Device Modeling at Cryogenic Temperatures: Effects of Incomplete Ionization

We present a device performance modeling methodology that self-consistently resolves device operation at cryogenic temperatures (T > 30 K) in conjunction with incomplete ionization effects that take into account the change in dopant activation energies as a function of doping. Using this methodology, we developed a device simulator that predicts n-channel MOSFET (NMOSFET) device characteristics for a wide range of temperatures by solving semiconductor equations, along with the Poisson equation. Comparison of our calculated results with measurements shows that proper inclusion of variations in activation energy as a function of doping level is necessary for accurately monitoring device operation at cryogenic temperatures. Using dopant activation energies that are independent of doping levels leads to current rolloff and eventually device turn-off at low-temperature simulations. However, activation energy models that give lower activation energies for higher doping levels result in improved NMOSFET performance at colder temperatures, which agrees with experiments. Furthermore, calculations indicate that different incomplete ionization models affect the NMOSFET characteristics mainly through changes in the resistances of the heavily doped source and drain regions, and the substrate.