Electronic DevicesKEEE 2224

Lecture 4BJT

Minority Carrier Distribution

Dr. Ghafour Amouzad Mahdiraji

October 2012

Minority Carrier• The goal is to calculate the currents in the bipolar transistor.• Since diffusion currents are produced by minority carriers, we must determine

the steady-state minority carrier distribution in each of the 3 transistor regions.

Forward-Active Mode

Base Region

Base Region

Base Region

Base Region

Example 1

Emitter Region

Emitter Region

Emitter Region

Example 2

Collector Region

Example 3

Cutoff Mode

Minority carrier distribution in an npn bipolar transistor in cutoff mode of operation

Saturation Mode

Minority carrier distribution in an npn bipolar transistor in saturation mode of operation

Inverse-Active Mode

Minority carrier distribution in an npn bipolar transistor in inverse-active mode of operation

Inverse-Active Mode

Cross section of an npn bipolar transistor showing the injection and collection of electrons in the inverse-active mode

Transistor Currents

and

Low-Frequency

Common-Base Current Gain

Particle Components

Particle current density or flux components in an npn bipolar

transistor operating in the forward-active mode.

Hole

e- flow

IC

IB

IE

p+ pn

1

2

345

IEp or IE1

IEn or IE2

IC

Particle ComponentsThe factor is the electron flux injected from the emitter into the base. As the electrons diffuse across the base, a few will recombine with majority carrier holes. The majority carrier holes that are lost by recombination must be replaced from the base terminal. This replacement hole flux is denoted by . The electron flux that reaches the collector is . The majority carrier holes from the base that are injected back into the emitter result in a hole flux denoted by . Some electrons and holes that are injected into the forward-biased B-E space charge region will recombine in this region. This recombination leads to the electron flux . Generation of electrons and holes occurs in the reverse-biased B-C junction. This generation yields a hole flux . Finally, the ideal reverse-saturation current in the B-C junction is denoted by the hole flux .

nEJ

RBJ

nCJ

pEJ

RJ

GJ

0pcJ

Current Components

Particle current density or flux components in an npn bipolar transistor operating in the forward-active mode.

Current Densities

Current Gain

Current Gain

Derivation of Transistor Current

Components and Current Gain Factor

Emitter Injection Efficiency Factor

Emitter Injection Efficiency Factor

Emitter Injection Efficiency Factor

Example 4

Base Transport Factor

Base Transport Factor

Example 5

Recombination Factor

Recombination Factor

Example 6

Summary

Summary

Expressions for the limiting factors assuming xB << LBand xE << LE .

Example 7

Example 8

Example 9

Example 10

Example 10

Exercises