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Bipolar Junction Transistor (BJT). Operation Theory. Engr.Usman Ali khan. Contents. BJT Formation BJT Terminals Biasing of BJT BJT Currents α of BJT. Formation of BJT. N. P. N. P. N. P. Two Junctions. Two Junctions.

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bipolar junction transistor bjt

Bipolar Junction Transistor (BJT)

Operation Theory

Engr.Usman Ali khan

contents
Contents
  • BJT Formation
  • BJT Terminals
  • Biasing of BJT
  • BJT Currents
  • α of BJT
formation of bjt
Formation of BJT

N

P

N

P

N

P

Two Junctions

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

C

E

Base

N

P

N

B

BJT Terminals

Base-Emitter Junction

Base-Collector Junction

bjt circuit symbols

C

C

E

E

N

P

P

N

N

P

B

B

BJT Circuit Symbols

C

E

C

B

B

E

C

E

C

B

B

E

forward biased junction operation

Opposing field due to battery

E

-

-

-

+

+

+

N

P

+

-

Forward Biased Junction Operation

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
Reverse Biased Junction Operation

Positive terminal attracts Minority Carrier electrons

Supporting field due to battery

Electrons Push Minority Carrier electrons to the other side

E

-

-

-

+

+

+

Current flows due to minority carriers

N

P

+

-

bjt biasing

-

+

+

BJT Biasing

N

P

N

Emitter

Collector

Base

-

+

+

-

VEE

VCC

ON

OFF

-

bjt currents

N

P

N

Emitter

Collector

Base

-

+

+

-

VEE

VCC

BJT Currents

Minority carriers are attracted to the positive terminal of the battery

When electrons come to Base, they become minority carriers

Negative Terminal pushes electrons to the other side

Some electrons escape to the ground

bjt currents1

C

E

N

P

N

B

BJT Currents

IE

IC (INJ)

IB

Conventional Current

-

+

IE = IB + IC(INJ)

Electron Current

collector base reverse current

C

E

N

P

N

B

Collector-Base Reverse Current

ICBO

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

Conventional Current

Electron Current

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

of bjt
α of BJT

α = IC(INJ)/IE

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

IC = IC(INJ)

α = IC/IE

home work
Home Work
  • Example: 4.1
  • End Problems: 4.1  4.5