Progress in SiC MOSFET Reliability

The low intrinsic carrier concentrations, high breakdown voltages, and high thermal conductivities of wide-bandgap semiconductors make them attractive alternatives to traditional Si-based power electronics, especially for high-temperature/power switching applications such as motor drives, hybrid electric vehicles, and photovoltaic inverters.  Silicon Carbide (SiC) is an especially strong candidate to replace Si devices due to widespread availability of SiC substrates, vertical device topologies, and the ability to thermally grow a silicon dioxide (SiO₂) layer on SiC to serve as a gate dielectric, similar to established Si technology.

SiC MOSFETs have been commercially available for over three years with blocking voltage of 1200V and excellent on-state resistances (~80 mΩ).  In addition, SiC MOSFETs may be able to operate at higher temperatures than competing Si IGBTs or MOSFETs. Currently the most significant hurdles to market penetration are reliability and cost.

SiC MOSFET reliability challenges stem mainly from poor SiC/SiO₂ interface quality. When operated under high gate field and temperatures, threshold voltage (V_T) instabilities are often observed both in SiC MOSFET [1] and MOS capacitor [2] structures, primarily due to the large number of electrically active SiO₂ bulk and SiC/SiO₂ interface states [3].

The application of a gate voltage (V_G) may result in electron (positive V_G) or hole (negative V_G) injection from the SiC into the oxide, populating traps at the SiO₂/SiC interface and/or within the SiO₂ bulk.  These populated traps alter the surface potential and shift the V_T necessary to switch the device.  Recent measurements on second-generation commercially available SiC MOSFETs indicate significant progress on this front compared to earlier devices, although appreciable V_Tshifts still exist for relatively moderate stresses (Fig. 1a).

We have characterized 1200 V SiC MOSFETs as well as Junction FETs (JFETs), which do not utilize a gate oxide, at high temperatures under both static and dynamic gate bias stress conditions.  SiC JFET devices demonstrate more stable V_T than SiC MOSFET devices for both types of gate bias stresses at high temperatures (Fig. 1b).  For static gate bias stresses, packaged JFET devices exhibited a negligible V_T shift  (ΔV_T < 2 mV) for temperatures up to 250^oC.  At higher temperatures, bare JFET die demonstrated ΔV_T < 10 mV up to 525^oC.  Further, in contrast to the SiC MOSFETs, the V_Tof the SiC JFETs was unaffected by dynamic gate bias stress over the test period, although the sub-threshold leakage current increased with time [4].

In addition to monitoring V_T shifts, we have calculated the change in density of SiC/SiO₂ interface traps (ΔD_IT) due to the application of gate stress to MOSFETs.  These profiles can be extracted from I-V curves for SiC MOSFETs based on the changes in sub-threshold slope. This technique can either be used to determine ΔD_IT (if body doping and gate capacitance are unknown) or absolute D_IT at specific energies within the bandgap (if doping and capacitance are known) [5].

This work was performed under the DOE Office of Electricity programs managed by Dr. Imre Gyuk. 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-AC04-94AL85000.

[1]  A. J. Lelis, et al., "Time Dependence of Bias-Stress-Induced SiC MOSFET Threshold-Voltage Instability Measurements," IEEE Trans. Electr. Dev., vol. 55 (8), pp. 1835-1840, 2008.

[2]  M. J. Marinella, et al., "Evidence of Negative Bias Temperature Instability in 4H-SiC Metal Oxide Semiconductor Capacitors," Appl. Phys. Lett., vol. 90 (25), pp. 253508-253508, 2007.

[3]  D. K. Schroder, "Progress In SiC Materials/Devices and Their Competition," Int. J. High Speed Electr. and Syst., vol. 21 (1), p. 1250009, Apr 2012.

[4]  J. Flicker, et al., "Performance and Reliability Characterization of 1200 V Silicon Carbide Power MOSFETs and JFETs at High Temperatures," High Temperature Electronics Conference (HiTEC), Albuquerque, NM, 2014.

[5]  D. R. Hughart, et al., "Sensitivity Analysis of a New Technique for Trapped Charge Extraction in SiC MOSFETs from Subthreshold Characteristics " International Reliability Physics Symposium (IRPS), Waikoloa, HI, 2014.