In this paper, on-resistance (Ron) degradation induced by off-state avalanche breakdown in a 40 V LDMOS with step-shaped gate oxide (SGO LDMOS) is investigated. Ron unexpectedly decreases at the beginning of stress, which is different from the phenomenon described in works on LDMOS with uniform gate oxide (UGO-LDMOS). Based on the experiment data and TCAD simulation results, two degradation mechanisms are proposed. That is the generation of positive oxide-trapped charges at the bird's beak region near source and formation of interface state at the bird's beak region near source and drain respectively. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.