Chapter 4

Bipolar Junction Transistors (BJTs)

Objectives• Describe the basic structure of the bipolar

junction transistor (BJT)

• Explain and analyze basic transistor bias and operation

• Discuss the parameters and characteristics of a transistor and how they apply to transistor circuits

4-1 Transistor Structure The BJT is constructed

with three doped semiconductor regions separated by two pn junctions.

The three region are called emitter (E),base (B) and collector (C)

4-1 Transistor Structure (cont.)

The BJT have 2 types:

1. Two n region separate by a p region – called npn

2. Two p region separated by a n region – called pnp

4-1 Transistor Structure (cont.) The pn junction joining

the base region and the emitter region is called the base-emiter junction

The pn junction joining the base region and the collector region is call base-collector junction

4-1 Transistor Structure (cont.)• The base region is

lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector region

4-2 Basic Transistor Operation To operate the transistor properly, the two pn junction

must be correctly biased with external dc voltages.

The figure shown the proper bias arrangement for both npn and pnp transistor for active operation as an amplifier.

4-2 Basic Transistor Operation (cont.)

The forward bias from base to emitter narrow the BE depletion region, and the reverse bias from base to collector widens the BC depletion region.

The heavily doped n-type emitter region is teeming with conduction-band (free ) electrons that easily diffuse through BE junction into the p-type base region where they become minority carriers.

4-2 Basic Transistor Operation (cont.)

The base region is lightly doped and very thin so that it has a very limited number of holes.

Thus only a small percentage of all the electrons flowing through the BE junction can combine with the available holes in the base.

4-2 Basic Transistor Operation (cont.)

A few recombined electrons flow out of the base lead as valence electrons, forming the small base electron current.

Most of electrons from the emitter diffuse into the BC depletion region.

Once in this region they are pulled through the reverse-biased BC junction by the electric field set up by the force of attraction between the positive and negative

4-2 Basic Transistor Operation (cont.)

• Once in this region they are pulled through the reverse-biased BC junction by the electric field set up by the force of attraction between the positive and negative

4-2 Basic Transistor Operation (cont.)

The electron now move through the collector region out through the collector lead into the positive terminal of the collector voltage source.

The operation of pnp transistor is the same as for the npn except that the roles of electrons and holes, the bias voltage polarities and the current directions are all reversed.

4-2 Basic Transistor Operation (cont.)

Illustration of BJT action:

4-2 Basic Transistor Operation (cont.)

Transistor Currents: The directions of the currents in npn transistor and pnp

transistor are shown in the figure.

4-2 Basic Transistor Operation (cont.)

The emitter current (IE) is the sum of the collector current (IC) and the base current (IB)

(4.1)

IB << IE and IC

The capital letter – dc value

CBE III

4-3 Transistor Characteristic & Parameters

• DC-Beta (DC)

The ratio of the dc collector current (IC) to the dc base current (IB) is the dc beta

Other name = dc current gain of transistorRange: 20 < DC < 200Usually designed as an equivalent hybrid (h)

parameter, hFE on transistor datasheethFE = DC

(4.2)B

CDC I

I

4-3 Transistor Characteristic & Parameters (cont.)

• DC Alpha (DC)

The ratio of the dc collector current (IC) to the dc emitter current (IE) is the dc alpha (DC) – less used parameter in transistor circuits

Range value: 0.95< DC <0.99 or greater , but << 1 (IC< IE )

E

CDC I

I

4-3 Transistor Characteristic & Parameters (cont.)

Current• dc base current, IB

• dc emitter current, IE

• dc collector current, IC

Voltage• dc voltage at base with

respect to emitter, VBE

• Dc voltage at collector with respect to base, VCB

• Dc voltage at collector with respect to emitter, VCE

4-3 Transistor Characteristic & Parameters (cont.)

Short Quiz: Draw location and direction of IB, IC, IE, VBE, VCB & VCE.

• 5 minutes

4-3 Transistor Characteristic & Parameters (cont.)

