5-6-4-1-2 Bipolar and Hybrid Power Rectifiers
For higher voltage applications, bipolar minority carrier charge injection (i.e., conductivity modulation) should enable SiC pn diodes to carry higher current densities than unipolar Schottky diodes whose drift regions conduct solely using dopant-atom majority carriers . Consistent with silicon rectifier experience, SiC pn junction generation-related reverse leakage is usually smaller than thermionicassisted Schottky diode reverse leakage. As with silicon bipolar devices, reproducible control of minority carrier lifetime will be essential in optimizing the switching-speed versus on-state current density performance trade-offs of SiC bipolar devices for specific applications. Carrier lifetime reduction via intentional impurity incorporation and introduction of radiation-induced defects appears feasible. However,
the ability to obtain consistently long minority carrier lifetimes (above a microsecond) has proven somewhat elusive as of this writing, indicating that further improvement to SiC material growth processes are needed to enable the full potential of bipolar power rectifiers to be realized .
As of this writing, SiC bipolar power rectifiers are not yet commercially available. Poor electrical reliability caused by electrically driven expansion of 4H-SiC epitaxial layer stacking faults initiated from basal plane dislocation defects (Table 5.2) effectively prevented concerted efforts for commercialization of 4H-SiC pn junction diodes in the late 1990s . In particular, bipolar electron–hole recombination that occurs in forward-biased pn junctions drove the enlargement of stacking disorder in the 4H-SiC blocking layer, forming an enlarging quantum well (based on narrower 3C-SiC bandgap) that effectively degrades transport (diffusion) of minority carriers across the lightly doped junction blocking layer. As a result, the forward voltages of 4H-SiC pn rectifiers required to maintain rated on-state current increase unpredictably and undesirably over time. As discussed in Section 5.4.5, research toward understanding and overcoming this material defect-induced problem has made important progress, so that hopefully SiC bipolar power devices might become commercialized within a few years .
A drawback of the wide bandgap of SiC is that it requires larger forward-bias voltages to reach the turn-on “knee” of a diode where significant on-state current begins flowing. In turn, the higher knee voltage can lead to an undesirable increase in on-state power dissipation. However, the benefits of 100× decreased drift region resistance and much faster dynamic switching should greatly overcome SiC onstate knee voltage disadvantages in most high-power applications. While the initial turn-on knee of SiC pn junctions is higher (around 3 V) than for SiC Schottky junctions (around 1 V), conductivity modulation enables SiC pn junctions to achieve lower forward voltage drop for higher blocking voltage applications .
Hybrid Schottky/pn rectifier structures first developed in silicon that combine pn junction reverse blocking with low Schottky forward turn-on should prove extremely useful in realizing applicationoptimized SiC rectifiers . Similarly, combinations of dual Schottky metal structures and trench pinch rectifier structures can also be used to optimize SiC rectifier forward turn-on and reverse leakage properties .