A new degradation mode of n-channel lateral double-diffused MOS (LDMOS) transistors has been investigated. The degradation, resulting in a large increase of the drain saturation current at the onset of strong inversion, is attributed to avalanche-generated hot holes injected and trapped in the gate oxide above the n-type drift region of LDMOS transistors operating at a high drain voltage and a low gate voltage near threshold. Worst-case static gate-bias condition, drain voltage dependence, maximum operating drain voltage, and the effect of varying some geometrical parameters of the device are studied. A method, based on gate-to-drain capacitance measurements, to characterize the spatial extension of the damaged region and the amount of trapped holes, is presented.