What is a Depletion-Type MOSFET?

There are two types of MOSFETS: depletion-type MOSFETs and enhancement-type MOSFETs.

Depletion-type MOSFETS are MOSFETs that are normally on. When you connect a depletion-type MOSFET, current flows from drain to source without any gate voltage applied. This is why it is called a normally on device. There is current flow even without a gate voltage. With a depletion-type MOSFET, maximum current flows from drain to source when no difference in voltage exists between the gate and source terminals (VGS=0).

However, if a voltage is applied to the gate lead of the MOSFET, the drain-source channel becomes more resistive. As the gate-source voltage increases more and more, the current flowing from drain to source decreases more and more, until all current flow from drain to source ceases.

A depletion-type MOSFET is so named a depletion device, because as the voltage to the gate increases, the current depletes more and more, until it ceases to flow at all.

A depletion-type MOSFET behaves very similar in action to a JFET.

The other type of MOSFET, an enhancement-type MOSFET, has the complete opposite behavior. Enhancement MOSFETs are normally off devices. They will not conduct current if there is no voltage applied to its gate lead. with an enhancement-type MOSFET, as you increase the gate-source voltage, the drain-source channel of the transistor becomes less resistive and current from drain to source increases.