Effects of Total Dose Irradiation on the Gate-Voltage Dependence of the 1/f 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 densitySVdis proportional to (Vg-Vt)-ß, whereVtis the threshold voltage,Vgis 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 ¿ = -¿lnSVd/¿lnfis 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 distributionDt(Ef) of these devices. Before irradiation,Dt(Ef) increases toward the valence band edge, but after irradiation, the distribution is typically more uniform. Moreover, for some moisture-exposed devices,SVd¿ ~ (Vg-Vt)-3after irradiation, indicating aDt(Ef) 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.