Metastable charge-trapping effect in SOI nMOSTs at 4.2 K

The effects of the application of channel hot-carrier (CHC) stress applied at 42 K to SOI nMOS transistors are investigated. A reduction of the threshold voltage is observed for typical stressing conditions, while simultaneously the maximum transconductance increases. Similar changes can be obtained nearly immediately by applying a strong pulse into multiplication, i.e. by pulsing the drain voltage beyond the kink. At 4.2 K this 'low'-threshold-voltage (V T ) state is metastable. These phenomena can be explained by considering hole trapping in the frozen-out film yielding a reduction of the depletion charge and an increase in the film potential. This results in a reduction of V T and of the transverse electric field. Due to the slow re-emission and to the steep forward diode characteristic, trapped charge will remain for a considerable time in the film, explaining the metastability of the hole-trapping-induced low-V T state at 4.2 K.