Operation of Depletion-Type MosFET
The application of a small positive voltage VDs, as shown in Figure 7-36(a), causes the flow of current ID through the channel from drain to source terminal (conventional flow). The resistance of the channel limits the amount of current ID. Thus, an increase in VDS causes a proportional increase in the drain current according to Ohm's law. However, this direct relationship between VDs and ID ends at the pinch-off voltage Vp, beyond which a further increase in VDs does not cause significant increase in ID. That is, the drain current starts leveling off and stays fairly constant at and beyond the pinch-off voltage. The drain current that flows with V VP (with VGs 0) is called the drain-to source saturation current IDss. Apparently, the operation of the depletion-type MOSFET is very similar to that of a JFET and has similar characteristics, with the only difference being that there is no reverse-biased p-n junction with depletion MOSFET. Hence, with VGs20, there is no formation of a depletion region and the channel width is controlled by the action of the electric field.