A thin film floating gate transistor with improved dielectric structure. The dielectric structure serves the purpose of encapsulating the floating gate and also interfacing with the semiconductor material, .alpha.-Si:H. It thus must meet a variety of requirements. In order to provide long memory retention times, the dielectric material, at least in the regions encapsulating the floating gate, must have a high resistivity, on the order of 10.sup.17 ohm-cm or better. Silicon dioxide is the preferred material for encapsulating the floating gate. However, since silicon dioxide creates a high density of defect state when interfaced with the .alpha.-Si:H layer. An interface layer, substantially free of oxide, is interposed between the high resistivity layer and the .alpha.-Si:H. Preferably, the interface portion of the dielectric layer is silicon nitride. In some cases, it is desirable to replace the entire dielectric structure, or at least the interface layer with aluminum nitride.