950V, 8.7mohm-cm2 High Speed 4H-SiC Power DMOSFETs
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0911-B13-04
950V, 8.7mohm-cm2 High Speed 4H-SiC Power DMOSFETs Sei-Hyung Ryu, Charlotte Jonas, Bradley Heath, James Richmond, Anant Agarwal, and John Palmour Cree, Inc, Durham, NC, 27703 ABSTRACT Fabrication and characteristics of high voltage, high speed DMOSFETs in 4H-SiC are presented. The devices were built on 1.2x1016 cm-3 doped, 6 µm thick n-type epilayer grown on a n+ 4H-SiC substrate. A specific on-resistance of 8.7 mΩ -cm2 and a blocking voltage of 950 V were measured. Device characteristics were measured for temperatures up to 300oC. An increase of specific on-resistance by 35% observed at 300oC, when compared to the value at room temperature. This is due to a negative shift in MOS threshold voltage, which decreases the MOS channel resistance at elevated temperatures. This effect cancels out the increase in drift layer resistance due to a decrease in bulk electron mobility at elevated temperature, resulting in a temperature stable on-resistance. The device operation at temperatures up to 300 oC and high speed switching results are also reported in this paper.
INTRODUCTION 4H-silicon carbide (4H-SiC) is a material of choice for high performance power switching devices because it offers a very high critical field and a high thermal conductivity. These advantages make high voltage switching devices very attractive in 4H-SiC because the drift layer of a 4H-SiC power switching device can be much thinner and have a much higher doping than that of a silicon device with comparable blocking capability, resulting in a significant reduction in specific on-resistance (up to a factor of 400 lower). This eliminates the need for the use of minority carrier injection, or conductivity modulation, in the drift layer, which is typically done in silicon devices. The conductivity modulation significantly reduces conduction losses, but results in reduced switching speeds and large switching losses, especially at elevated temperatures. As a result, 4H-SiC unipolar power device offers a very low conduction and switching losses. These performance advantages enable very high speed switching ( > 10 MHz), and eliminate the needs for snubber circuits, which can be bulky for high power systems. The other advantage of 4H-SiC is its wide bandgap. Due to the wide bandgap, carrier generation in 4H-SiC is negligible for temperatures ranging up to 300oC. This results in a very small leakage current in a reverse biased pn junction, and enables 4H-SiC devices to operate at much higher temperatures compared to devices fabricated in conventional semiconductor materials such as silicon and gallium arsenide. Power MOSFETs in 4H-SiC provide excellent switching performance and offer easy MOS gate control, and have attracted interests of many engineers in high power, high temperature applications. However, it was suggested that the operating temperature of a 4H-SiC MOSFET should be limited to 200oC due to poor long-term reliability of the gate oxide at elevated temperatures. Despite the limitations, it is important to demonstrate short-term 300oC o
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