Definition of Depletion MOSFET and it's Working

Depletion MOSFET

Depletion MOSFET may be fabricated from the basic MOSFET structure. An n-type channel is obtained by diffusion between N⁺ type source and drain in an n-channel MOSFET. The below figure shows the structure of depletion MOSFET.

    n-channel Depletion MOSFET

Working of Depletion MOSFET

When the gate source voltage is zero considerable drain current flows. When the gate is applied with negative voltage. Positive charges are induced in the n-channel through the Si O₂ layer of the gate capacitor. The conduction in n-channel FET is due to electrons i.e. the majority carriers.
Therefore the induced positive charges make the n-channel less conductive. The drain current therefore gets reduced with increase in the gate bias voltage. The distribution of charges in the channel results in depletion of majority carriers. That is why this type of FET is called depletion MOSFET. The voltage drop due to the drain current causes the channel region nearer to the drain more depleted than the region due to the source. This is similar to the pinch off in JFET.

                                                          Drain charecteristics of an n-channel dual mode MOSFET

The depletion MOSFET can also be operated in enhancement mode simply by applying a positive voltage to the gate. Application of positive gate voltage results in induced negative channel in the n-type channel. Thus the conductivity of the channel gets increased. The n-channel depletion MOSFET can be used as in enhancement mode by changing the gate voltage polarity. When a MOSFET is operated this way, we call it as dual mode MOSFET. The above fig show the drain characteristics.