Alatise, Olayiwola M., Kennedy, Ian, Petkos, George, Heppenstall, Keith, Parkin, Jim, Khan, Khalid, Dr., Koh, Adrian and Rutter, Philip. (2010) The impact of trench depth on the reliability of repetitively avalanched low-Voltage discrete power trench nMOSFETs. IEEE Electron Device Letters, Vol.31 (No.7). pp. 713-715. ISSN 0741-3106

Abstract

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.

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