Current and Voltage Analysis:

When the BE junction is forward-biased, like a forward biased diode and the voltage drop is

Since the emitter is at ground (0V), by Kirchhoff’s voltage law, the voltage across is: …….(1)

Also, by Ohm’s law: ……..(2)

From (1) & (2) :

Therefore, the dc base current is:

VVBE 7.0

BR BEBBR VVVB

BBR RIVB

BBBEBB RIVV

B

BEBBB R

VVI

(4-3)

(4-4)

4-3 Transistor Characteristic & Parameters (cont.)

Current and Voltage Analysis:

The voltage across resistor RC is

Since the drop across is:

The dc voltage at the collector with respect to the emitter is:

where

The dc voltage at the collector with

respect to the base is:

CECCR VVVC

CCCCCE RIVV

BECECB VVV (4-6)

CR CCRC RIV

(4-5)

BDCC II

4-3 Transistor Characteristic & Parameters (cont.) Collector Characteristic Curve:

Using a circuit as shown in below, we can generate a set of collector characteristic curve that show how the collector current, Ic varies with the VCE voltage for specified values of base current, IB.

variable voltage

4-3 Transistor Characteristic & Parameters (cont.)

Assume that VBB is set to produce a certain value of IB and VCC is zero.

At this condition, BE junction and BC junction are forward biased because the base is approximately 0.7V while the emitter and the collector are zero.

4-3 Transistor Characteristic & Parameters (cont.)

The IB is through the BE junction because of the low impedance path to ground, therefore IC is zero.

When both junctions are forward biased – transistor operate in saturation region.

4-3 Transistor Characteristic & Parameters (cont.)

As VCC is increase gradually, IC increase – indicated by point A to B.

IC increase as VCC is increased because VCE remains less than 0.7V due to the forward biased BC junction.

4-3 Transistor Characteristic & Parameters (cont.)

When VCE exceeds 0.7V, the BC becomes reverse biased and the transistor goes into the active or linear region of its operation.

In this time, IC levels off and remains constant for given value of IB and VCE continues to increase.

4-3 Transistor Characteristic & Parameters (cont.)

Actually, IC increase very slightly as VCE increase due to widening of the BC depletion region

This result in fewer holes for recombination in the base region which effectively caused a slight increase in BDCC II

4-3 Transistor Characteristic & Parameters (cont.)

When VCE reached a sufficiently high voltage, the reverse biased BC junction goes into breakdown.

The collector current increase rapidly – as indicated at the right point C

4-3 Transistor Characteristic & Parameters (cont.)

The transistor cannot operate in the breakdown region.

When IB=0, the transistor is in the cutoff region although there is a very small collector leakage current as indicated – exaggerated on the graph for purpose of illustration.

4-3 Transistor Characteristic & Parameters (cont.)

4-3 Transistor Characteristic & Parameters (cont.) Transistor Operating Regions:

1.Cutoff region:• Both transistor junctions are reverse biased• All terminal current are approximately equal to zero

2.Active region: • The BE junction is forward biased and the BC junction is reverse biased• All terminal currents have some measurable value• The magnitude of IC depends on the values of and IB• VCE is approximately 0.7V and VCE falls in ranges VBE<VCE<VCC

3.Saturation:• Both transistor junctions are forward biased• IC reaches its maximum values- determine by the component in the CE circuit, and independent of the values of and IB• VBE is approximately 0.7V and VCE < VBE

leakage current isneglected

4-3 Transistor Characteristic & Parameters (cont.) DC Load Line: Cutoff and saturation can be illustrated in relation to

the collector characteristic curves by the use of a load line. DC load line drawn on the connecting

cutoff and saturation point. The bottom of load line is ideal

cutoff where IC=0 & VCE=VCC. The top of load line is saturation

where IC=IC(sat) & VCE =VCE(sat)

In between cutoff and saturation

is the active region of transistor’s

operation.

