacteristic and so it can be employed as anoscillator.

Operation of a UJT

Imagine that the emitter supply voltage is turned down tozero. Then the intrinsic stand-off voltage reverse-biasesthe emitter diode, as mentioned above. If VB is the barriervoltage of the emitter diode, then the total reverse biasvoltage is VA + VB = Ƞ VBB + VB. For silicon VB = 0.7 V.

Now let the emitter supply voltage VE be slowly increased.When VE becomes equal to Ƞ VBB, IEo will be reduced to zero.With equal voltage levels on each side of the diode, neitherreverse nor forward current will flow. When emitter supplyvoltage is further increased, the diode becomes forward-biased as soon as it exceeds the total reverse bias voltage(Ƞ VBB + VB). This value of emitter voltage VE is called thepeak-point voltage and is denoted by VP. When VE = VP,emitter current IE starts to flow through RB1 to ground, thatis B1. This is the minimum current that is required totrigger the UJT. This is called the peak-point emittercurrent and denoted by IP. Ip is inversely proportional tothe interbase voltage, VBB. Now when the emitter diodestarts conducting, charge carriers are injected into the RBregion of the bar. Since the resistance of a semiconductormaterial depends upon doping, the resistance of region RBdecreases rapidly due to additional charge carriers (holes).With this decrease in resistance, the voltage drop across RB

also decrease, cause the emitter diode to be more heavilyforward biased. This, in turn, results in larger forwardcurrent, and consequently more charge carriers are injectedcausing still further reduction in the resistance of the RBregion. Thus the emitter current goes on increasing until itis limited by the emitter power supply. Since VA decreaseswith the increase in emitter current, the UJT is said tohave negative resistance characteristic. It is seen that thebase-2 (B2) is used only for applying external voltage VBBacross it. Terminals E and B1 are the active terminals. UJTis usually triggered into conduction by applying a suitablepositive pulse to the emitter. It can be turned off byapplying a negative trigger pulse.

UJT Characteristics

The static emitter characteristic (a curve showing therelation between emitter voltage VE and emitter current IE)of a UJT at a given inter base voltage VBB is shown infigure.  From figure it is noted that for emitter potentials

to the left of peak point, emitter current IE never exceedsIEo . The current IEo corresponds very closely to the reverseleakage current ICo of the conventional BJT. This region, asshown in the figure, is called the cut-off region. Onceconduction is established at VE = VP the emitter potential VE

starts decreasing with the increase in emitter current IE.This Corresponds exactly with the decrease in resistance RB

for increasing current IE. This device, therefore, has anegative resistance region which is stable enough to be usedwith a great deal of reliability in the areas ofapplications listed earlier. Eventually, the valley pointreaches, and any further increase in emitter current IE

places the device in the saturation region, as shown in thefigure. Three other important parameters for the UJT are IP,VV and IV and are defined below:

Peak-Point Emitter Current. Ip. It is the emitter current atthe peak point. It represents the rnimrnum current that isrequired to trigger the device (UJT). It is inverselyproportional to the interbase voltage VBB.

Valley Point Voltage VV The valley point voltage is theemitter voltage at the valley point. The valley voltageincreases with the increase in interbase voltage VBB.

Valley Point Current IV The valley point current is theemitter current at the valley point. It increases with theincrease in inter-base voltage VBB.

Special Features of UJT. The special features of a UJT are :

1. A stable triggering voltage (VP)— a fixed fraction ofapplied inter base voltage VBB.

2. A very low value of triggering current.

3. A high pulse current capability.

4. A negative resistance characteristic.

5. Low cost.