Fundamentals of Materials Science and Engineering: An Integrated Approach, 3e

GTBL042-12 GTBL042-Callister-v2 August 13, 2007 18:22

494 • Chapter 12 / Electrical Properties

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-modep-type MOSFET of Figure 12.26), minority carriers do play

a role with junction transistors (i.e., injected holes in then-type base region, Figure

12.25).

Concept Check 12.8

Would you expect increasing temperature to influence the operation ofp–njunction

rectifiers and transistors? Explain.

[The answer may be found at http://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 utilized 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).

Within this framework, numbers are represented by a series of two states (sometimes

designated 0 and 1). Now, transistors and diodes within a digital circuit operate as

switches that also have two states—on and off, or conducting and nonconducting;

“off” corresponds to one binary number state, and “on” to the other. Thus, a single

number may be represented by a collection of circuit elements containing transistors

that are appropriately switched.

Microelectronic Circuitry^9

During the past few years, the advent of microelectronic circuitry, where millions

of electronic components and circuits are incorporated into a very small space,

has revolutionized the field of electronics. This revolution was precipitated, in part,

by aerospace technology, which necessitated computers and electronic devices that

were small and had low power requirements. As a result of refinement in process-

ing and fabrication techniques, there has been an astonishing reduction in the cost

of integrated circuitry. Consequently, at the time of this writing, personal comput-

ers are affordable to large segments of the population in many countries. Also,

integrated circuit the use ofintegrated circuitshas become infused into many other facets of our

lives—calculators, communications, watches, industrial production and control, and

all phases of the electronics industry.

(^9) Web Module E discusses materials that are used in one integrated circuit design.