Q. Sketch the depletion MOSFET drain characteristics

The depletion-type MOSFET has a structure similar to that of the enhancement-type MOSFET with one important difference: The depletion MOSFET has a physically implanted channel. An n-channel depletion type MOSFET has an n-type silicon region connecting the n+ source and drain regions at the top of the p-type substrate. Therefore, if a voltage Vds is applied between drain and source, a current Id flows for Vgs=0. There is no need to induce a channel, unlike the case of the enhancement MOSFET.

The channel depth and its conductivity can be controlled by Vgs in exactly the same manner as in the enhancement-type device. Applying a positive Vgs enhances the channel by attracting more electrons into it, however, applying a negative Vgs, causes electrons to be repelled from the channel; and the channel becomes shallower and its conductivity decreases. The negative Vgs is said to deplete the channel of its charge carriers, and this mode of operation (negative Vgs) is called depletion mode. As the magnitude of Vgs is increased in the negative direction, a value is reached at which the channel is completely depleted of charge carriers and Id is reduced to zero even though Vds may be still applied. This negative value of Vgs is the threshold voltage of the depletion-type MOSFET.