High Current [200 A], Low Resistance (0.87 mΩ-cm/sub 2/) Normally-off SiC VJFETs for Power Switching Applications

Abstract

Silicon Carbide (SiC) power transistors offer large benefits for high power, low loss power switching applications. The 10× increase in critical breakdown field and 2.5× increase in thermal conductivity of SiC compared to silicon make SiC a far superior material for power switching. The Vertical Junction Field-Effect Transistor (VJFET) possesses many advantages compared to other SiC power devices, including the lack of a critical SiC/SiO2 interface, the speed of majority carrier switching, and the ability to reliably parallel many devices to achieve high currents. The challenge that VJFETs present is making them normally-off with low on-state resistance. In this work, we report the design and fabrication of normally-off SiC VJFETs with very low specific on-resistance (0.87 m: -cm 2 ) and very high current levels (200 A). These are some of the highest total current levels and lowest specific on-state resistances reported in SiC power transistors to date. The devices block voltages ranging from 50- 250 V and are optimized for use as the normally-off device in a high-voltage all-SiC cascode switch. Fabrication: The ion-implanted VJFET structure is shown in Figure 1 (1). Baseline devices had a 2.7 Pm, 5x10 15 cm -3 drift and 1.4Pm, 2x10 16 cm -3 channel. Gates and sources were defined by etching source pillars and then using the same mask to implant self-aligned, recessed Al gates. For a normally-off VJFET, the p + implants must be close enough that the built-in depletions regions pinch-off the channel with no applied gate bias. The total widths used to mask the implant ranged from 0.8-1.2 Pm. Ohmic contacts were formed using nickel silicide and interconnection done with gold. Individual device cells ranged from 6.4x10 -4 to 4.5x10 -3 cm 2 in area. Device characteristics: A baseline device blocking 60 V normally-off (VGS=0V) is shown in Figure 2a. By adjusting the spacing of the p + layers, normally-off blocking voltages up to 250 V have been achieved. Figure 2b show the on-state characteristics of the 60 V device. The specific on-state resistance at VGS=2.5 V, VDS=0.5 V is 1.74 m: -cm 2 . Device yields were very high for the normally-off VJFETs, with values as high as 82% for a 3 wafer. Using device simulation,