GaAs and AlGaAs layers grown by molecular beam epitaxy at low temperatures were used as the insulator in a metal-insulator-semiconductor diode simulating the gate structure of a GaAs metal-insulator-semiconductor field-effect transistor (MISFET). The diode current increases after the high-temperature annealing at 800 degrees C for 10 s, a schedule commonly used for ion-implantation activation, and the amount of the increase depends strongly on the insulator material and the growth temperature. It appears that the low-temperature-grown (LTC) GaAs grown at 200 degrees C and the LTG Al0.43Ga0.57As grown at 300 degrees C, both embedded between AlAs barrier layers grown at a normal temperature, are the two best gate insulators when the high-temperature annealing is a required process for the MISFET. For a diode with a 300-Angstrom-thick Al0.43Ga0.57As insulator and 100-Angstrom-thick AlAs barriers, 1.42 V forward bias results in a leakage current of 1 nA/mu m(2). This low diode current proves that the LTG insulator is suitable for the gate of MISFETs.