Silicon carbide (Sic) MESFETs-with gate lengths from 0.3 mum to 1.5 mum-were fabricated on a semi-insulating substrate, and their RF characteristics were measured. These measurements showed that when the gate length is less than 1.0 mum, the cutoff frequency f(t) and maximum oscillation frequency f(max) of the SiC-MESFETs is less dependent on gate length. It was also found that the degradation of these RF characteristics is apparently caused by the short-channel effect. To uncover the mechanism underlying this finding, static electrical characteristics, such as the transconductance and threshold voltage of the fabricated short-channel SiC-MESFETs, were investigated; consequently, it was found that the short-channel effect occurred when the aspect ratio of gate length to channel thickness was five or less.