JFETS AND MOSFETS
Instructor
Engr: Mir Muhammad Lodro
Lecturer
Department of Electrical (Telecomm) Engineering
Sukkur IBA
OUTLINES
JFETs N-channel JFET P-channel JFET Operation of JFET JFET terminals JFET drain curves JFET trans-conductance JFET biasing
Ohmic region Active region
MOSFET Depletion mode Enhancement mode
Fig: JFET biased for conduction (common source configuration)
JFET OPERATION
DRAIN CHARACTERISTIC CURVES FOR JFET
VGS controls ID
For the JFET in the following figure, VGS(off)= - 4v and IDSS=12mA. Determine minimum value of VDD required to put the device in constant current area of operation.
Since VGS(off) = -4 V ,VP=4V. The minimum value of VDS for the JFET to be in its constant current area is
In the constant current area with VGS=0 V,
Drop across drain resistor
Apply kirchhoff’s law around the drain
circuit
This is the required value of VDD to put the transistor in constant current area
Sol:
EXAMPLE:
JFET FORWARD TRANS-CONDUCTANCE
JFET BIASING
Self-bias Voltage-divider bias
SELF-BIAS JFETS
For n-channel JFET,IS=ID and VG=0 ,VS=IDRD gate to source voltage is
Thus
Drain voltage with respect to ground is
Since VS=IDRS , the drain to source voltage is
Find VDS and VGS for following JFET circuit, internal parameter values are such as gm, VGS(off), and IDSS are such that a drain current of approximately 5mA is produced . Another JFET even of the same type may not produce the same results when connected in this circuit due the variations in parameter value.
Solution
Here ID=IS
VGS is –ve for n-channel
EXAMPLE
MID-POINT BIAS
Usually desirable to bias a JFET near mid-point of its transfer characteristic curve where ID=IDSS/2Mid-point allows maximum amount of drain current swing between IDSS
and 0.When VGS=VGS(off)/3.4 = ID=IDSS/2
Select resistor values for RD and RS to setup an approximate mid-point bias. For this particular JFET, the parameters are IDSS =12mA and VGS(off)= -3 V. VD should be approximately 6 V.
EXAMPLE
SolutionFor mid-point biasing
Then
VOLTAGE DIVIDER BIAS
Fig: n-channel JFET with voltage-divider bias (IS=ID)
Voltage at source of JFET must be more positive than voltage at the gate in order to keep the gate-source junction reverse-biased
Gate voltage is set by resistors R1 and R2
Source voltage
Gate to source voltage is
And source voltage is
The drain current can be expressed as
EXAMPLE
Determine ID and VGS for the JFET with voltage divider bias given that for this particular JFET the internal parameter values are such that VD=7V.
Solution
Calculate gate to source voltage
For voltage divider Q-point is determined as follows For ID=0
For VGS=0
The point at which load line intersects the transfer characteristic curve is known as Q-point
Fig: Generalized dc load line for a JFET with voltage-divider bias
EXAMPLE
Determine the approximate Q-point for the JFET with voltage-divider bias , given that this particular device has transfer characteristic curve of
Solution
First establish two points for the bias line i-e ID and VGS
Approximate Q-point values are ID=1.8mA and VGS=-1.8 V
MOSFET (METAL OXIDE SEMI-CONDUCTOR FET)
MOSFET is another category of field effect transistor. The MOSFET differs from JFET in that it has no pn junction structure.Gate is insulated from channel by silicon dioxide (SiO2) layer
MOSFETModes
Depletion (D-MOSFET)
Enhancement (E-MOSFET)
D-MOSFET (DEPLETION MOSFET)
N-channel MOSFET operates in depletion mode when a negative gate to source voltage is applied and in enhancement mode when positive gate to source voltage is applied
These devices are generally operated in Depletion mode
D-MOSFET OPERATION
Greater the negative voltage on gate, greater the depletion of n-channel electrons
With positive gate voltage, more electrons are attracted into the channel, thus increasing channel conductivity.
E-MOSFET (ENHANCEMENT-MOSFET)
•E-MOSFET operates only in enhancement mode and has no depletion mode•E-MOSFET has no structural channel (substrate extends completely to SiO2 layer)•Positive gate voltage induces a channel by creating a thin layer of negative charges in substrate region adjacent to SiO2 layer. •For gate voltage below threshold there is no channel • conductivity of channel is enhanced by increasing gate-to-source voltage and thus pulling more electrons into the channel area
Fig: E-MOSFET schematic symbols
Broken lines symbolize the absence of physical channel
THANKS
Drs Ian Munro Ross (front) and G.C Dacey jointly developed experimental procedure for measuring characteristics of Field Effect Transistor in 1955.
(AT&T archives)