Preswitching and postswitching phenomena in amorphous semiconducting films

Abstract

Low-duty-cycle pulsed dc switching experiments have been performed on a variety of thin-film Te-based semiconducting glasses. No premonitory effects are observed in the current-time profile of the nonswitching off state. If, however, the threshold voltage is exceeded, a continuous current increase with time throughout the normal delay-time regime (preswitching off state) is always observed prior to switching. In a given device or film this current rise is interrupted, independent of overvoltage, by the rapid switching transition at about the same relative current increment above the current background extrapolated to zero time (leakage current). In the switched or on (filamentary conduction) state the current level, as limited by the series load resistor, can condition the response of the device in the subsequent preswitched off state. For loads in excess of ∼1 kΩ, there is no effect on the preswitching off state characteristics whereas, for appreciably smaller loads, there is a continuous increase in delay time with increased pulse length (time spent in the previous on state). The results (i) indicate that the development of a filamentary instability begins at the onset of the pulse and (ii) show that the operating threshold voltage can be raised (returned to its virgin value) when the operating current is large for a sufficiently long time. These features are consistent with switching models that treat the ``formed'' switching element as a high-conductance-formed filamentary region (of not necessarily uniform composition) surrounded by a low-conductivity glass.