Chapter 10. Bipolar junction transistor fundamentals
Invented in 1948 by Bardeen, Brattain and Shockley
Contains three adjoining, alternately doped semiconductor regions:
Emitter (E), Base (B), and Collector (C)
The middle region, base, is very thin compared to the diffusion
length of minority carriers
Two kinds: npn and pnp
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Schematic representation of pnp and npn BJTs
Emitter is heavily doped compared to collector. So, emitter
and collector are not interchangeable.
The base width is small compared to the minority carrier
diffusion length. If the base is much larger, then this will
behave like back-to-back diodes.
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BJT circuit symbols
IE = IB + IC
and
VEB + VBC + VCE = 0
VCE =  VEC
As shown, the currents are positive quantities when the
transistor is operated in forward active mode.
3
BJT circuit configurations and output characteristics
4
BJT biasing modes
5
Cross sections and simplified models of discrete and IC
npn BJTs
6
Electrostatic variables for a pnp BJT at equilibrium
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Qualitative discussions of operation
Consider two diodes, one forward biased and one reverse biased.
p

n
E
E
h
e
e
e
h
h
E
forward
e
p
n
h
e
E
h
e
h
h
e
reverse
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Qualitative discussion of transistor action
Combine the two diodes!
VF
VR
h
p

p
n
e
I forward
I reverse
No transistor action
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Consider very thin base width: Transistor action
E
h
e
Hole concentration is zero here, reverse biased
IE
IE
( 1   )I E
R
VF
VR
The collector current IC is almost equal to IE, and collector current
is controlled by the E-B junction bias. The loss, i.e.  < 1
corresponds to the recombination of holes in base.
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PNP under forward active mode
 holes
3
IEP
IE


–IEN
 electrons
5
1

IBR
2
IC
4
–IBE –IBR
Electron
flux
IB
Hole
flux
and current
IE = IEP + IEN = 1 + 2 + 5
IB = IBR + IBE = IBR + IEN = – (4 + 5)
IC = IEP – IBR = 2
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Current components
1 = hole current lost due to recombination in base, IBR
2 = hole current collected by collector, ~ IC
1 + 2 = hole part of emitter current, IEP
5 = electrons injected across forward biased E-B junction, (– IBE);
same as electron part of emitter current, – IEN
4 = electron supplied by base contact for recombination with
holes lost, – IBR (= 1)
3 = thermally generated e & h making up reverse saturation
current of reverse biased C-B junction. (generally neglected)
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Performance parameters (Consider pnp)
Neglect the reverse leakage (electron) current of C-B junction
Emitter efficiency:
 
I EP
I EP  I EN

I EP
IE
Fraction of emitter current carried by holes.
We want  close to 1.
Base transport factor:
αT 
IC
I Ep
Fraction of holes collected by the collector.
We want T close to 1.
Common base dc current gain:
I C   T I EP   T  I E   dc I E
 dc   T 
Note that  is less than 1.0 but close to 1.0 (e.g.  = 0.99)
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Performance parameters (Consider pnp)
Common emitter dc current gain, dc:
I C   dc I B
But,
I C   dc I E   dc ( I C  I B )
IC
 dc 
 dc
1   dc

  dc
 
 1   dc

 I B

 T
1   T
Note that  is large (e.g.  = 100)
For npn transistor, similar analysis can be carried out. However,
the emitter current is mainly carried by electrons.
Example:  
I EP
I EP  I EN
etc .
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