Amorphous—Crystalline heterojunction transistors

The characteristics of a new hybrid amorphous-crystalline three-terminal device are described in detail. The device uses a threshold-switching-type chalcogenide glass as the emitter, with a p-type crystalline-silicon base and an n-type silicon collector. When the amorphous emitter is in the off state, the small-signal current gain is significantly smaller than unity. However, once the voltage across the glass is sufficient to produce the on state, the device operates as a transistor with gains well in excess of unity. (Small-signal gains of up to 15 have already been observed in unoptimized configurations.) Under certain conditions, the high-gain on state can be preserved after the switching pulse is removed, suggesting memory-type applications. Previous results on amorphous-crystalline heterojunctions are used to construct a band model for the transistor. This model is used to analyze the steady-state and pulsed-mode characteristics of the device. Recent studies of the on-state current density and carrier concentration are used to calculate the expected gain as a function of current and base doping concentration, with results in good agreement with the experimental data. The behavior of the devices provides another confirmation of the electronic nature of threshold switching in chalcogenide glasses.