Effects of Total Dose Irradiation on the Gate-Voltage Dependence of the $\hbox{1}/f$ Noise of nMOS and pMOS Transistors

Sarah A Francis,Daniel M Fleetwood,Aritra Dasgupta

doi:10.1109/ted.2009.2036297

Abstract

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 ¿ = -¿ln S Vd /¿ln f 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.

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