MOSFETs and IGBTs
The two workhorse switch families and why each was chosen for different eras of EV drives.
You'll learn
- Explain mosfets and igbts at engineering level
- Read the numbers that define the behaviour
- Apply the idea to a real drive system
Overview
Every switch in an inverter is either a MOSFET or an IGBT (or, increasingly, a wide-bandgap variant of a MOSFET). Understanding how each turns on and off explains most of what separates a good drive from a mediocre one.
In depth
Two different on-states
A MOSFET conducts through a resistive channel: its on-state voltage drop rises linearly with current, described by an on-resistance in milliohms. An IGBT conducts by injecting minority carriers into a drift region, giving it a roughly constant saturation voltage drop regardless of current — good at very high current, but with a hard floor on loss even at low current.
MOSFET vs. IGBT
| Silicon MOSFET | IGBT | |
|---|---|---|
| Conduction behaviour | Resistive (I²R) | Roughly constant voltage drop |
| Switching speed | Fast | Slower, tail current on turn-off |
| Best at low current | Yes | No, fixed drop dominates |
| Best at very high current | Silicon MOSFETs less common here | Yes, historically |
| Typical automotive role | DC/DC, OBC, SiC traction | Traction inverters (legacy and mainstream) |
Key takeaways
Remember this
- MOSFETs conduct resistively; IGBTs conduct with a near-constant voltage drop.
- IGBTs suffer turn-off tail current that MOSFETs avoid.
- The choice between them is a function of current level, switching frequency and cost.
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