Reliability of Commercially Available SiC Power MOSFETs

With the introduction of SiC power MOSFETs into the commercial market place, it is critically important to determine the reliability of these devices.  A number of potential issues need to be addressed, including the stability of the device threshold voltage, and the reliability of both the body diode and the gate oxide. 

An ideal switch has minimal leakage current in the OFF state and very low resistance in the ON state.  But an excess negative shift of the threshold voltage under high-temperature reverse-bias (HTRB) conditions can lead to a critical increase in OFF-state leakage current and potential device failure [1].  In a similar fashion, a large positive shift of the threshold voltage may occur under high-temperature gate-bias (HTGB) conditions, wherein a positive bias-temperature stress is applied.  This can lead to a significant increase in the ON-state resistance.

This work focuses on the stability of the device threshold voltage when subject to bias-temperature stressing, although the reliability of the body diode will also be discussed in the full paper.  The primary defects that contribute to shifts in the threshold voltage are near-interfacial oxide traps [2, 3].  A certain number of defect states become activated during device processing.  This can vary depending on the details of the processing steps, or may be a function of device design.  Typical instabilities of the threshold voltage at room temperature are about 0.25 V.  Much larger threshold-voltage instabilities are observed following bias-temperature gate-bias stressing.  This is very likely due to the activation of additional oxide traps, which can then participate in the oxide-trap charging process [3].  Since this activation is a function of time at temperature (and bias), more stressful processing conditions can quicken the onset of significant shifts in the threshold voltage during bias-temperature stressing.  (Similarly, stressing at higher temperatures also leads to an earlier onset of significant increases in threshold-voltage instability.)

As a result, we have observed a marked difference in the onset of significant increases in threshold-voltage instability between different commercial vendors.  This difference in bias-temperature stress time can vary in some cases by two orders of magnitude.  Detailed results will be provided in the full paper.

References

 

1.    Lelis, et al., Mater. Sci. Forum, vols. 679-680, p. 599 (2011).

2.    Lelis, et al., IEEE Trans. Elec. Dev., 55:8, 1835 (2008).

3.    Lelis, et al., ECS Transactions, Vols. 41(8), p. 203 (2011).