Switching times variation of power MOSFET devices after electrical stress

The switching performance of three power MOSFET devices with different oxide thicknesses is studied after several periods of electrical stress. The thickest oxide reveals a large accumulation of positive charges in the oxide bulk after small periods of stress. These charges affect the switching parameters by increasing the rise time and by decreasing the fall time. Larger periods of stress reduce the effect of positive charges by increasing the number of interface states. The threshold voltage is decreased by the effect of a positive oxide charge and increases with the appearance of interface states. All these phenomena are less observable as we reduce the oxide thickness.