Temperature dependent threshold behavior of depletion mode MOSFETs. Characterization and simulation

During the study of depletion mode MOSFET behavior at low temperatures, unusual changes in the threshold characteristics of the devices were observed. First, the effectiveness of the donor implantation in producing a negative threshold voltage shift was significantly reduced. At the same time the substrate sensitivity was found to be substantially reduced. A third observation was the existence of an unusual structure in the subthreshold region of the device at low temperatures. Computer simulation is used to explore these observations and to demonstrate that they are caused by impurity freezeout as temperature is reduced. The computer simulation program, usable over the temperature range 50-350 K, is discussed, and a threshold definition suitable for numerical analysis of devices with arbitrary channel structures is developed. © 1979.