GAAs High-Speed Devices: Physics, Technology, and Circuit Applications

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METAL-TO-GaAs CONTACTS

7.1 ELECTRICAL PROPERTIES OF METAL-SEMICONDUCTOR CONTACTS

Metallization systems are a fundamental component of all semiconductor devices and integrated circuits. They provide the electrical connection between the active regions of the semiconductor and other circuit elements of an IC and also serve as the link to the external circuit, by interconnecting with functional electrodes and bonding pads [Wel88]. Two types of metal-semiconductor contacts have been identified by researchers. These are ohmic (low resistance) and Schottky (rectifying) type contacts [Hen84].

The series resistance at the metal-semiconductor interface of the ohmic contact is very small and can be neglected during device operation. The Schottky barrier contact has a high-resistance region devoid of mobile carriers. This is the depletion region, and the contact exhibits rectification properties that are in some respects similar to those of a p-n junction diode. The choice of metal for a second-level interconnection is dictated by the desire for high conductivity, excellent bondability, adherence to oxides and nitrides, and compatibility with the ohmic and Schottky metal systems. This chapter will discuss the general electrical properties of the metal-GaAs systems, Schottky barrier metals, and refractory silicide metallization; then for ohmic contacts to n⁺-GaAs and p-GaAs alloyed and nonalloyed ohmic contacts, interconnect design considerations, measurement of contact resistance r_c, interconnect ...