Figure 9 shows an n-channel power MOSFET circuit where the input signal VGS is applied across gate to source and the output signal VDS is obtained from drain. The current flow from drain to source ID is controlled by gate signal. Usually the source terminal is common between the input and output of a power MOSFET.
The transfer characteristic of an n-channel power MOSFET is depicted in Figure 10. The drain current ID is a function of gate-to-source voltage VGS. When the gate-to-source voltage is less than threshold voltage VGS(th), the current flow from drain to source is zero. The value of VGS(th) for power MOSFET is about few volts (2 to 3 V). Since ID is equal to zero, the drain to source is open circuit and the device should able to hold the supply voltage VDD.
It is clear from Figure 11 that the drain-to-source breakdown voltage VDSS must be greater than
VDD to keep away from the device breakdown. If the drain to source voltage is larger than VDSS, the power MOSFET will be breakdown due to avalanche breakdown of the drain-to-body (n–p) junction.
The output characteristics of a power MOSFET is shown in Fig. 3.23. The drain current ID is a function of drain-to-source voltage VDS when gate-to-source voltage VGS is constant. It is clear form Figure 11 that the output characteristics consist of three regions such as cut-off, active and ohmic regions.
When the drain-to-source voltage is small, the relation between ID and VDS is linear and power MOSFET operates in the ohmic region. When the power MOSFET is used as a switch to control the flow of power to the load, the ID – VDS characteristics of the device must be traversed from the cut-off region to the ohmic region through the active region. The device turns OFF in the cut-off region and turns on in the ohmic region. In the active region the drain current ID does not depend on the drain-source voltage VDS but it varies with the gate to source voltage VGS. Since the current ID is known as saturated current, this region is also called the saturation region.