High-temperature diffusion leakage-current-dependent MOSFET small-signal conductance

The existence of finite small-signal drain-body and source-body conductances g_{r}(T) associated with drain and source to body diffusion leakage currents in MOSFET's is reported. These currents and therefore these conductances increase as n\min{i}\max{2}(T) with increasing temperature. For devices of geometries commonly encountered in analog MOS integrated circuits, these conductances typically become comparable in magnitude to the conventional output (g d ) and body effect (g mb ) conductances between 200° and 250°C. The origin of these conductances is traced to the voltage modulation of the effective areas of the junctions under consideration. This modulation occurs through the reverse-bias voltage (V r ) induced modulation of the depletion region thicknesses w(V_{r}, T) of the junctions. Expressing leakage currents as I(V_{r}, T) = J(V_{r}, T)' \cdot A(V_{r}, T) where J(V_{r}>, T) and A (V_{r}, T) are voltage- and temperature-dependent current density and junction area, respectively, we find that whereas with generation-recombination leakage currents (dominating in the range 25° to 150°) the g_{r}(T) result from the voltage dependence of J(V_{r}, T) ; it is the area A(V_{r}, T) whose modulation by voltage dominates that of J(V_{r}, T) in the case of diffusion leakage currents (T > 150° C). A modified small-signal MOSFET model is proposed which includes the aforementioned conductances g_{r}(T) , and consequences of these for high-temperature analog MOS IC design are highlighted. Experimental data supporting the reported interpretation are presented.