Analysis of performance of SiC bipolar semiconductor devices for grid-level converters

Abstract

Recent commercialization of SiC bipolar devices, including SiC BJT, SiC MPS diode and SiC PiN diodes have enabled potential candidates to replace their SiC unipolar counterparts. However, the prospects of 4H-SiC power bipolar devices still need further investigation. This thesis compares the static and dynamic performance and reliability for the commercial SiC bipolar devices including SiC BJT, SiC MPS diode and SiC PiN diode and their similarly rated Silicon counterparts mainly by means of experimental measurements.
Through comprehensive double-pulse measurements, the turn-on and turn-off transition in Silicon BJT is seen to be much slower than that of the SiC BJT while the transient time will increase with temperature and decreases with collector currents. The common-emitter current gain (β) of SiC BJT is also found to be much higher than its Silicon counterpart. Significant turn-off delay is observed in single Si BJT which becomes worse when in parallel connection as it aggravates the current mismatch across the two devices, while this delay is almost non-existent in SiC devices. The current collapse seen in single SiC BJT is mitigated by parallel connection. These are dependant on temperature and base resistance, especially in the case of Silicon BJT.
The static performance of power Silicon and SiC BJT has also been evaluated. It has been found that the higher base-emitter junction voltage of SiC BJTs enables quasi-saturation mode of operation with low on-resistance, which is also the case for Silicon BJTs only at high base currents. In terms of DC gain measured under steady state operation, the observed negative temperature coefficient (NTC) of β in SiC BJTs and the positive coefficient (PTC) in Silicon BJTs can make the β of SiC BJT lower than that in Silicon at high temperatures. It has been found that parallel connection promotes both the on-state conductivity and current gain in Silicon BJTs and conductivity in SiC BJTs.
The characterization of power diodes reveals that the superior switching performance of the SiC MPS & JBS diode when compared with the Si PiN diode is due to the absence of the stored charge. This also leads to the larger on-state voltage in both SiC diodes and becomes worse at high currents under high temperatures. Through comprehensive Unclamped Inductive Switching (UIS) measurements, it is seen that the avalanche ruggedness of SiC MPS & JBS diodes outperform that of the closely rated Silicon PiN diode taking advantage of the wide-bandgap properties of SiC. Higher critical avalanche energy and thus better avalanche ruggedness can also be observed in SiC JBS diode compared with the SiC MPS diode. SiC MPS diodes can compete with Si PiN diodes in terms of the surge current limits, while the SiC JBS diode failed under a lower electrothermal stress. This is observed by the dramatic increase in its reverse leakage current at lower voltages.
The 15 kV SiC PiN diodes feature smaller device dimensions, less reverse recovery charge and less on-resistance when compared to the 15 kV Silicon PiN diodes. Nevertheless, when evaluating its long-term reliability by using the aggravated power cycling configuration, the high junction temperature together with the dislocation defects in the SiC PiN diode accelerate its degradation. Such degradations are not observed in Silicon PiN diodes for the same junction temperature and high-temperature stress periods.

Date of Award	5 Dec 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Saeed Jahdi (Supervisor) & Phil H Mellor (Supervisor)