Comprehensive Study of Impact Ionization Coefficients of 4H-SiC
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J9.3.1
Comprehensive Study of Impact Ionization Coefficients of 4H-SiC T. Hatakeyama1, T. Watanabe1, K. Kojima2, N. Sano3, T. Shinohe1, and K. Arai2 1
Corporate Research & Development Center, Toshiba Corporation, 1, Komukai Toshiba-cho, Saiwai-ku, Kawasaki 212-8582, Japan 2
National Institute of Advanced Industrial Science and Technology, Power Electronics Research Center, Tsukuba Center 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan 3
Institute of Applied Physics, University of Tsukuba 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan ABSTRACT The electric field dependence and anisotropy of the impact ionization coefficients of 4H-SiC are investigated by means of the avalanche breakdown behavior of p+n diodes. The breakdown voltages as a function of doping density and the multiplication factors of a leakage current are obtained using p+n diode fabricated on (0001) and (11 2 0 ) 4H-SiC epitaxial wafers. The obtained impact ionization coefficients show large anisotropy; the breakdown voltage of a p+n diode on (11 2 0) wafer is 60% of that on (0001) wafer. We have shown that anisotropy of the impact ionization coefficients is attributable to the anisotropy of saturation velocity originated from the electronic structure of 4H-SiC. INTRODUCTION Among many wide band gap semiconductor materials, 4H silicon carbide (4H-SiC) has great potential for use as the material for power devices owing to its crystal maturity and superior electrical properties such as isotropic mobility and high breakdown electric field. Impact ionization coefficients are important material properties for power devices because the avalanche breakdown of a power device is caused by the impact ionization phenomena and the physical model of impact ionization coefficient is indispensable for the device simulation of power devices. However, the reports of measurements of the impact ionization coefficient of 4H-SiC are few, and they are not in agreement with one another [1, 2]. Recently, it was shown that a significant reduction of the breakdown field in 4H-SiC occurs when the electric field is applied perpendicular to the c-axis, but the impact ionization coefficients were not reported [3]. In order to predict the breakdown voltage of a real power device precisely, we have to consider the anisotropy of impact ionization coefficients because the direction of the electric field at the field crowding part is not necessarily parallel to c-axis when the reverse bias is applied, even if the device is fabricated on (0001) face. In this paper, we present the impact ionization coefficients of 4H-SiC for 0001 and 11 2 0 directions that reproduce avalanche breakdown behavior of p+n diodes on (0001) and (11 2 0 ) epitaxial 4H-SiC wafers. We also discuss the origin of anisotropy of impact ionization coefficient of 4H-SiC based on the microscopic description of the impact ionization and the transport physics under high electric field.
Ar laser (350nm) Polyimid Ni
np+
4142B
p+ sub. (0001) or (1120)
Reverse Leakage Current (A)
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