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 semiinsulating 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 square microns. 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.