VJFET Based All-SiC Normally-Off Cascode Switch for High Temperature Power Handling Applications

Abstract

High-voltage vertical-junction-field-effect-transistors (VJFETs) are typically designed normally-on to ensure low-resistance voltage-control operation at high current-gain. To exploit the high-voltage/temperature capabilities of VJFETs in a normally-off voltage-controlled switch, high-voltage normally-on and low-voltage normally-off VJFETs were connected in the cascode configuration. The cascode gate’s threshold voltage decreases from 2.5 V to 2 V as the temperature increases from 25°C to 225°C, while its breakdown voltage increases from -23 V to -19 V. At 300°C, the drain current of the cascode switch is 21.4% of its 25°C value, which agrees well with the reduction of the 4H-SiC electron mobility with temperature. The VJFET based all-SiC cascode switch is normally-off at 300°C, with its threshold voltage shifting from 1.6 V to 0.9 V as the temperature increases from 25°C to 300°C. This agrees well with the measured reduction in VJFET built-in potential. Finally, the reduction in cascode transconductance with temperature follows that of the theoretical 4H-SiC electron mobility. Overall, the measured thermally-induced cascode parameter shifts are in excellent agreement with theory, which signifies fabrication of robust SiC VJFETs for power switching applications.