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The concept of quasi-Fermi level and expansion of faulted loops in SiC under minority carrier injection



Degradation of 4H-SiC PiN diodes – a rapid increase in the forward voltage drop with operation time – has been a focus of research since it was first reported over a decade ago. It was soon discovered that associated with this degradation is a rapid increase in the number and extent of intrinsic stacking faults (SFs) in SiC. Further research showed that the expansion of SFs is due to the injection of electron–hole pairs (ehps) in the active region of the device under forward biasing. This effect indicated that recombination-induced dislocation glide – a phenomenon that was known to occur in many other semiconductors and in SiC – may play an important role in enhancing the motion of the leading partial dislocations and thus in expanding SFs in PiN diodes operated under forward bias conditions. This was later supported by enhanced nucleation and expansion of SFs by bandgap optical excitation in virginal SiC crystals itself – whether the material was of the 4H or 6H polytype – rather than in forward-biased SiC diodes. At the same time, we used the concept of quasi-Fermi energy – as the transient value of the electron population during excited states of 4H- or 6H-SiC – to explain the driving force for expansion of SFs. Recently, Caldwell et al. have used this concept to develop various experimental observations that have been made on these diodes, including SF expansion under forward biasing, saturation of the maximum forward voltage drift, annealing-induced contraction of SFs so generated and consequent drift recovery of the forward voltage drop. In this paper, we expand the concept of quasi-Fermi level to get a better understanding of faulted loop expansion in 4H-SiC bipolar devices and in virgin 4H- and 6H-SiC crystals.