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Threshold voltage reduction from hot-hole injection during repetitive unclamped inductive switching is investigated in low-voltage discrete power trench nMOSFETs with different trench depths. Power nMOSFETs with 21 mm2 of active area, breakdown voltages of 25 V, oxide thicknesses of 76 nm, and in TO-220 packages have been fabricated with 1.3-, 1.55-, 1.75-, and 2-μm trench depths. The reduction in the threshold voltage ( VGSTX) is shown to be a function of the number of avalanche cycles (N) through a power law, i.e., VGSTX =A Nn, where A is the prefactor and n is the exponent. After 100 million cycles of repetitive avalanche at a mounting base temperature of 150°C, an avalanche current of 160 A, and an avalanche duration of 100 s, the power law prefactor (A) is shown to increase from 3 × 10-14 to 1 × 10-12 as the trench depth is increased from 1.3 to 2 μm. This is due to the increased hot-hole injection into the gate dielectric, which increases with the trench depth as a result of increased oxide exposure to hot carriers and increased electric fields with deeper trenches. However, deeper trench MOSFETs have the benefit of a reduced on-state resistance.
Date of Publication: July 2010