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ECSE-2210 Microelectronics Technology
Class Activity 20 – Solution
1. In a BJT, indicate what happens (increase, decrease or no-effect: choose one and
understand “why”):
a. to
γ
if we increase the emitter doping?
Increases. It is the fraction of emitter current carried by the emitter majority
carriers. If we increase the majority carriers in the emitters, this fraction increases.
In a pnp transistor,
ENEP
EP
II
I
+
=
γ
. By increasing the emitter doping, IEP increases
and as a result the fraction increases.
b. to
γ
if we increase the base doping?
Since the reverse injection of carriers from the base increases, the emitter-
injection efficiency (
γ
) decreases. In a pnp transistor,
ENEP
EP
II
I
+
=γ . By
increasing the base doping, IEN increases and this reduces the fraction. As a result,
the emitter-injection efficiency (
γ
) decreases.
c. to
γ
if we increase the collector doping?
No direct effect (to a first order).
d. to
α
T if we increase the base width?
Decreases since the distance the minority carriers have to travel increases. With
the increase in the base width more carriers recombine and the base transport
factor reduces.
e. to
α
T if we increase the lifetime in the base?
Increases since the minority carriers can be collected by the collector with
negligible recombination. The carriers spend more time in the base before they
recombine, increasing the probability of being collected by the base.
f. to
α
T if we increase the C-B reverse voltage?
Increases since increasing the reverse bias of the C-B junction effectively reduce
the base width. With a reduction of the base width the
α
T increases.
2. The base transport factor,
α
T, is IC/IEP. Using the equations derived in class for IC and
IEP, show that the base transport factor is given by (see equation 11.42 in the
textbook)
2
B
2
B
T
2
1
1
L
W
+
=α
