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Chapter (3)
Transistors and Integrated Circuits
BIPOLARJUNCTIONTRANSISTOR
BJT in contrast to the "unipolar" FET
• Both minority and majority carries play
significant roles.
• Permits  greater gain  better highfrequency performance.
Alloy Structure consist of
• Collector (C)  n-type chip  less than 1 mm square
• Base (B)  p-type  thinner than this paper
• Emitter n-type  alloyed to the base
• The result is a pair of pn junctions  separated by a base
region,  npn junction transistor.
Chapter (3)
Transistors and Integrated Circuits
Planar Structure
npn  BJT
• (n)  grown upon a heavily doped (n+)
substrate.
• oxidation of the surface
• window  opened to diffuse impurities
 (P) into the crystal to form the pn
junction.
• A smaller window for emitter in opened
to diffuse emitter region (n).
Biasing Parameters
BJT  C  E  B  VEB  VCB  iE  iC  iB
Chapter (3)
Transistors and Integrated Circuits
Operation
• The emitter junction  forward biased 
VEB reduced the barrier at emitter 
electrons injection into B where they are
minority carriers.
• The collector junction is reverse biased
 VCB increase the barrier at C
• B is very thin  most electron pass from
E to C
• The net result  transfer electron from E to C.
• Large RL insertion in C  large voltage 
voltage amplification
• Variation of the base current iB  large iC 
current amplification
• Switching operation used in digital signal
processing.
Chapter (3)
Transistors and Integrated Circuits
DC Behavior
• E forward  C reverse biased.
• iE consist  electron across
np J  holes from B.
• g  almost unity  iE 
nearly electrons.
• a varies from 0.90 to 0.999
where a typical value is 0.98.
• Most of these electrons in B
diffuse to C  B is very
narrow.
• iC consist of  iE and a very
small current due to thermally
generated minority carrier
ICBO
iC = -aiE + ICBO

I CBO e
iB = -iE -iC
ev / KT

1  I CBO
Chapter (3)
Transistors and Integrated Circuits
Common - Base Characteristics
• CB configuration
• B common  input E  output C.
• The emitter-base section  forward-biased
diode.
Input Characteristics
• Input characteristics Fig.( b ) similar to
Fig. (a)  diode characteristics
• The effect of VCB  small
• +VCB  emitter open-circuited  IE = 0
 C section  reverse-biased junction
Chapter (3)
Transistors and Integrated Circuits
Output Characteristics
• The collector
characteristic  reverse
bias diode  iE = - 5 mA,
 iC = - aiE @ + 5 mA.
• The slope of the curves
in  Fig C  due to an
effective increase in a
as VCB increases.
• a  always less 1
Chapter (3)
Transistors and Integrated Circuits
The common base
configuration is not good for
practical current amplification
Chapter (3)
Transistors and Integrated Circuits
Chapter (3)
Transistors and Integrated Circuits
 Input and output  CE Characteristics
 iB  depends on VBE only .
 iB = 0  iC = iCEO
 a = 0.98  b = 49,
 A very small increase in iB  large
increase in iC
 A very small increase in a  much
greater change in b.
Practice Problem 6-3
Chapter (3)
Transistors and Integrated Circuits