4.4.1 Schottky Barrier Diodes

SiC Schottky barrier diodes (SBDs) or metal-semiconductor diodes [37,40] are attractive because they provide rectification without significant switching loss. Switching loss occurs when a diode switches from the conducting state to the blocking state. In p-i-n diodes [37,40] the on-state current is comprised of both holes and electrons injected into the lightly doped region. These carriers must be removed before the diode can turn off. This gives rise to a reverse recovery transient, where a large reverse current flows as carriers are extracted from the lightly doped region of the diode. Schottky diodes are metal/semiconductor junctions, and conduction current in these devices consists only of electrons injected from the n-type semiconductor into the metal. Since no holes are injected into the semiconductor, there is no stored charge and no reverse recovery transient, so the SBD turns off very rapidly. The reverse current transient of the diode causes power dissipation in other components of the switching system, typically a switching transistor, and this switching loss is attributed to the diode since the current transient is caused by the diode. Substitution of a p-i-n diode by a SBD effectively eliminates the switching loss.

Although Schottky diodes are desirable because of their low switching loss, it is not feasible to build high voltage SBDs in silicon. This is because of the relatively low barrier heights between common metals and silicon. The bandgap energy of silicon is 1.12 eV and the barrier heights of metal contacts to silicon are typically less than 0.5 eV. Under large reverse bias, the barrier is further reduced by Schottky barrier lowering. Since electron injection from the metal into the semiconductor increases exponentially as the barrier height is reduced, a very large reverse current is observed in silicon SBDs at modest reverse voltages. This is not the case with SiC because it is easy to fabricate SBDs with barrier heights as high as 1.5 eV (see Section 3.1.6.2). Since the reverse current depends exponentially on the barrier height, the reverse leakage in SiC SBDs is orders of magnitude lower than in silicon, even at high voltages.
T. Ayalew: SiC Semiconductor Devices Technology, Modeling, and Simulation