Transient current instabilities in a-Si: HP+ni structures

It has been demonstrated that amorphous silicon p+ni junctions exhibit nonvolatile polarity dependent memory switching after initial conditioning by means of a high applied potential (`forming¿). The memory on-state is due to the presence of a highly conducting filament, whose physical properties are not well understood. Recent work has shown that in junctions where the i-layer is either thin or lightly n-doped the forming process may be preceded by a transient current instability, which decays even when the voltage across the device is maintained. Unlike the forming process, the occurrence of a current instability does not necessarily modify the device, although progressive changes may result after repeated operation. The magnitude of the current maximum shows only a weak dependence on device area, which suggests that conduction during the current instability is localised within an incipient filament. In certain respects this behaviour resembles the off to on transition observed in notionally analogous crystalline threshold switches, although in our case the high conducting on-state cannot be maintained by a holding voltage. A qualitative model which accounts for most of the observed features is presented and discussed.