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We have found that the room-temperature 1/f-noise gate-voltage and frequency dependences of pMOS transistors are affected significantly by moisture exposure and total dose irradiation. The voltage noise power spectral density S Vd is proportional to (V g - V t)-Ã, where Vt is the threshold voltage, V g is the gate voltage, and Ã is a measure of the gate-voltage dependence. For the pMOS devices, preirradiation Ã ranges from 0.4 to 0.9, and the frequency exponent Â¿ = -Â¿lnS Vd/Â¿lnf is greater than unity. Postirradiation, gate-voltage, and frequency dependences change significantly, with Ã >> 1 and Â¿ much closer to unity. For nMOS devices, preirradiation Ã Â¿ 1.6 and Â¿ Â¿ 1, with little change after irradiation. We attribute these observed changes in pMOS noise to changes in the trap density and energy distribution D t(E f) of these devices. Before irradiation, D t(E f) increases toward the valence band edge, but after irradiation, the distribution is typically more uniform. Moreover, for some moisture-exposed devices, S Vd Â¿ ~ (V g - V t)-3 after irradiation, indicating a D t(E f) that increases toward midgap. We conclude that irradiation and/or moisture exposure can greatly affect the defect energy distributions for these devices and that the observed nMOS and pMOS noise can be described by a simple trapping model with an energy-dependent trap distribution.
Date of Publication: Feb. 2010