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 (Rsd), 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 (gm) of the device thus more nearly approaches its high intrinsic value. At 77 K, the best (Al,Ga) As FET's had Rsd= 0.69 Ω . mm and gm= 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.