(Invited) Updated Key Issues for SiC MOSFET Reliability
Authors: Lelis Aivars , Habersat Daniel , Urciuoli Damian , Schroen Erik , Nouketcha Franklin , Green Ron 
Published: December 2020

This work will provide an updated overview of our understanding of several key reliability issues for SiC power MOSFETs, including threshold-voltage stability, body-diode robustness, and short-circuit current robustness. We will also discuss recent results regarding the modeling of high-field breakdown in SiC and other power semiconductor devices.

Threshold-voltage instability likely occurs due to the charging and discharging of near-interfacial oxide traps in response to a change in gate bias. If enough traps change charge state, it can lead to significant shifts in the threshold voltage, which may in turn lead to increases in leakage current in the off state, or increases in on-state resistance in the on state. An important question is, what is the best—or most appropriate—test method to use to determine the reliability of these devices. A key factor is the measurement speed used to determine the magnitude of the threshold-voltage instability. It has recently been observed that one commercial device manufacturer has large instabilities, even in previously unstressed devices, and that such large instabilities can result in significant increases in on-resistance during normal switching conditions, thereby degrading circuit performance.

Some suppliers of SiC power MOSFETs are producing devices with low basal plane dislocation (BPD) density, resulting in decreased bipolar degradation during normal operation of its integral body-diode. However, not all suppliers manage their BPD densities to the same degree, resulting in a variation in responses observed across suppliers. Even devices from a single supplier with the same date code exhibited varying levels of degradation for identical body-diode stress and measurement conditions. Unlike other extended defects, BPDs when faulted, leads to degradation in both the body-diode and MOSFET characteristics. These faulted BPDs induce stacking faults that expand over time and limits the reliability of some commercially available SiC MOSFETs. This degradation phenomenon necessitates the development of an appropriate reliability test standard for SiC to separate good devices from bad ones. Three of the five major suppliers of 1200-V SiC MOSFETs showed varying levels of bipolar degradation in both their body-diode forward voltages and their MOSFET on-resistances. Two out of three suppliers of 1700-V SiC MOSFETs showed significant bipolar degradation issues with one particular supplier’s devices showing forward voltage changes of 1.3 V to 2.5 V from their pre-stress values. Their on-state resistance values were also significantly degraded.

Another important aspect of SiC MOSFET reliability is its robustness to short-circuit current conditions that can occur in many power electronics applications utilizing a half-bridge topology. Since both planar and trench device architectures are commercially available, electrical results have been generated to highlight their short-circuit capabilities under similar circuit conditions. Our electrical results show that trench devices have lower robustness in comparison to planar devices of similar ratings. These differences in short-circuit withstand times can be attributed to the smaller heat conduction volume of the trench design coupled with its higher saturation drain current for the same gate-drive voltage. The existing test method for short-circuit withstand time in the MIL-STD may overestimate the actual withstand time because it relies on a single gate-pulse method. Electrical results indicate that an incremental gate-pulse method may be more accurate in determining the actual withstand time, and it has proven to be more effective in determining latent failures missed by the standard method.

Another key issue for power devices in general is the trade-off between on-state resistance and blocking voltage. One of the key advantages of wide bandgap (WBG) materials such as SiC is that it can block the same voltage as Si using a much thinner, and more highly-doped, blocking layer. This results in a much lower on-state resistance at the same rated voltage. The superior high-field properties of WBG (and ultra WBG) power semiconductor materials also enable them to block much higher voltages than Si is able—as long as the background doping is low enough. We will present recent work, using impact ionization coefficients from the literature, to calculate avalanche breakdown as a function of epi-layer thickness and doping, thereby determining on-state resistance versus breakdown voltage for SiC and GaN. We will compare these results to Si, as well as to UWBG materials such as diamond, AlGaN, and Ga2O3. It is observed that SiC and GaN have very similar characteristics in terms of critical field, and in their relationships between on-state resistance and blocking voltage.