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of a donor impurity per 10^9 parts of germanium increases its conductivity by a factor

of nearly 10^3. Silicon and germanium are not the only semiconducting materials with

practical applications: another important class of semiconductors consists of compounds

of trivalent and pentavalent elements, such as GaAs, GaP, InSb, and InP.

10.7 SEMICONDUCTOR DEVICES

The properties of the p-njunction are responsible for the microelectronics

industry

The operation of most semiconductor devices is based upon the nature of junctions

between p- and n-type materials. Such junctions can be made in several ways. A method

especially adapted to the production of integrated circuits involves diffusing impuri-

ties in vapor form into a semiconductor wafer in regions defined by masks. A series of

diffusion steps using donor and acceptor impurities is part of the procedure for

manufacturing circuits that can contain millions of resistors, capacitors, diodes, and

transistors on a chip a few millimeters across. The limiting factor in this method is the

wavelength of the light that is shined through masks to expose and thereby harden the

The IBM PowerPC 601 microprocessor chip is 10.95 mm square

and contains 2.8 million transistors. The functions of the various

parts of the chip are indicated.

+

+

Missing

electron

+

Missing

electron

Missing

electron

Figure 10.28Current in a p-type

semiconductor is carried by the

motion of “holes,” which are sites

of missing electrons. Holes move

toward the negative electrode as a

succession of electrons move into

them.

Conduction band

Valence band

eF Forbidden band

Acceptor

impurity

levels

Figure 10.29A trace of gallium in a silicon crystal provides acceptor levels in the normally forbidden

band, producing a p-type semiconductor.

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