TABLE OF CONTENTS This chapter will cover the physics, material science and practical aspects of metal/semiconductor interfaces that are fabricated for ohmic contacts to GaAs. The material presented in this chapter focuses on the generalised GaAs-based ohmic contact, its ideal properties and a wide range of implementations. Chapters 8 and 9 on FETs and HBTs also include sections on ohmic contacts where contacts are discussed in the context of specific types of high frequency transistors.
Section 6.2 introduces the necessary basics including a physical description of metal/semiconductor interfaces and the principles of the common metallurgical reactions. Section 6.3 covers the basic methods of fabrication and testing. Section 6.4 surveys the different types of n-type ohmic contacts to GaAs, their intended use and the motivation for engineering contacts with different properties. Section 6.5 surveys the different types of p-type ohmic contacts.
Metals and semiconductors come into close contact, and physical or chemical bonds are formed at an interface between them. Such contacts are of two generic types, Schottky and ohmic. Ohmic contacts are those that follow Ohm's law that current increases linearly with the applied voltage. This behaviour implies that a low barrier to current conduction exists at the metal/semiconductor interface in ohmic contacts.
In fact, almost all metal/semiconductor interfaces have barriers to mobile carrier conduction. The key to making an ohmic contact is to design these barriers to be low enough that current can cross the interface by thermionic emission or to be narrow enough...
