ANALYTICAL MODEL FOR THE KINK IN NMOSTS OPERATING AT LIQUID-HELIUM TEMPERATURES (LHT)

From a detailed investigation of nMOST characteristics at liquid helium temperatures (T < 40 K), it is inferred that the kink in the drain current ID can be interpreted as a switching from a high to a low threshold voltage Vt. The low Vt state corresponds with an increase in the substrate (well) potential VBS of about 1.1 V, i.e. the "on" voltage of the source diode. This increase is caused by the space charge limited flow of the substrate current IB through the well impedance. Based on these observations a simple, one-dimensional analytical model is constructed, relating the kink amplitude δID to the square root of IB. From this model, the influence of device dimensions, well/substrate contact separation and temperature T on the kink is derived. As will be shown, other mechanisms like the "forced depletion layer formation" contribute to the kink/hysteresis behaviour, especially at temperatures below 10 K. © 1990.