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High withstand voltage (reverse voltage) characteristics of SiC SBDs

Silicon carbide (SiC) is a wide-bandgap semiconductor with a bandgap of 3.26 eV, much higher than that of silicon (Si) (=1.12 eV). SiC provides high electric breakdown field and high thermal conductivity because of high atomic bond due to a low lattice constant (i.e., a short atom-to-atom distance).

Comparisons of physical properties between Si and SiC

Characteristics Unit Si 4H-SiC
Bandgap eV 1.12 3.26
Electron mobility, μe cm2/Vs 1400 1000/1200
Hole mobility, μh 600 120
Electric breakdown field, Ec V/cm 3.0×105 2.8×106
Thermal conductivity, λ W/cmK 1.5 4.9
Saturation electron drift velocity, Vsat cm/s 1.0×107 2.2×107
Relative dielectric constant, ε   11.8 9.7/10.2

When an SBD with a conventional structure is reverse-biased, the depletion region extends into the semiconductor as shown below. The area of the triangle formed by the electric breakdown field and the depletion region width represents the withstand voltage of an SBD. The depletion region depth is inversely proportional to the dopant concentration. Increasing the dopant concentration helps reduce the resistance of silicon and therefore the forward voltage (VF) of the SBD, but at the expense of withstand voltage (i.e., triangle area). The electric breakdown field of SiC is nearly 10 times that of silicon. As shown below, it is therefore possible to increase the withstand voltage (i.e., triangle area) of a SiC SBD relative to a Si SBD, even if it is heavily doped.

In addition, since the depletion layer is less stretched by the higher concentration, the thickness of the chip can be thinner than  in the case of Si. The thickness of the semiconductor (Si or SiC) can be considered as series resistance in the forward direction, and so the forward voltage can be improved by reducing the thickness.

Si SBD Withstand voltage/Depletion Region

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