GOLD-GERMANIUM-BASED OHMIC CONTACTS TO THE TWO-DIMENSIONAL ELECTRON GAS AT SELECTIVELY DOPED SEMICONDUCTOR HETEROINTERFACES.

Results of a study of ohmic contacts to the two-dimensional electron gas (2-DEG) at N** plus -n III-V semiconductor heterointerfaces are presented. In a comparison of alloyed metallizations based on the Au-27at. %Ge eutectic system, the addition of Ni and the method of deposition were found to have the largest effects in lowering the contact resistance. The Ni-Ge-Au ohmic contact reproducibly gives a (width-normalized) contact resistance of less than 0. 2 OMEGA -mm, which is adequate for MESFET applications using these structures. MESFETs fabricated with (Al, Ga)As and (Al, In, Ga)As heterojunction 2-DEG channels and Ni-Ge-Au contacts have source-drain resistances (R//s//d), which decrease dramatically at low temperature as a result of the mobility enhancement in the 2-DEG channel and the small contribution of contact resistances. The transconductance (g//m) of the device thus more nearly approaches its high intrinsic value. AT 77 K, the best (Al, Ga)As FETs had R//s//d equals 0. 69 OMEGA -mm and g//m equals 309 mS/mm with gate lengths of 1. 5 mu m and a source-drain spacing of 9 mu m. A microwave gain of 11 dB at 6. 4 GHz has been obtained at room temperature for these devices.