12.1 Introduction

So far, all of the digital integrated circuits considered have been realized using silicon, which has a valence of four. There are other semiconductor materials that can also be used to realize digital integrated circuits. Many of these alternative semiconductors are based on crystals composed of two different atoms, one having a valence of three and one having a valence of five. Examples of these III-V compound semiconductor materials include gallium arsenide (GaAs), indium phosphide (InP), aluminum arsenide, (AlAs), and indium arsenide (InAs), as well as others. Of these alternatives, semiconductors based on GaAs have become the most important commercially, at present.

The primary use of GaAs integrated circuits is for very high-speed applications; historically, commercially available GaAs circuits have operated at about twice the maximum speed of commercially available Si circuits. They are also somewhat more resistant to radiation than silicon-based ICs.

There are a number of reasons why GaAs-based integrated circuits have been faster than Sibased ICs. The velocity of electrons in n-doped regions of GaAs is greater than that of electrons in n regions of silicon, especially at lower electric fields. At a somewhat typical electric field of 1 V/ ?m, electrons in GaAs have slightly less than double the velocity of electrons in Si. Plots of carrier velocities versus electric fields taken from Sze (1981) are shown in Fig. 12.1. At higher fields, the velocity advantage of electrons in GaAs over electrons in Si is not as large because...