[NiHaoMike]

20V gate IGBTs are pretty unusual. Most common are 10V to 15V on and 0 to -10V off, the latter being especially variable depending on application - generally high speed switching of more than a few tens of kHz is where negative gate drive makes sense.

It's also worth noting that a lot of IGBTs indeed do not like operating in a "half on" state for much more than the few microseconds it takes for them to switch off. One of my friends who does power electronics says that "a half on IGBT makes a poor imitation of Tiffany Yep" (a "hot" joke) but that's only if the IGBT doesn't just self destruct.