Callister - An introduction - 8th edition

752 • Chapter 18 / Electrical Properties
Figure 18.26
Schematic crosssectional
view of a
MOSFET transistor.
Source Gate Drain
p-Type channel
p-Type Si
p-Type Si
SiO 2 insulating layer
n-Type Si substrate
The MOSFET
One variety of MOSFET 8 consists of two small islands of p-type semiconductor
that are created within a substrate of n-type silicon, as shown in cross section in
Figure 18.26; the islands are joined by a narrow p-type channel. Appropriate metal
connections (source and drain) are made to these islands; an insulating layer of
silicon dioxide is formed by the surface oxidation of the silicon. A final connector
(gate) is then fashioned onto the surface of this insulating layer.
The conductivity of the channel is varied by the presence of an electric field
imposed on the gate. For example, imposition of a positive field on the gate will
drive charge carriers (in this case holes) out of the channel, thereby reducing the
electrical conductivity. Thus, a small alteration in the field at the gate will produce a
relatively large variation in current between the source and the drain. In some respects,
then, the operation of a MOSFET is very similar to that described for the junction
transistor. The primary difference is that the gate current is exceedingly small in
comparison to the base current of a junction transistor. MOSFETs are, therefore, used
where the signal sources to be amplified cannot sustain an appreciable current.
Another important difference between MOSFETs and junction transistors is that
although majority carriers dominate in the functioning of MOSFETs (i.e., holes for
the depletion-mode p-type MOSFET of Figure 18.26), minority carriers do play a role
with junction transistors (i.e., injected holes in the n-type base region, Figure 18.25).
Concept Check 18.8
Would you expect increasing temperature to influence the operation of p–n junction
rectifiers and transistors? Explain.
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]
Semiconductors in Computers
In addition to their ability to amplify an imposed electrical signal, transistors and
diodes may also act as switching devices, a feature used for arithmetic and logical
operations, and also for information storage in computers. Computer numbers and
functions are expressed in terms of a binary code (i.e., numbers written to the base 2).
8 The MOSFET described here is a depletion-mode p-type.A depletion-mode n-type is also
possible, wherein the n- and p-regions of Figure 18.26 are reversed.