This document discusses the bipolar junction transistor. It describes the emitter, collector, and base regions and their doping levels. The current gain parameters alpha and beta are defined as the ratios of various currents. Alpha is approximately 1 due to some carrier recombination and leakage currents. Beta is much greater than 1. Injection, transport, and collection efficiencies are also defined. The document discusses base width modulation and the Early effect where the base width decreases with increasing collector voltage. Punch-through breakdown occurs when the depletion regions merge.

1. Bipolar Junction
Transistor
Arpan Deyasi
Dept of ECE, RCCIIT, Kolkata, India
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Emitter
. It supplies carrier (for NPN, electron; for PNP, hole)
. Most heavily doped area
Collector
. It collects carrier (for NPN, electron; for PNP, hole)
. lighter doped than emitter, heavier doped than base
3
Base
. It forms the p-n junctions both with collector and emitter
. less doped than both of the regions
. thinnest region
. BE junction is always forward-biased (npn)
. BC junction is always reverse-biased (npn)

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Gain parameters of Transistor
CB current gain C
E
I
I
α =

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‘α’ is close to 1 because
small fractions of injected electrons from emitter to base
are lost due to recombination with holes
small hole current is injected from base to emitter
hole current from collector to base is negligible
C EI I≤ 1α ≈

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CE current gain
C
B
I
I
β =
Gain parameters of Transistor
C BI I
1β 

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Relation between α and β
C C
B E C
I I
I I I
β= =
−
1
1
C
E
C
E
I
I
I
I
α
β
α
 
 
 = =
− 
− 
 
1
β
α
β
=
+

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Gain parameters of Transistor
Injection efficiency is defined as
the fraction of emitter current
due to electron injection from emitter to base
1
1
nE nE
pEpE nE pE
nE
I I
II I I
I
γ= = =
+  
+  
 
pE nEI I 1γ ≈

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Gain parameters of Transistor
Transport efficiency is defined as
the fraction of electrons
injected from emitter to base which reach the collector
1nC nE rec rec
T
nE nE nE
I I I I
I I I
α
−
= = = −
rec nEI I 1Tα ≈
Transport efficiency is also known as Transport Factor

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Gain parameters of Transistor
Collection efficiency is defined as
the ratio of collector leakage current to electron leakage current
from base to collector in reverse biased base collector junction
1
nC pC pCC
nC nC nC
I I II
M
I I I
+
= = = +
pC nCI I 1M ≥
Collector efficiency is also known as Collector Multiplication Factor

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Base-Width Modulation
variation in the width of the base in a bipolar transistor due
to a variation in the applied base-to-collector voltage

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Base-Width Modulation
VBE < VCE1 << VCE2
Effective base width is reduced ---- Early effect

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Base narrowing has two consequences that affect the
current:
There is a lesser chance for recombination within the
“smaller” base region
The current of minority carriers injected across the
emitter junction increases
Base-Width Modulation
Early Effect

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To countervail the Early- effect is a lightly doping of the collector
region and a heavy doping of the emitter region
Base-Width Modulation

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The CB current gain (α), taking into account the effective width of
the base quasi-neutral region (assuming γ=1) is then given by
Base-Width Modulation
2
1
1
2
eff
T
B
W
L
α α
 
= ≈ −  
 
CE current gain (β) can be approximated with
2
2
1
B
eff
L
W
α
β
α
 
= =   −  

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Punch-through Breakdown
The punch-through breakdown occurs when the reverse-bias C-B
voltage is so large that the C-B and the E-B depletion regions merge
Punch-through voltage is usually much larger than the
avalanche breakdown voltage

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Punch-through Breakdown