The basic circuit diagram for n-channel power MOSFET is shown in Fig(a), where voltages and currents are as indicated.

(a) Transfer characteristics: This characteristic shows the variation of drain current $I_D$ as a function of gate-source voltage V05. Fig (b) shows typical transfer characteristic for n-channel power MOSFET. It is seen that there is threshold voltage $V_{GST}$ below which the device is off. The magnitude of $V_{GST}$ is of the order of 2 to 3 V.

(b) Output characteristics: Power MOSFET output characteristics shown in Fig (c) indicate the variation of drain current $I_{D}$ as a function of drain-source voltage $V_{G S}$ as a parameter. For low values of $V_{D S}$ , the graph between $I_{D}-V_{D S}$ is almost linear; this indicates a constant value of on-resistance $R_{D S}=V_{D S} / I_{D} .$ For given $V_{G S},$ if $V_{D S}$ is increased, output characteristic is relatively flat indicating that drain current is nearly constant. A load line intersects the output characteristics at A and B. Here A indicates fully-on condition and B fully-off state. Power MOSFET operates as a switch either at A or at B just like a BJT.

(c) Switching characteristics: The switching characteristics of a power MOSFET are influenced to a large extent by the internal capacitance of the device and the internal impedance of the gate drive circuit. At turn-on, there is an initial delay $t_{d n}$ during which input capacitance charges to gate threshold voltage $V_{G S T}$ . Here $t_{d n}$ is called turn-on delay time.

There is further delay $t_{r}$ , called rise time, during which gate voltage rises to $V_{G S P}$ , a voltage sufficient to drive the MOSFET into on state. During $t_{r}$ , drain current rises from zero to full on current $I_{D}$ . Thus, the total turn-on time is $t_{o n}=t_{d n}+t_{r}$ . The turn-on time can be reduced by using low-impedance gate drive source.

As MOSFET is a majority carrier device, turn-off process is initiated soon after removal of gate voltage at time $t_{1}$ . The turn-off delay time, $t_{d f}$ is the time during which input capacitance discharges from overdrive gate voltage $V_{1}$ to $V_{G S P}$ . The fall time, $t_{f}$ is the time during which input capacitance discharges from $V_{G S P}$ to threshold voltage. During $t_{f}$ , drain current falls from $I_{D}$ to zero. So when $V_{G S} \leq V_{G S T},$ MOSFET turn-off is complete. Switching waveforms for a power MOSFET are shown in Fig (d)