Amorphous semiconductor threshold on−state properties as functions of decay time, ambient temperature, and polarity

Abstract

The on−state conductance and radiative emission properties of an amorphous semiconductor threshold switch have been investigated. The transient on−state conductance GT and differential conductance G̃T are found to be constant for time intervals τ of up to 120 nsec during which the voltage is below the low−frequency holding voltage but above the knee voltage. The knee voltage is the voltage at which the transient on−state current−voltage curve coalesces into the transient off−state curve in the down−voltage direction. For τ≳120 nsec, GT and G̃T vary asymptotically with time. As ambient temperature is increased from 300 to 400 °K, G̃T is observed to remain constant or increase slightly as would be expected in dGT/dT for a metal. If the on−state is assumed to have a band gap, conductance vs temperature measurements indicate that the gap would have a value less than 1.2×10−4 eV, and thus for practical purposes the gap could be considered to be essentially zero. The intensity of the nonthermal radiative emission which is observed during the on−state of a threshold event is found to be polarity dependent such that when the Nesa electrode adjacent to the detector is of positive polarity, the intensity of the emission is 25% greater than when the polarity is reversed. These observations are interpreted as indications that the on state is maintained electronically rather than thermally, in a state analogous to a metal or semimetal with a zero indirect electrical band gap but a finite direct optical gap.