ALLOYED AND NONALLOYED OHMIC CONTACTS FOR ALINAS/INGAAS HIGH-ELECTRON-MOBILITY TRANSISTORS

AuGe/Ni/Au alloyed and WSi non-alloyed ohmic contacts are investigated for AIInAs/InGaAs high electron mobility transistors (HEMTs). For the alloyed contact, a contact resistance (R(c)) lower than 0.03 Ohm mm is obtained at an alloy temperature of 300 degrees C. The value of R(c) drastically increases with alloy temperatures above 300 degrees C and exceeds 0.15 Ohm mm at 380 degrees C. Auger analysis and analytical cross-sectional transmission electron microscopy have revealed significant outdiffusion of In in the epitaxial layer into the top Au layer and the formation of polycrystalline GaAs in the epitaxial layer, which cause the increase ofR(c) with alloy temperature. For the refractory WSi non-alloyed ohmic contact, R(c) remains lower than 0.1 Ohm mm under annealing temperatures up to 380 degrees C. The extrinsic maximum transconductance (g(m)) of 600 mS/mm is obtained for the HEMT device with the WSi ohmic contact.