Uniformity evaluation of MESFET´s for GaAs LSI fabrication

Studies were made on GaAs MESFET I-V characteristic scattering using self-aligned FET´s on semi-insulating substrates. Surface treatment before gate-metal evaporation was found to have a satisfactory affect on FET drain current. The two main factors of threshold-voltage scattering in self-aligned FET´s were clarified. One is the lack of uniformity in gate lengths, and the other is substrate nonuniformity. An analytical method was proposed to distinguish between threshold-voltage dispersions attributed to the factors without direct measurement of the gate lengths. Threshold-voltage scattering due to crystal inhomogeneity was estimated for both LEC and HB substrates, and for both the entire area of a 2-in wafer and an area as small as 400 µm². It was confirmed that the dislocations making up the firm network structure in LEC crystal affect the threshold voltage of self-aligned FET´s and give rise to large dispersion even in the small area. High uniformity was recognized in the small area on HB substrates.