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 (2DEG) at N
⁺
-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 Ω . mm, which is adequate for MESFET applications using these structures. MESFET's fabricated with (Al,Ga) As and (Al,In,Ga) As heterojunction 2DEG channels and Ni-Ge-Au contacts have source-drain resistances (R
sd
), which decrease dramatically at low temperature as a result of the mobility enhancement in the 2DEG 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 FET's had R
sd
= 0.69 Ω . mm and g
m
= 309 mS/mm with gate lengths of 1.5 µm and a source-drain spacing of 9 µm. A microwave gain of 11 dB at 6.4 GHz has been obtained at room temperature for these devices.