4-3 Transistor Characteristic & Parameters (cont.)More About beta, :

-Important parameter for BJT

-Varies both IC & temperature

-Keeping the junction temperature

constant, IC cause

-Further increase in IC beyond this

max. point cause to decrease

Maximum Transistor Ratings:

-Specified on manufacturer’s data sheet

-Given for VCE,VBE,VBC,IC & power dissipation

-The product of VCE and IC must not exceed the max. power dissipation

-Both VCE and IC cannot be max. at the same time.

CE

DC V

PI (max)

FEDC h,

DC

DC

4-3 Transistor Characteristic & Parameters (cont.)

Derating :

-Specified at 25°C

-Data sheet often give derating factor for determining at > 25°C

-Example: derating factor of 2mW/°C indicates that the max. power

dissipation is reduced 2mW for each degree increase in temperature.

Transistor Data Sheet:

-See Figure 4-20, pg. 179

-Max. VCEO = 40V – indicated that the voltage is measured from C to E

with the B is open

-The max. IC is 200mA

- for several values of IC

-VCE(sat) is 0.2V max for IC(sat) = 10mA

(max)DP

(max)DP

DC

4-3 Transistor Characteristic & Parameters (cont.)

Assignment• Download/find/buy/copy any BJT

transistor datasheet and elaborate on each parameter.

• Use handwriting / attach datasheet• Due: Next friday

Basic ElectronicChapter 4

4-4 Transistor as an Amplifier• Amplification of a small

ac voltage by placing the ac signal source in the base circuit.

• Vin is superimposed on the DC bias voltage VBB by connecting them in series with base resistor RB.

• Small changes in the base current circuit causes large changes in collector current circuit.

BDCC II

Voltage gain

b

c

V V

VA

Ccc RIV

'ee

Ce

b

c

V rI

RI

V

VA

'e

C

V r

RA

ec II resistanceemitter ac internal 'er

4.5 Transistor as a switch

A transistor when used as a switch is simply being biased so that it is in

1. cutoff (switched off)

2. saturation (switched on)

Conditions in Cutoff

CCcutoffCE VV )(

C

satCECCsatC

R

VVI

)()(

Conditions in Saturation

DC

satCB

II

)(

(min)

Troubleshooting

Troubleshooting a live transistor circuit requires us to be familiar with known good voltages, but some general rules do apply. Certainly a solid fundamental understanding of Ohm’s law and Kirchhoff’s voltage and current laws is imperative. With live circuits it is most practical to troubleshoot with voltage measurements.

Troubleshooting

Internal opens within the transistor itself could also cause transistor operation to cease.

Erroneous voltage measurements that are typically low are a result of point that is not “solidly connected”. This called a floating point. This is typically indicative of an open.

More in-depth discussion of typical failures are discussed within the textbook.

Opens in the external resistors or connections of the base or the circuit collector circuit would cause current to cease in the collector and the voltage measurements would indicate this.

TroubleshootingTesting a transistor can be viewed more simply if you view it as testing two diode junctions. Forward bias having low resistance and reverse bias having infinite resistance.

TroubleshootingThe diode test function of a multimeter is more reliable than using an ohmmeter. Make sure to note whether it is an npn or pnp and polarize the test leads accordingly.

Troubleshooting

In addition to the traditional DMMs there are also transistor testers. Some of these have the ability to test other parameters of the transistor, such as leakage and gain. Curve tracers give us even more detailed information about a transistors characteristics.

Summary The bipolar junction transistor (BJT) is

constructed of three regions: base, collector, and emitter. The BJT has two p-n junctions, the base-emitter junction and the base-collector junction.

The two types of transistors are pnp and npn.

For the BJT to operate as an amplifier, the base-emitter junction is forward biased and the collector-base junction is reverse biased.

Of the three currents IB is very small in comparison to IE and IC.

Beta is the current gain of a transistor. This the ratio of IC/IB.

Summary A transistor can be operated as an electronics

switch. When the transistor is off it is in cutoff condition (no current).

When the transistor is on, it is in saturation condition (maximum current).

Beta can vary with temperature and also varies from transistor to transistor.