ABSTRACT
In the 1960s, when Gunn devices were first introduced, low contact resistance was not critical, simple low-temperature alloying metal layers were used. The researchers’ focused attention on Au-Ge systems as these provide a low alloying temperature. As newer devices like metal-semiconductor field-effect transistors (MESFETs) were being developed in the 1970s, the need for lower contact resistance became apparent. With the introduction of high-electronmobility transistors (HEMTs) and HBTs, requirements became even more demanding. The most common metal systems show electrical degradation with further processing and time. Ohmic contacts introduce parasitic resistance in field-effect transistors (FETs) and HBTs, which is not desired for high-frequency performance and low-noise figures. Improvements have been sought over the last few decades, achieving perfect low resistance, and temperature stability has been an elusive goal. However, contact metallurgies that fulfil the needs of III-V and III-N compound semiconductor circuits have been developed and are in production. The most extensively used technique to make ohmic contacts to GaAs is to deposit metal alloys containing dopants on the surface and then diffuse the dopants in by a process of melting called alloying. If the metal does not melt, then the contact is sintered to achieve intermixing. To improve high-temperature stability, metal systems that use high-temperature alloying or sintering, as well as refractory and barrier metals, have been used to suppress continued diffusion (e.g., that of Au) during operation. Table 11.1 lists some common metallization systems that have been tried for making contacts to GaAs. For extensive reviews of ohmic contact techniques and mechanisms, readers are referred to reviews by Rideout [1], Yoder [2], Braslau [3], and Murakami [4].
