Hot-electron-induced degradation in MOSFET's at 77 K

Hot-electron-induced degradation of transconductance and of threshold voltage at 77 K of n-channel enhancement metal-gate MOSFET's was investigated as a function of electrical stress applied at liquid nitrogen temperature. After stress, the threshold voltage was found to have increased at low drain voltages but to have remained unchanged at higher drain voltages, and the saturation transconductance was virtually unchanged for operation in the normal mode. For operation in the inverse mode (source and drain interchanged), the threshold voltage was found to have increased, independent of drain voltage, while the saturation transconductance was decreased. The threshold voltage for inverted operation increased monotonically with stress time, while the saturation transconductance decreased initially and then saturated. This saturation corresponds to an order of magnitude decrease in carrier mobility in the channel near the drain. These results are interpreted using a model in which the threshold voltage and channel mobility are position-dependent. While hot-electron-induced degradation may not be a problem for devices operated only in the forward saturation region, it could be a serious problem for devices such as bilateral switches.