Vertical metal-oxide-semiconductor field-effect transistors (MOSFETs) have evolved into dominant members in the power transistor family. Reliability issues continue to be a major concern, as economic requirements drive towards miniaturization, and higher power ratings for these devices. The charge pumping method offers a simple, direct and powerful way of assessing interface damage for planar structures. Absence of an independent substrate contact in the vertical power structure implies that conventional charge pumping, which requires a substrate contact as well as three additional contacts to the remaining terminals of the MOSFETs, cannot be applied directly to these devices. In this article, we propose an adaptation of the charge pumping technique that enables its application to three terminal devices, in general. The modified form of charge pumping was applied to assess effects of Fowler-Nordheim stressing on production level U-shaped trench-gated MOSFETs. A good correlation between transfer characteristic measurements and the proposed method has been observed. (c) 2005 American Vacuum Society.