A new semiconductor-insulator-semiconductor field-effect transistor has been fabricated. The device consists of a heavily doped n-type GaAs gate with undoped (Al,Ga)As as the gate insulator, on an undoped GaAs layer. This structure gives the device a natural threshold voltage near zero, well suited for low-voltage logic. The threshold voltage is, to first order, independent of Al mole fraction and thickness of the (Al,Ga)As layer. The layers were grown by MBE and devices fabricated using a self-aligned technique involving ion-implantation and rapid thermal annealing. A transconductance of 240 mS/mm and a field-effect mobility of about 100 000 cm2/V-s were achieved at 77 K.