Five Ways SiC MOSFET Technology Differs from IGBTs

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2. Manufacturing Process 

A downside of SiC is the wafer fabrication cost, said in 2021 to be 30-50x for the same size Si wafer used for IGBTs ^[2], but this can be expected to reduce over time. The effect is offset by the die size – for the same headline performance, a SiC MOSFET die could be around 5x less area than an IGBT. As a material, SiC is more expensive and slower to manufacture and with lower die yield than silicon. SiC wafers are now proven to be viable at 200mm, providing potential further economies of scale, but current production is at the smaller wafer sizes where defect rate is lower. Significantly, IGBT wafers have recently been announced at 300mm, providing further production cost savings for the old technology, offsetting the gains that SiC provides. Factors affecting the relative cost of SiC MOSFETs and IGBTs are very fluid, but it is likely that devices in the new technology like-for-like, will remain significantly higher than IGBTs. When system costs are considered, however, the story can change dramatically, for example, SiC switching at perhaps 10x the frequency of IGBTs can use associated magnetics that are far smaller, lighter and cheaper. 

3. Switching Speed 

Switching speed must be split into two considerations – edge rate and cycle frequency. For sure, both can be dramatically higher in SiC MOSFETs compared with IGBTs, but whether it gives an overall benefit depends on the application. For a given cycle frequency, faster edge rates with SiC reduce transient dissipation or ‘switching loss’. However, EMI is likely to be higher, voltage overshoots higher and layout far more critical. Spurious voltages are generated by high di/dt through trace and wiring inductances which can cause unreliability or even damage, especially if coupled into the gate circuit. High dV/dt can be equally problematic with coupling through device and stray capacitances, for example causing ‘phantom turn-on’ through device miller capacitance. Measures to mitigate all these effects such as adding snubbers and gate protection can add to the cost and size of the end product. Cycle or switching frequency can be higher with SiC than IGBTs, but dynamic losses increase proportionally to a level where there is no advantage over IGBTs. If the benefit of smaller magnetics is leveraged, there can be a large size and weight advantage to high-frequency switching but some applications do not see this – for example motor drives, where the magnetic element is the motor itself. In this application, high frequency and edge rate can be a disadvantage, causing leakage currents, breakdown and bearing wear. 

4. The Gate Driver 

The insulated gates of SiC MOSFETs and IGBTs are nominally similar but practical considerations mean the drive circuits are quite different. IGBTs are very robust, and the drive voltages are not critical – typically +12/15V is used for the on-state with full saturation of the associated bipolar transistor and with a breakdown rating of around +/-30V there is a good safety margin. Both SiC MOSFETs and IGBTs are off for 0V gate drive, but a small negative voltage is sometimes used to counter spurious turn-on from miller capacitance, and voltage transients from common inductance in the gate/emitter loop. An IGBT gate drive can be very simple, just a transformer secondary in some cases. A SiC MOSFET gate, however, needs around +17V for full saturation and this can be alarmingly close to the absolute maximum, which could be around 20/22V. This necessitates close control of the positive drive voltage to avoid damage. Again, a negative gate drive is sometimes actually necessary for the off state because of the high edge rates seen.  

Despite the potential simplicity of an IGBT gate drive, it can sometimes need to provide significant power, proportional to cycle frequency, gate charge and total voltage swing. This can be a watt or more for a large device with a high gate charge. SiC MOSFETS on the other hand have gate charges in the nanocoulomb range and require much less power though if operated at a very high frequency this increases.  

5. Threshold voltage and RDS(on) 

The gate threshold voltage for an IGBT is around 5V and the device can be driven to its full rated collector current with a voltage of around 10V. Higher gate voltages have little effect on collector saturation voltage. In contrast, a SiC MOSFET starts to conduct at perhaps 3V gate voltage but needs 17/18V for full enhancement of the channel. Anything less risks high dissipation and thermal runaway with the positive coefficient of the channel resistance.