  Crossbar array architecture is an essential design element for densely
connected Non-Volatile Memory(NVM) applications. To overcome intrinsic sneak
current problem of crossbar arrays, each memory unit is serially attached to a
selector unit with highly nonlinear current-voltage (I-V) characteristics.
Recently, Ovonic Threshold Switching (OTS) materials are preferred as selectors
due to their fabrication compatibility with PRAM, MRAM or ReRAM technologies;
however, OTS selectors suffer from the temporal drift of its threshold voltage.
First, based on Poole-Frenkel conduction, we present time and temperature
dependent model that predicts temporally evolving I-V characteristics,including
threshold voltage of OTS selectors. Second, we report an ultrafast saturation
($\sim 10^3$ seconds) of the drift and extend the model to predict the time of
drift saturation. Our model shows excellent agreement with OTS devices
fabricated with 8 nm technology node at 25{\deg}C and 85{\deg}C ambient
temperatures. The proposed model plays a significant role in understanding OTS
device internals and the development of reliable threshold voltage jump table.
