4H-SIC Dmosfets for High Frequency Power Switching Applications
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4H-SIC DMOSFETS FOR HIGH FREQUENCY POWER SWITCHING APPLICATIONS Sei-Hyung Ryu, Anant K. Agarwal, James Richmond, and John W. Palmour Cree, Inc., 4600 Silicon Drive Durham, NC 27703 ABSTRACT Very high critical field, reasonable bulk electron mobility, and high thermal conductivity make 4H-Silicon carbide very attractive for high voltage power devices. These advantages make high performance unipolar switching devices with blocking voltages greater than 1 kV possible in 4H-SiC. Several exploratory devices, such as vertical MOSFETs and JFETs, have been reported in SiC. However, most of the previous works were focused on high voltage aspects of the devices, and the high speed switching aspects of the SiC unipolar devices were largely neglected. In this paper, we report on the static and dynamic characteristics of our 4H-SiC DMOSFETs. A simple model of the on-state characteristics of 4H-SiC DMOSFETs is also presented. INTRODUCTION 4H-silicon carbide (4H-SiC) is an ideal material for high performance, high voltage power devices because it has a very high critical field, a reasonable electron mobility, and a high thermal conductivity. These advantages make high voltage ( ≥ 600V ) switching devices very attractive in 4H-SiC because the drift layer resistance of a 4H-SiC high voltage device can be up to a factor of 400 lower than that of a silicon device with the same voltage rating and the same active area [1,2]. High voltage silicon switching devices reduce drift layer resistance by utilizing conductivity modulation in the drift layer to reduce the forward drop. This significantly reduces conduction losses, but results in reduced switching speeds and large switching losses, especially at elevated temperatures. A 4H-SiC unipolar device, when fully developed, is expected to have a low forward drop which matches that of a silicon bipolar device at the same on-current density, in addition to fast, temperature independent switching characteristics which result in very low switching losses. Exploratory devices, which includes several vertical MOSFETs structures [3,4], have been reported in SiC. However, in most of the previous works, the high speed switching aspects of the SiC devices did not receive much attention. In this paper, we report on the static and dynamic characteristics of 4H-SiC DMOSFETs. ANALYSIS OF DMOSFET ON STATE CHARACTERISTICS Figure 1 shows a simplified cross-sectional view of a DMOSFET cell. Electrons flow from the source contact into the n+ source regions, then flow laterally through the MOS channel regions. The electrons then flow vertically through JFET regions, which are formed by two adjacent p-well regions, through the lightly doped drift layer, and then finally, to the drain contact. The total on-resistance of a 4H-SiC DMOSFET includes contact resistance, source resistance, MOS channel resistance, accumulation resistance, JFET resistance, spreading resistance, drift resistance, and substrate resistance. Contact, source, accumulation, and substrate
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