Au/Ge/Ni/Au and Pd/Ge/Ti/Au Ohmic contacts to AlxGa1-xAs/InGaAs (x=0.75) pseudomorphic high electron mobility transistor

Abstract

The optimal Ohmic contact to an AlxGa1-xAs/lnGaAs pseudomorphic high-electron-mobility-transistor (PHEMT) structure with a high A1 mote fraction (x = 0.75) was developed from comparative studies of Au/Ge/Ni/Au and Pd/Ge/Ti/Au contacts for various etching depths of the cap layer and annealing temperatures. A minimum Ohmic contact resistivity (rho(c)) of 2.0 x 10(-6) Ohm-cm(2) was obtained by annealing (T = 450 degreesC) Au/Ge/Ni/Au metals deposited on undoped PHEMT substrate. The rho(c) of Au/Ge/Ni/Au increased with the etching depth, but that of Pd/Ge/Ti/Au showed the opposite trend. This was due to deeper penetration of the Au/Ge/Ni/Au contact compared with the Pd/Ge/Ti/Au contact. The penetrating Art reacted with the PHEMT substrate to form Au2Al and Au2Ga, which generated group-III vacancies acting as donors. The device fabricated using the optimal Ohmic contact showed a low knee voltage and a negligible gate leakage current at high gate bias, which are due to the good Ohmic contact and the large conduction band offset between AlGaAs and InGaAs.