Callister - An introduction - 8th edition

18.15 Semiconductor Devices • 751
Figure 18.24
Schematic diagram
of a p–n–p junction
transistor and its
associated circuitry,
including input and
output voltage–time
characteristics
showing voltage
amplification.
(Adapted from A. G.
Guy, Essentials of
Materials Science,
McGraw-Hill Book
Company, New York,
1976.)
Forward-biasing voltage
0.1
Input
voltage
(mV)
+
–
Time
Junction 1
Input
voltage
Emitter
p
Base
n
10
Output
voltage
(mV)
Junction 2
Collector
p
Load
Output
voltage
Time
–
+
Reverse-biasing voltage
Figure 18.25 illustrates the mechanics of operation in terms of the motion of
charge carriers. Because the emitter is p-type and junction 1 is forward biased,
large numbers of holes enter the base region. These injected holes are minority
carriers in the n-type base, and some will combine with the majority electrons.
However, if the base is extremely narrow and the semiconducting materials have
been properly prepared, most of these holes will be swept through the base without
recombination, then across junction 2 and into the p-type collector. The holes
now become a part of the emitter–collector circuit. A small increase in input voltage
within the emitter–base circuit produces a large increase in current across
junction 2. This large increase in collector current is also reflected by a large
increase in voltage across the load resistor, which is also shown in the circuit
(Figure 18.24). Thus, a voltage signal that passes through a junction transistor experiences
amplification; this effect is also illustrated in Figure 18.24 by the two
voltage–time plots.
Similar reasoning applies to the operation of an n–p–n transistor, except that
electrons instead of holes are injected across the base and into the collector.
p-Type n-Type p-Type
+ + +
+ +
+ +
+ + +
+
+
+
+
+ +
+ +
(a)
+
+
Figure 18.25 For a junction transistor (p–n–p
type), the distributions and directions of
electron and hole motion (a) when no
potential is applied and (b) with appropriate
bias for voltage amplification.
Junction 1 Junction 2
Emitter Base Collector
+ +
+ +
+
+
+
+
+ + + +
+ + –
+
+ +
+ + + +
–
(b)
+