So with any conventional transistor including the GaN HEMT, the input/gate capacitance must be charged in order for the transistor to turn on. The turn on and off (discharging the input and the output capacitance) is predictable, and is the job of a gate driver integrated circuit.

It appears to me that while the effective input capacitance is reduced with HZO, the gate driver would need to be special to deal with the changing capacitance.

The article implies that the Berkeley team has been researching the use of feroelectrics in Si devices for 20 years. Are there Si mosfets in production with this material on the gate?