SiC Power Devices

The SiC power MOSFET adopts the planar DMOS base structure of its silicon counterpart (see the fundamentals chapter "The MOSFET"), but exploits the much higher breakdown field strength of SiC to make the drift region significantly thinner and more highly doped at the same blocking voltage. Concretely: while a 1200 V Si device needs a drift region several tens of micrometers thick with comparatively low doping, an equivalent SiC device gets by with a drift region only a few micrometers thick and considerably more highly doped.

This lowers the specific on-resistance R_on·A by more than an order of magnitude compared to an equivalent Si MOSFET – while simultaneously allowing a higher blocking voltage (600 V to over 3.3 kV) and a noticeably smaller chip area per ampere. In practice, however, the drift region alone does not limit the total resistance: channel resistance at the SiC/SiO2 interface (see the last paragraph of this chapter) and the JFET resistance between the p-wells contribute a substantial, sometimes dominant, share of the total resistance in modern planar SiC MOSFETs – a circumstance that has significantly driven the development of alternative cell concepts such as the trench MOSFET (see below). Commercial SiC MOSFETs today cover the range from 650 V to over 3.3 kV and have replaced the Si IGBT as the standard switching device in many applications.

A Si pn diode needs time when switching off to clear the minority charge stored during conduction – the so-called reverse recovery time, which causes switching losses that become more significant at higher frequencies. Physically, this is because a bipolar pn junction injects minority carriers into the adjacent region during conduction; when blocking, these must first be removed through recombination or sweep-out before the diode actually blocks – during which a non-negligible reverse current briefly flows.

A SiC Schottky diode, by contrast, is a pure majority-carrier device: there is virtually no stored charge and therefore almost no reverse recovery current, since the metal-semiconductor junction operates without minority carrier injection. This only becomes practical with SiC, because its high breakdown field strength allows Schottky contacts to sustain high blocking voltages with acceptable conduction losses – with silicon, Schottky diodes are only practical up to a few hundred volts, since the drift region would need to become so thick at higher voltages that the on-resistance would become unacceptably high. SiC Schottky diodes are now manufactured as standard in the 600 V to 1700 V range and are typically used as freewheeling diodes in combination with SiC or even Si MOSFETs, where their virtually loss-free switching behavior noticeably improves overall system efficiency.