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Trench depth is important in low-voltage trench MOSFETs because it affects the switching losses through the gate-drain capacitance (CGD). The dependence of CGD on the trench depth is investigated by analytical modeling and experimental characterization. An analytical model that relates the trench depth, trench bottom oxide thickness, n- layer doping, and drain voltage (VD) to CGD is developed and validated by experimental measurements. Trench MOSFETs with thick bottom oxides have been fabricated with 1.3-, 1.5-, 1.7-, and 2-μm deep trenches. CV measurements show that CGD is proportional to the trench depth at low VD and becomes increasingly independent of trench depth as VD is increased. The model is used to show that this is due to CGD being dominated by the oxide capacitance at low VD and the depletion capacitance at high VD. The fact that the average thickness of the trench bottom oxide decreases as the trench depth increases (because of additional sidewall oxide overlapping the drain) means that the impact of the trench depth is the highest at low VD , where the depletion capacitance is ineffective.