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

摘要

We have found that the room-temperature $hbox{1}/f$-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 $S_{rm Vd}$ is proportional to $(V_{g} - V_{t})^{-beta}$, where $V_{t}$ is the threshold voltage, $V_{g}$ is the gate voltage, and $beta$ is a measure of the gate-voltage dependence. For the pMOS devices, preirradiation $ beta$ ranges from 0.4 to 0.9, and the frequency exponent $alpha = -partialln S_{rm Vd}/partial ln f$ is greater than unity. Postirradiation, gate-voltage, and frequency dependences change significantly, with $beta gg hbox{1}$ and $alpha$ much closer to unity. For nMOS devices, preirradiation $beta geq hbox{1.6}$ and $alpha approx hbox{1}$, 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 ed-nge, but after irradiation, the distribution is typically more uniform. Moreover, for some moisture-exposed devices, $S_{rm Vd} propto sim(V_{g} - V_{t})^{-3}$ 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.