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SiC Schottky barrier diodes (SBDs) with low switching loss

An SBD is a unipolar device formed by the junction of a semiconductor with a metal. In principle, SBDs have no reverse recovery time that causes a problem with pn junction diodes. Therefore, SBDs help to considerably reduce the switching loss during turn-off.

A pn junction diode is bipolar as conduction is due to both holes and electrons. The pn junction diode has reverse recovery time since minority carriers (electrons in the case of the p layer and holes in the case of the n layer) remain at the junction during turn-off. In contrast, being unipolar, SBDs do not have reverse recovery time. However, since the depletion region at the junction of a metal and a semiconductor has capacitance, current due to its charging and discharging flows. Although this capacitance is a function of temperature and reverse bias voltage, it is hardly affected in the high-voltage region where typical SiC SBD applications operate.

In addition, this capacitance is a not function of current. Unlike the capacitance of pn junction diodes, it is hardly dependent on current at less than the rated DC current. However, SiC SBDs with an MPS structure might be affected by minority carriers at a current exceeding the rated DC value.

The capacitance of an SBD is calculated as follows:

C(VR) = S x [ ε x q x Nd/{2 x (Vbi + VR)}]^0.5
 

Vbi: Built-in voltage (= work function of a metal – work function of an n-type (p-type) semiconductor)

S: Junction area, ε: Dielectric constant, q: Unit charge, Nd: Dopant concentration

VR: Reverse bias voltage
 

Although Vbi is a function of temperature, it is negligible because Vbi << VR.

 

SiC vs Si  Turn-off waveform  (125℃)

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