The Impact of Trench Depth on the Reliability of Repetitively Avalanched Low-Voltage Discrete Power Trench nMOSFETs

摘要

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 $hbox{mm}^{2}$ 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- $muhbox{m}$ trench depths. The reduction in the threshold voltage $(Delta V_{rm GSTX})$ is shown to be a function of the number of avalanche cycles $(N)$ through a power law, i.e., $Delta V_{rm GSTX} =A cdot N^{n}$, where $A$ is the prefactor and $n$ is the exponent. After 100 million cycles of repetitive avalanche at a mounting base temperature of 150 $^{circ}hbox{C}$, an avalanche current of 160 A, and an avalanche duration of 100 $muhbox{s}$, the power law prefactor $(A)$ is shown to increase from $hbox{3} times hbox{10}^{-14}$ to $hbox{1} times hbox{10}^{-12}$ as the trench depth is increased from 1.3 to 2 $ muhbox{m}$. This is due to the increased hot-hole injection into the gate dielectric, which increases with the tren-n-nch 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.