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

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Abstract
The existence of finite small-signal drain-body and source-body conductancesg_{r}(T)associated with drain and source to body diffusion leakage currents in MOSFET's is reported. These currents and therefore these conductances increase asn\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 (gd) and body effect (gmb) 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 (Vr) induced modulation of the depletion region thicknessesw(V_{r}, T)of the junctions. Expressing leakage currents asI(V_{r}, T) = J(V_{r}, T)' \cdot A(V_{r}, T)whereJ(V_{r}>, T)andA (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°) theg_{r}(T)result from the voltage dependence ofJ(V_{r}, T); it is the areaA(V_{r}, T)whose modulation by voltage dominates that ofJ(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 conductancesg_{r}(T), and consequences of these for high-temperature analog MOS IC design are highlighted. Experimental data supporting the reported interpretation are presented.

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