05 BJT Basics

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05 Bipolar Junction Transistors (BJTs) basics

There are two types of bipolar transistors: the NPN transistor, in which aP-type region is sandwiched between two N-type regions, and the PNP

transistor, where N-type silicon is confined between two P-type regions.

Emitter Base Collector

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Long-base device

If the width of the neutral base, is large enough, all the electrons injected by the emitter into

the base recombine in the P-type material, because the base width is larger than the electron

diffusion length in the base. There is no interaction between both junctions and therefore no

current flowing between emitter and collector. Neglecting the small reverse current in the

collector-base junction, the only current flowing through the device is between the base andthe emitter:

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Short-base device

The term "short base" implies that the neutral base width is smaller than the electron

diffusion length: WB < LnBLet the emitter-base junction be forward biased VBE = VB VE > 0

and the collector-base junction be reverse biased VBC = VB VC

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WB

In modern BJTs 99% or more of the electrons injected by the emitter into the basereach the collector.

The magnitude of current flowing in the collector does not depend on magnitude of

the collector voltage; the collector-base junction simply needs to be reverse biased.

This effect, in which the current in a junction is controlled by the bias applied to

another junction, is called "transistor effect".

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Symbolic representation, applied bias, and currents

in an NPN bipolar transistor.

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A BJT transistor with a forward-biased emitter-base junction and a reverse-biased

collector-base junction is said to operate in theforward active mode. If both junctions are forward biased the transistor is said to be insaturation. In that

case electrons are injected from the emitter through the base into the collector and

from the collector through the base into the emitter.

If both junctions are reverse biased there is no current flow at all and the device is

in thecut-off mode.

If the emitter junction is reverse biased and the collector junction is forward

biased the transistor operates in thereverse active mode.

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BJT Current gain

The current flowing through the emitter junction is given by the sum of the holecurrent injected from the base into the emitter and the electron current injected from

the emitter into the base . The ratio between these two current components

where NaB and NdE are the doping concentrations in the base and the emitter, respectively

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BJT circuit configurations

Common-base

configuration

Common-emitter

configuration

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BJT Currents

The collector current, InC is due to the diffusion through the base of electrons injectedby the emitter into the base.

InC = InE IrB,

where IrB is the current due to the recombination of electrons in the base.

The base current is equal to IpE + IrB;

For the convention for

current direction as shown

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Common base gain,F

or,

BJT Current Gain

Common emitter gain,F

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BJT fabrication

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Amplification using a bipolar transistor

For typical Si BJT in the forward active mode,

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Amplification using a bipolar transistor

( ) x IC =

power supplied

by the power

supply

power dissipated

in the load

resistor

power loss

(the price one has to

pay to obtain

amplification by the

transistor.

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Ebers-Moll model

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Ebers-Moll model

05 Bi l J ti T i t (BJT ) b i

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Ebers-Moll model

A is the area of the cross section


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Ebers-Moll model

Defining the emitter and the collector junctions

reverse saturation currents


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Ebers-Moll model

Combining these expressions the Ebers-Moll Equations are

or, in the matrix form:


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Ebers-Moll model


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Emitter efficiency


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Transport factor in the base

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05 BJT Basics