Large Area Silicon Carbide Vertical Junction Field Effect Transistors for High Temperature Power Conditioning Applications

SiC VJFETs are excellent candidates for reliable high power/temperature switching as they only use PN junctions in the active device area where the high electric fields occur. VJFETs do not suffer from forward voltage degradation, exhibit excellent short circuit performance, and operate at 300degC. 0.19 cm² 1200 V normally-on and 0.15 cm² low-voltage normally- off VJFETs were fabricated. The 1200 V VJFET outputs 53 A with a forward drain voltage drop of 2 V and a low specific on-state resistance of 5.6 mOmega cm². The low-voltage VJFET outputs 38 A with a forward drain voltage drop of 3 V and a specific on- state resistance of 10 mOmega cm². The 1200 V SiC VJFET was connected in the cascode configuration with a Si MOSFET and with a low-voltage SiC VJFET to form normally-off power switches. At a forward drain voltage drop of 2 V, the SiC/MOSFET cascode switch outputs 31 A and the all-SiC cascode switch outputs 19 A.

I. Introduction

Wide band gap semiconductors like Silicon Carbide (SiC) and the III-IV nitrides are currently being developed for high power/temperature applications. Silicon carbide (SiC) is ideally suited for power conditioning applications due to its high saturated drift velocity, its mechanical strength, its excellent thermal conductivity, and its high critical field strength. For power devices, the tenfold increase in critical field strength of SiC relative to Si allows high voltage blocking layers to be fabricated significantly thinner than those of comparable Si devices. This reduces device on-state resistance and the associated conduction and switching losses, while maintaining the same high voltage blocking capability. Fig. 1 shows the theoretical specific on-resistance of blocking regions designed for certain breakdown voltages in Si and 4H-SiC, under optimum punch-through conditions [1]. The specific on-resistance of 4H-SiC is approximately 400 times lower than that of Si at a given breakdown voltage. This allows for high current operation at relatively low forward voltage drop. In addition, the wide band gap of SiC allows operation at high temperatures where conventional Si devices fail. Forward voltage drop vs. current density of Northrop Grumman's all-SiC Vertical Junction Field Effect Transistor (VJFET) based cascode switch, and those of commercial Si MOSFET, Si IGBT, and Si CoolMOS switches are shown in Fig. 1.

Theoretical specific on-resistance of blocking regions designed for certain breakdown voltages in si and 4H-sic, under optimum punch-through conditions.