Intrinsic current overshoot during thermal-runaway threshold switching events in TaOx devices

The response of a TiN/TaOx/TiN S-type threshold switching device to a rectangular source voltage pulse was simulated using an electrothermal finite element model. At the start of the pulse, the current density within the device was uniform. As the device transitioned to the steady state, the current followed the load line of the series resistor and the current density became increasingly more constricted. Unexpectedly, the temperature and current density evolution was not monotonic. At low values of the load resistor, the current density constricted more during the transition than at the steady state. The temperature at the point of maximum constriction can exceed the steady state temperature by 100 degrees C if the time of the threshold switching event is shorter than the thermal time constant of the device. The magnitude of the overshoot decreases with decreasing device size. The reported effect can have a significant impact on the reliability of threshold switches and the electroformation of memory devices.