Depletion mode :
When the gate voltage is positive with respect to source, the depletion type MOSFET (metal oxide semiconductor field effect transistor) operates in depletion mode.
The construction of depletion type N channel MOSFET (metal oxide semiconductor field effect transistor) is shown in above figure. It consist the conducting bar of N type silicon material with an insulated gate on the left and P region on the right. An N channel is formed by distribution between the source and drain. The type of impurity for the channel is the same as for the source and drain. Here is only a single P region which is called substrate or body. It physically reduces the conducting path to a narrow channel. A thin layer of silicon dioxide is deposited on the left side of the N type channel. This layer insulates the gate from the channel. It has no PN junction.