Effects of Total Dose Irradiation on the Gate-Voltage Dependence of the Noise of nMOS and pMOS Transistors

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)^-ß, where Vt is the threshold voltage, V _g is the gate voltage, and ß is a measure of the gate-voltage dependence. For the pMOS devices, preirradiation ß ranges from 0.4 to 0.9, and the frequency exponent ¿ = -¿lnS _Vd/¿lnf is greater than unity. Postirradiation, gate-voltage, and frequency dependences change significantly, with ß >> 1 and ¿ much closer to unity. For nMOS devices, preirradiation ß ¿ 1.6 and ¿ ¿ 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 ¿ ~ (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.