AbstractA recess gate depletion‐type InGaAs MISFET has been fabricated by using a thick photo‐CVD SiO2outer layer and on ultrathin MBE Si interface control layer (Si ICL) after HF treatment of air‐exposed InGaAs surface treated by HF. After optimum annealing of the MISFET at 350°C in hydrogen atmosphere for 1 hr, a significant increase of the transconductance was obtained and with a very stable drain current. Maximum effective mobility of 3850 cm2/V.s and a transconductance of 61 mS/mm for a gate length 6 μm were achieved.Based on the observed dependence of gate length on the transconductance, it is found that transconductance as much as 250 mS/mm can be expected for 1 μm gate length even after considering the velocity saturation effect. A drain current drift was controlled below 5 percent after 104s after a gate step voltage was applied.The performance obtained in the present study is comparable to that of photo‐CVD SiO2/Si ICL/InGaAs MISFET fabricated in an in‐line process in ultrahigh vacuum. the result clearly demonstrates that an HF treatment of air‐exposed InGaAs surface is useful for a MISFET