The switching characteristics of power MOSFET the delay time and rise time are responsible to remove the effect of internal capacitance of the device and internal impedance of the gate drive circuit. These two times give the turn on time of power MOSFET.
During the delay td input capacitance charges to gate threshold voltage Vgt and during rise time td to gate voltage rises to Vgp that will be the sufficient voltage to turn on the power MOSFET.
During the turn off delay time tod the gates voltage is removed hence input capacitance of the device discharges from Vp to Vgp where VP is the overdrive gate voltage. The time during which this capacitance discharges from Vgp to Vgt is known as fall time. In this time drain current falls from IP to zero. Power MOSFET is completely turned off if VGS < Vgt.