I have a question that in motor driving application, by using power MOSFET, it must be in saturation region, is this right? And for IGBT, in which region it must be to act as a switching device?
by using power MOSFET, it must be in saturation region, is this right ?
No, when a MOSFET is in saturation it will determine (regulate) the amount of current that flows. Due to this regulation the MOSFET will get very hot. For driving a motor this is bad as you want the motor to determine the current by applying the full supply voltage across it. For that you need the MOSFET to be in linear mode (also called triode mode).
You want the MOSFET to act as a switch and then you must use it in linear mode.
Probably you're confused by Bipolar transistors because for bipolars the mode where the transistor acts as a switch is called saturation mode.
Since the main switching device in a IGBT is a Bipolar transistor, it will operate in saturation mode.