2017.10.12 SiC Power Device
From this point forward, we compare SiC-MOSFETs with other individual power transistors.
This time, we describe their differences with Si-MOSFETs. Rather than just presenting detailed data, readers who have not yet used SiC-MOSFETs will probably have questions about how they differ from Si-MOSFETs with respect to driving methods and so on. Here we will explain two important points relating to driving when comparing SiC-MOSFETs with Si-MOSFETs.
Differences with Si-MOSFETs: Driving Voltage
Compared with their Si counterparts, SiC-MOSFETs have a lower drift layer resistance but a higher channel resistance, and so the higher the gate-source voltage Vgs, which is the driving voltage, the lower is the on-resistance. The following graph indicates the relation between on-resistance and Vgs for SiC-MOSFETs.
The on-resistance changes (declines) gradually from a Vgs of about 20 V, and approaches a minimum. The driving voltage Vgs for IGBTs and Si-MOSFETs in general is around 10 to 15 V, but in the case of SiC-MOSFETs, driving at Vgs = 18 V or so is recommended in order to obtain a fairly low on-resistance. In other words, one important difference with Si-MOSFETs is the need for a high driving voltage. When replacing a Si-MOSFET with a SiC-MOSFET, the gate driving circuit must also be studied.
Differences with Si-MOSFETs: Internal Gate Resistance
The internal gate resistance Rg of an SiC-MOSFET (chip) itself depends on the sheet resistance of the gate electrode material and the chip size. For a given design, the gate resistance value is inversely proportional to the chip size, so that the resistance is higher for smaller chips. Because, for devices with equivalent performance, a SiC-MOSFET chip is small compared with an Si device, the gate capacitance is small, but the internal gate resistance is higher. For example, a device rated at 1200 V and 80 mΩ (an S2301 bare-die product) has an internal gate resistance of about 6.3 Ω.
While not a statement that is limited to SiC-MOSFETs, the MOSFET switching time depends on the total gate resistance value, which is sum of the external gate resistance and the internal gate resistance discussed above. The internal gate resistance of a SiC-MOSFET is higher than that of a Si-MOSFET, and so in order to achieve fast switching, the external gate resistance must be kept as low as possible, on the order of a few Ohms or so.
However, the external gate resistance also serves as protection from surge voltages that may be applied to the gate, and so the resistance should be chosen with due attention paid to surge protection.
・In order to obtain a low on-resistance for a SiC-MOSFET, the Vgs must be set higher than that for a Si-MOSFET, to around 18 V or so.
・The internal gate resistance of a SiC-MOSFET is higher than that of a Si-MOSFET, and so the external resistance Rg is set low; but surge protection should also be considered.