MOSFET switching characteristics

The turn ON and OFF times of MOSFET gets affected by its internal capacitance and the internal impedance of the gate drive circuit but it does not affect during steady-state operation. 

To understand switching characteristics of MOSFET we can take the simple equivalent circuit for an n-type MOSFET is given below :

Turn ON Process :

• The gate voltage is made positive to turn ON the MOSFET. When the gate voltage has applied the gate to source capacitance C_GS starts charging.
• When the voltage reached through C_GS certain voltage level is called the Threshold voltage ( V_GST ) at the same time drain current I_D begins to increase. 
• The time needed to charge C_GS to the threshold voltage level is known as a turn-on delay time.
• The C_GS charges from threshold to the full gate (V_GSP) voltage. The time required for this charging is known as rising time (t_r).
• During this period, the drain current increases to its full value of I_D because of that the MOSFET is fully turned ON.
• MOSFET turn-on time is given by, 

t_ON = t_[d(on)] + t_r

• The turn-on time can be reduced using a low-impedance gate drive source. 

Turn OFF Process:

• The MOSFET can be turned off with a negative or zero gate voltage. Due to this, the gate to source voltage decreases from V_I to V_GSP.
• C_GS discharges from V_I to V_GSP gate voltage. The time required for this discharge is called a turn-off delay time, during which the drain current also begins to decrease.
• The C_GS continues to discharging and its voltage equals a threshold voltage (V_GST).
• The time required to discharge C_GS from V_GSP to V_GST is called fall time (t_f). The drain current will be zero when the voltage V_GS < V_GST then it is said to have turned off. 
• MOSFET turn-off time is given by,  

t_OFF = t_[d(off)] + t_f

MOSFET switching waveform is shown in the figure below :