Non-alloyed thermally stable Pd/Sn and Pd/Sn/Au Ohmic contacts have been utilized for the fabrication of GaAs MESFETs for the first time. MESFETs with son-alloyed Pd/Ge Ohmic contacts are also investigated for comparison. All contacts were deposited using a resistance heating evaporator and were furnace annealed in a conventional graphite strip annealer. Contact resistivities, rho(c), of the metallizations were measured utilizing the conventional transmission line model (cTLM) method. Metallization samples were characterized utilizing scanning electron microscopy (SEM) and current-voltage (I-V) measurements. Morphological characteristics display significant impact on the edge uniformity of the contacts. The maximum amplitude of the undulation with Pd/Sn/ Au contacts is similar to 100 nm, MESFETs with Pd/Sn/Au metallizations show improved extrinsic transconductance, g(m), when compared to the MESFETs with Pd/Sn and Pd/Ge contacts. The calculated g(m) for the MESFETs with Pd/Sn/Au metallizations is more than 100 mS/mm for a gate length, L-G, of 2 mu m Experimental results show that nonalloyed thermally stable Pd/Sn/Au Ohmic contacts appear to be promising candidates for GaAs devices. (C) 2000 Elsevier Science Ltd. All rights reserved.