A MISFET which consists of $n^+$ GaAs/$N^-$ AlGaAs structure was fabricated. In order to obtain good ohmic contact for heterostructure, alloying condition was investigated. Good condition was 430℃, 15∼20min. Using this condition, Transistor whose gate length and gate width are 0.8μm and 240μm was measured. Extracted max-transconductance, $Gm_{max}$ was 96mS/mm. And its schottky barrier $\phi_b$ was 1.1eV. It shows good schottky characteristics of AlGaAs.