Diotec Semiconductor AG

Qrr and Qc Describe Different Physical Phenomena

For a conventional silicon PN diode, the reverse recovery charge (Qrr) represents the charge stored by minority carriers during forward conduction. Before the diode can block reverse voltage, these carriers must be removed, resulting in reverse recovery current and switching losses.

A SiC Schottky diode behaves fundamentally differently. Since conduction does not rely on minority carriers, there is essentially no reverse recovery charge. Instead, manufacturers specify the capacitive charge (Qc), which is associated with charging and discharging the device's junction capacitance during switching.

Although both parameters are expressed in coulombs, they do not describe the same physical process.

Why a Numerical Comparison Can Be Misleading

As an example, the SIT12C065, a 650 V, 12 A SiC Schottky diode, specifies a capacitive charge (Qc) of 28 nC at 400 V.

Placed next to the Qrr value of a silicon diode, this figure may appear directly comparable. However, interpreting the two values as equivalent can lead to incorrect conclusions about switching behaviour.

Unlike Qc, the Qrr of a silicon diode increases with forward current and junction temperature - the very conditions under which power converters typically operate. The absence of minority-carrier storage allows the SiC Schottky diode to avoid this mechanism entirely, resulting in consistently fast switching performance.

Evaluate Switching Behaviour, Not Just Datasheet Rows

The most meaningful comparison between silicon and SiC diodes is therefore not based on a single datasheet parameter, but on the behaviour of the device within the intended application.

Metrics such as switching energy, converter efficiency, electromagnetic interference (EMI) and thermal performance provide a much more accurate picture of the benefits offered by SiC Schottky technology than a direct comparison of Qrr and Qc values.

Looking Beyond the Datasheet

Datasheets accurately document measured device characteristics, but they do not indicate whether similarly named parameters represent the same underlying physical phenomenon.

When selecting a silicon replacement, engineers should therefore verify not only the numerical values in the specification table, but also what those values actually measure. Understanding this distinction helps prevent misleading comparisons and enables a more realistic assessment of switching performance at the system level.