SiC Power Electronic Devices, MOSFETs and Rectifiers
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whole range of MOS devices and integrated circuits known in silicon [5]. These exciting
properties have given impetus to significant device development programs in Europe, Japan, and the United States. The current device development activities are taking place in an environment where many basic fabrication and material science issues are still unresolved. The situation is reminiscent of the early days of silicon technology, and indeed the SiC industry today is in many ways comparable to the silicon industry of the 1950's. The prospects are exciting, the problems are real, and the future is uncertain. In this review, we focus on one aspect of SiC device and technology development, unipolar (or majority carrier) power switching devices, specifically power MOSFETs and Schottky rectifiers. These devices are expected to be among the first SiC devices to enter commercial production early in the next decade. In the sections that follow, we will outline the basic device designs, review the present status of device development, indicate the relationship between material science issues and device performance, and identify the most critical performance and material science issues still to be resolved.
Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
2. MOTIVATION FOR POWER MOSFETs AND SCHOTI'KY RECTIFIERS Unipolar devices such as the power MOSFET and the Schottky rectifier are attractive for electronic power switching applications for several reasons. Unlike bipolar devices such as the insulated-gate bipolar transistor (IGBT) or the pin diode, the on-state current in unipolar devices does not flow through a forward-biased pn junction. The voltage drop across a forward-biased pn junction in 4H-SiC is about 2.8 V. Assuming a current density of 200 A/cm2 , SiC IGBTs and pin diodes will dissipate 560 W/cm2 just to establish current flow. This dissipation is absent in power MOSFETs and Schottky diodes. Second, since unipolar devices do not store minority carrier charge in the conducting state, they exhibit minimal reverse recovery transients during switching. In high-frequency switching applications, power dissipated during switching transients can be the dominant power loss in the system. 3. SCHOTTKY RECTIFIERS 3.1 BASIC DESIGN Figure 1 shows the cross section of a Schottky rectifier in SiC [6]. The starting wafer is 4H-SiC, cut approximately 80 off axis to enable step-controlled epitaxy [7], and polished on the (0001) silicon face. The n-type substrate is doped with nitrogen, with resistivity about 10-20 rrt-cm and thickness of 300 - 350 Rim. A lightly-doped n-type -pilayer is grown on the substrate with doping and thickness chosen to provide the Jesired blocking voltage while minimizing on-resistance. For diodes designed for 1500 V operation, the epilayer is about 10 pgm thick, doped 4 - 8x10"5 cm-3 . knplanted Edge
Aode
Termination
Schottky Barrier
N- Epilayer
N
M
I
iCathode
Figure 1. Cross section of a high-voltage SiC Schottky diode. 3.2 SELECTION OF SCHOTTKY METAL The Schottky metal is chos
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