1.8 kV, 10 mOhm-cm 2 4H-SiC JFETs
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0911-B12-02
1.8 kV, 10 mOhm-cm2 4H-SiC JFETs James D. Scofield1, Sei-Hyung Ryu2, Sumi Krishnaswami2, Husna Fatima2, and Anant Agarwal2 1 AFRL/PRPE, Air Force Research Laboratory, 1950 Fifth St, WPAFB, OH, 45433 2 Advanced Devices, CREE, Inc, 4600 Silicon Dr, Durham, NC, 27703 ABSTRACT Fabrication and characteristics of high voltage, normally-on JFETs in 4H-SiC are presented. The devices were built on 5x1015 cm-3 doped, 12 µm thick n-type epilayer grown on a n+ 4H-SiC substrate. A specific on-resistance of 10 mΩ-cm2 and a blocking voltage of 1.8 kV were measured. Device characteristics were measured for temperatures up to 300oC. An increase of specific on-resistance by a factor of 5 and a decrease in transconductance were observed at 300oC, when compared to the value at room temperature. This is due to a decrease in bulk electron mobility at elevated temperature. A slight negative shift in pinch-off voltage was also observed at 300oC. The devices demonstrated robust DC characteristics for temperatures up to 300oC, and stable high temperature inverter operation in a power DC-DC converter application, using these devices, is 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. In addition, 4H-SiC has negligible intrinsic carrier generation at temperatures up to 300oC, due to its’ large bandgap. This results in very small leakage currents in reverse biased junctions, and enables 4HSiC devices to operate at much higher temperatures compared to devices in conventional semiconductor materials. Several types of 4H-SiC power devices have been demonstrated [1-3] and characterized at elevated temperatures. Power MOSFETs in 4H-SiC are very attractive devices. However, the operating temperature of a 4H-SiC MOSFET is limited to 200oC due to poor reliability of the gate oxide at elevated temperatures. Bipolar Junction Transistors, BJTs, in 4H-SiC can operate at higher temperatures (300oC) because their operation does not depend on the gate dielectric, but requires a static base current, necessitating a more complex drive circuit. 4H-SiC Junction FETs, JFETs, are viable high temperature alternatives because they are voltage controlled devices without critical gate dielectric functionality. In this paper, we present our latest results in high voltage 4H-SiC JFET development.
DEVICE STRUCTURE AND FABRICATION Figure 1 shows a simplified cross-sectional view of a JFET cell. Electrons flow from the source contact into the n+ source regions, then flow laterally through the JFET channel. The electrons then flow through vertical 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 thickness of the lateral JFET channel is defined by the thickness of the n-epilayer and the widths of depletion regions formed by the n-channel and p-well (bottom), and the n-channel and top p+
gate (top). At t
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