BJT Basics 1

 

C

B

 

E

Dr. D G Borse  

The BJT – Bi polar Junction Transistor  Bipolar Note: Normally Emitter layer is heavily doped, Base layer is lightly doped and Collector layer has oderate doping.

The T!o Types o" BJT Transistors Transistors:: npn pnp E

n

p

n

C C

Cross #ection

E

p

n

C C

Cross #ection #ection

B

B B

#chematic #ym$ol

B #chematic #ym$ol

•

E Collector doping is usually % &' ( Base doping is slightly higher % &' &' – &'&&

•

Emitter doping is much higher % &'&)

•

p

 

E

Dr. D G Borse  

ltage  E-uations BJT Current * +o +oltage E E



 

C

+ / +CE /

E 

C

E

C /

+EC



C

/ +BE

B

+BC

/

+EB 

/

B

/

+CB

B  B

npn

pnp

E  0 / C 0 B  /

E  0 0 B  / / C

+CE  0 0 +BC  / / +BE

+EC  0 0 +EB   +CB

 

Dr. D G Borse  

n

 co

+CB



nc

/ 

p - Electrons + Holes

/ +BE 

pe

ne

/ n

Bul1recom$ination Current

 Figure : Current flow (components) for an n-p-n BJT in the active region. region . NOTE: Most of the current is due to electrons moving from the emitter through base to the collector. Base current consists of holes crossing from the base into the emitter and of holes that recombine with electrons in the base.

 

Dr. D G Borse  

Physicall Structure Physica •

•

•

 

Consists of 3 alternate layers of n- and p and p-type semiconductor called emitter ( E ), ), base ( B  B)) and collector (  (C  C ). ). Majority of current enters collector, crosses base region and exits through emitter..  small current also enters emitter e nters base terminal, crosses base-emitter junction and exits through throug h emitter.

Carrier transport in the acti!e base region directly beneath the hea!ily doped (n ( n+) emitter dominates i-v i-v characteristics  characteristics of "#$.

Dr. D G Borse  

c

C

2ecom$ination

+CB /

     n             

 4 

-   Electrons

B

 

/ /  4 

   / 

     /  p

B +BE





-  

     -    -       -     -  

-         

E

 

E

n

/ 3oles

Dr. D G Borse  

5or CB Transistor E0 ne/ pe c0 nc co

Bul1 recom$ination current

C<

nc

6nd c0  7E / Co CB Current Gain, 7   8 8 9c co . 9E '   5or CE Trans., C  0 0 ;$  / / 9&/; co   !here ; 8   7   , & 7   is CE Gain

pe

ne

5igure:  An npn transistor with variable biasing sources (common-emitter ( common-emitter configuration).

 

Dr. D G Borse  

CommonEmitter Circuit Diagram +CE C

/

+CC  4 

CollectorCurrent Curves C

B

6ctive 2egion

B

2egion o" Description