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Absfruct-State-of-the art Sic MESFET's showing a record high fmax of 26 GHz and RF gain of 8.5 dB at 10 GHz are described in this paper. These results were obtained by using high-resistivity S i c substrates for the first time to minimize substrate parasitics. The fabrication and characterization of these devices are discussed.HE S i c devices are emerging as promising candidates T for high temperature and high power RF operation due to the unique combination of high electron velocity, high thermal conductivity, and electric breakdown strength of Sic. Significant progress has been recently achieved in this material system and MESFET's with fmax of 11 GHz in 6H-SiC, and 12.9 GHz in 4H-material have been developed by us and by other groups [l], [2]. However, the high frequency performance in this system has thus far been limited by increased device parasitics mainly arising due to low resistivity substrates and buffer layers. In this paper, we present for the first time development of S i c MESFET's on high resistivity substrates to eliminate these parasitic effects. These devices show a record high fmax of 25 GHz, and RF gain (MAGMSG) of 8.5 dB at 10 GHz, while operating at a high drain voltage of 40 V.MESFET structures were fabricated on high resistivity N -S i c substrates of 6H-polytype. Single crystals of S i c weighing up to 300 g were grown at rates of 0.25 to 1 m d h r in the c-(OOOl) axial direction with diameters up to 50 mm. The crystals were grown using a modified sublimation technique which has been described elsewhere [3]. High resistivity was obtained by using high purity growth without any intentional doping of deep level compensating impurities [3]. Polished substrates with diameters of 40 mm and surface normals oriented 3 degrees off the (0001) direction were fabricated from the crystals. Substrates exhibited as-grown resistivities in the 1500-2000 ohm-cm range, as determined by using a Lehighton Model 13 10 contactless measurement system.Epitaxial layers were grown on the high resistivity substrates at 1450°C using an atmospheric pressure vapor phase epitaxial reactor (VPE) employing silane and propane as the reagents IEEE Log Number 9405996. Fig. 1. curve is for 1, = 3 V. I-V characteristics of a 0.5 irm x 320 p m S i c MESFET. The top and nitrogen as n-type dopant [4], [ 5 ] . The MESFET structures consisted of a 2 pm thick unintentionally doped (< 5 x 10l4/cm3) buffer layer, 0.4 p m thick channel layer doped to 2.5 x 10"/cm", and a 0.1 lLm n+ contact layer doped to -1019/cm3.Device fabrication started with etching of mesas down to the buffer layer for device isolation. Reactive Ion Etching (RIE) with A1 as the mask was used to fabricate the mesas. A 2 / m wide channel recess was fabricated by etching the n+ and part of the active layers again using RIE. Ni defined by lift-off and sintered using a rapid thermal process was used to form the ohmic contacts. A multilayer metal system was used as the gate to reduce the gate resistance and was defined by lift-off. The gate length was 0.5 pm and the ...