DEPLETION MODE MOSFET:

In the depletion-mode type, charge carriers are present in the channel at "Zero" gate bias and the device is, therefore, conducting.  The application of a gate voltage, which depletes the number of charge carriers in the channel thus reduces the conductivity and is called reverse gate bias.  This type of operation is similar to that of the JUGFET previously discussed.  Because the gate is insulated from the channel it is also possible to apply the forward gate bias without drawing gate current.  This bias draws more charge carriers into the channel and results in an increase in conductivity.

Figure: shows an P-channel Depletion type MOSFET.