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Performance and Reliability Characterization of 1200 V Silicon Carbide Power JFETs at High Temperatures
Keywords: Silicon Carbide, JFET, Threshold Voltage Shift
The low intrinsic carrier concentration and high thermal conductivity of the wide-bandgap semiconductor Silicon Carbide (SiC) make it a strong candidate for high-temperature power switching applications. Much attention has been paid to the SiC power Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) due to the ability to thermally grow a Silicon Dioxide (SiO2) gate oxide on SiC and due to the positive device threshold voltage (Vt) of the device. However, the small band offset between SiC and SiO2, coupled with a high density of electrically active bulk and interface states, results in Vt instability and potentially unreliable device operation at high temperature. Conversely, SiC Junction FETs (JFETs) are constructed without a gate oxide layer and are therefore expected to demonstrate greater Vt stability than similarly-rated MOSFETs. However, JFETs are inherently normally-on devices with Vt < 0. In this work, we have characterized 1200 V SiC MOSFETs and JFETs at high temperatures. For the JFETs (obtained from a collaborating manufacturer), the threshold shift (ΔVt) was measured under both static and dynamic voltage stress to be less than 2mV (which is within the measurement margin of error) at temperatures up to 250oC for stress times as long as 200 hours. As a comparison, commercially available SiC MOSFETS demonstrated shifts of up to 300mV at 175oC after 30 minutes of static gate stress. Even small changes in device threshold leakage may be undesirable, depending on the application. For example, simulation program with integrated circuit emphasis (SPICE) models of a photovoltaic inverter show that a 300mV threshold shift of a SiC device in the H-bridge may result in upwards of 2% DC injection into the nominal AC signal. In addition to packaged parts, results from unpackaged JFET die at temperatures up to 600oC will be included in the final report. Although Vt remained unchanged for the duration of the test for both static and dynamic stress conditions, under dynamic stress conditions the JFET packaged parts demonstrated a linear increase in threshold leakage of around 0.2 nA per hour. Potential causes of this increase (including packaging) are being investigated. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC0494AL85000. This work was performed under funding from the DOE Energy Storage Program managed by Dr. Imre Gyuk of the DOE Office of Electricity Delivery and Energy Reliability. The authors thank John Hostetler, Chris Dries, and Anup Bhalla of United SiC for providing JFET samples and for useful discussions.
Jack Flicker, Postdoctoral Appointee
Sandia National Laboratories
Albquerque, NM
USA


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