Operation Theory

BIPOLAR JUNCTION TRANSISTOR (BJT)

Engr.Usman Ali khan

CONTENTS

1. BJT Formation

2. BJT Terminals

3. Biasing of BJT

4. BJT Currents

5. α of BJT

FORMATION OF BJT

P N P

Two Junctions

N P N

Two Junctions

N-Type material has excessive free electrons and P-Type material has excessive holes

NPN and PNP BJTs have same characteristics. The only difference is in their polarities of applied voltages. The polarities are reversed for the two types of transistor.

BJT TERMINALS

N P N

Emitter Collector

Base

N NPE C

B

Base-Emitter Junction Base-Collector Junction

BJT CIRCUIT SYMBOLS

N NPE C

B B

CE

C

E

B

P PNE C

B B

CE

C

E

B

FORWARD BIASED JUNCTION OPERATION

+

+

+

-

-

-

E

NP

+ -

Opposing field due to battery

Electrons Push electrons to the other side

Positive terminal attracts electrons

Flow of electron current; but in semiconductor electronics the flow of positive or conventional current is considered

REVERSE BIASED JUNCTION OPERATION

+

+

+

-

-

-

E

NP

+-

Supporting field due to battery Electrons Push Minority Carrier electrons to the other side

Positive terminal attracts Minority Carrier electrons

Current flows due to minority carriers

BJT BIASING

N P N

Emitter Collector

Base- +

- +

VEE VCC

- +

+-

ON

OFF

BJT CURRENTS

N P N

Emitter Collector

Base- +

- +

VEE VCC

Negative Terminal pushes electrons to the other side

When electrons come to Base, they become minority carriers

Minority carriers are attracted to the positive terminal of the battery

Some electrons escape to the ground

BJT CURRENTS

N NP

EC

B

Conventional Current

Electron Current

-+

IB

IE IC (INJ)

IE = IB + IC(INJ)

COLLECTOR-BASE REVERSE CURRENT

N NP

EC

B

ICBO

Conventional Current

Electron Current

When Collector-Base Junction is reverse biased and Emitter-Base Junction is Open, a reverse current flows in this reverse biased junction which is called ICBO

Total Collector current is = IC = IC(INJ) + ICBO

Α OF BJT

α = IC(INJ)/IE

IC = IC(INJ) + ICBO ICBO<<IC(INJ)

IC = IC(INJ)

α = IC/IE

HOME WORK• Example: 4.1

• End Problems: 4.1 4.5