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DMT 121 – ELECTRONIC DEVICES

CHAPTER 3BIPOLAR JUNCTION TRANSISTOR (BJT)

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BJT Structure

• BJT is constructed with 3 doped semiconductor regions separated by 2 p-n junctions (Base-Collector & Base-Emitter).

• 3 regions are called emitter, base and collector.• Emitter (E) – most heavily doped region.• Base (B) – thin and lightly doped region.• Collector (C) – largest and moderately doped region.

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Type/Symbol

npn transistor

pnp transistorPointing inNot pointing

in

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BJT OperationIn normal operation, the base-emitter is forward-biased

while the base-collector is reverse-biased.

For npn type, the collector is more positive than the base, which is more positive than the emitter.

For pnp type, the voltages are reversed to maintain the forward-reverse bias.

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Operation pnp transistor

Forward-biased junction

Reverse-biased junction

IE = IC + IB

IC = IC majority + ICO (minority)

ICO (minority) is called leakage current

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Operation npn transistor

• heavily doped n-type emitter region has a very high density of free electrons.

• free electrons easily diffuse through BE junction into lightly doped and thin base region

• base has low density of holes (majority carriers).

• electrons that have recombined with holes as valance electrons leave the crystalline structure of the base, they become electrons in the metallic base lead and produce the external base current.

• most free electrons don’t recombine with holes as the base is very thin→move toward BC junction.

•Swept across into collector region by attraction of +ve supply.

• free electrons move through collector region into external circuit.

• then return into emitter region along with the base current.

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BJT Currents

IE = IC + IB

The emitter current is the sum of the collector current and the small base current.

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Basic Operation• For both npn and pnp transistors, VBB forward-biases the BE junction

and VCC reverse-biases the BC junction.• Look at this one circuit as two separate circuits, the base-emitter

(left side) circuit and the collector-emitter (right side) circuit. • Note that the emitter leg serves as a conductor for both circuits. • The amount of current flow in the base-emitter circuit controls

the amount of current that flows in the collector circuit. • Small changes in base-emitter current yields a large change in

collector-current.

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BJT Characteristics & Parameters

DC is usually equivalent hybrid (h) parameters hFE on transistor datasheets.

hFE = DC

DC is ratio of collector current (IC) to the emitter current (IE). Less used parameter than beta in transistor circuits.

DC = IC/IE

The collector characteristic curves illustrate the relationship of the 3 transistor currents. By setting up other values of base current, a family of collector curves is develop. Beta() is the ratio of collector current to base current.

DC= IC/IB

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Beta (β)

Relationship between amplification factors and

1β

βα

1α

αβ

Relationship Between Currents

BC βII BE 1)I(βI

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• The beta for a transistor is not always constant. Temperature and collector current both affect beta, not to mention the normal inconsistencies during the manufacture of the transistor.

• There are also maximum power ratings to consider. • The data sheet provides information on these

characteristics.

BJT Characteristics

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EXAMPLE

What is the βDC for the transistor shown?

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SOLUTION

1. Choose a base current near the center of the range, in this case IB3.

2. Read the corresponding collector current.3. Calculate the ratio.

16730

5

A

mA

I

I

B

C

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BJT Characteristics• The collector characteristic curves

show the relationship of the 3 transistor currents.

• Curve shown is for a fixed based current.

• Saturation region – collector current has reached a maximum and is independent of the base current.

• Ideally, when VCE exceeds 0.7 V, the BC junction become reverse-biased and transistor goes into active/linear region. IC increases very slightly as VCE increases due to widening of BC depletion region.

• When VCE reaches a sufficiently high voltage, reverse-biased BC junction goes into breakdown; and IC increases rapidly as point C.

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• Cutoff – condition in which there is no base current, IB=0 which results in only an extremely small leakage current (ICEO) in the collector circuit. For practical work, ICEO is assumed to be 0. So VCE=VCC.

BJT Characteristics

In cutoff, neither the BE junction nor the BC junction are forward-biased.

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• Saturation – condition in which there is maximum IC. The saturation current is determined by the external circuit (VCC and RC in this case) because the emitter-collector voltage is minimum (≈0.2 V).

BJT Characteristics

In saturation, an increase of base current has no effect on the collector circuit and the relation IC=βDCIB is no longer valid.

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BJT Characteristics

• DC load line – represent circuit that is external to the transistor. Drawn by connecting saturation and cutoff point.

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EXAMPLE

1. What is the saturation current and the cutoff voltage for the circuit?

2. Is the transistor saturated?Assume VCE = 0.2 V in saturation.

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SOLUTION

VVV

mAkR

VI

CCCO

C

CCSAT

15

48.43.3

2.0152.0

Q1

Q2

mAAII

Ak

I

BC

B

09.2)45.10(200

45.10220

7.03

Since IC < ISAT, the transistor is not saturated.

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BJT Configuration

There are three key dc voltages and three key dc currents to be considered. Note that these measurements are important for troubleshooting.

IB: dc base current

IE: dc emitter current

IC: dc collector current

VBE: dc voltage across base-emitter junction

VCB: dc voltage across collector-base junction

VCE: dc voltage from collector to emitter

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BJT Configuration

For proper operation the base-emitter junction is forward biased by VBB and conducts just like a diode.

The collector-base junction is reverse biased by VCC and blocks current flow through it’s junction just like a diode.

Remember current flow through the base-emitter junction will help establish the path for current flow from the collector to emitter.

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BJT Configuration

Analysis of this transistor circuit to predict the dc voltages and currents requires use of Ohm’s law, Kirchhoff’s voltage law and the beta for the transistor.

Analysis begins with the base circuit to determine the amount of base current. Using Kirchhoff’s voltage law, subtract the 0.7 VBE and the remaining voltage is dropped across RB. Determining the current for the base with this information is a matter of applying of Ohm’s law. VRB/RB = IB

The collector current is determined by multiplying the base current by beta.

DC = IC/IB

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BJT ConfigurationBase-Emitter (Forward Bias)

0.7 VBE will be used in most analysis examples.

B

BEBBB

BEBBBB

R

VVI

VRIV

0

BECECB

BEBBBB

VVV

VRIV

0

Collector - Emitter

BDCC

CCCCCE

CECCCC

II

RIVV

VRIV

0

Collector – Base (Reverse Bias)

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BJT Configuration

Previously explained for npn transistor, what about pnp ???

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BJT Configuration

What we ultimately determine by use of Kirchhoff’s voltage law for series circuits is that:

1. VBB is distributed across the base-emitter junction and RB in the base circuit.

2. VCC is distributed proportionally across RC and the transistor (VCE) in the collector circuit.

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BJT Configuration

Common-base

Common- Emitter

Common- Collector

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BJT Amplifiers• BJT amplifies AC signals by converting some of the DC

power from the power supplies to AC signal power.• An AC signal at the input is superimposed in the dc bias

by the capacitive coupling.• The output AC signal is inverted and rides on a DC level

of VCE.

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BJT Switches

A transistor when used as a switch is simply being biased so that it is in cutoff (switched off) or saturation (switched on). Remember that the VCE in cutoff is VCC and 0V in saturation.

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Datasheet

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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 (important).

• With live circuits it is most practical to troubleshoot with voltage measurements.

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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.

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

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Troubleshooting

Testing 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.

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Troubleshooting

The 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.

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Summary Bipolar Junction Transistor (BJT) is constructed of

three regions: base, collector, and emitter. BJT has two pn 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.

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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.