A thin‐film field‐effect transistor has been fabricated using glow‐discharge amorphous silicon as the semiconductor and silicon nitride as the insulator. The transistor operates in the electron (ntype) accumulation mode and by changing the gate potential from zero to only 3 V a change in the source‐drain conductance of greater than four orders of magnitude is obtained. The results imply upper limits to the density of gap states in amorphous silicon and interface states at the amorphous silicon‐silicon nitride interface of 3×1016cm−3 eV−1and 5×1011cm−2 eV−1, respectively.