Metal Oxide Semiconductor Field Effect Transistor (MOSFET).

Like the JFETS these devices have three terminals, Source, Gate and Drain . Also ID can be controlled with VGS in a similar way to JFETs. However MOSFETs have different construction and operating principles. Another important difference is that there are two types of MOSFETs, depletion and enhancement types. i.e. when VGS is applied, it depletes the conduction ability of one type, but enhances the conduction of the second type.

In both types of MOSFET there is a thin layer of oxide that insulates the gate from the conducting channel. For this reasons these devices are also referred to as Insulated Gate Field Effect Transistors or IGFETs.

Depletion type MOSFET.

Despite their name these types of FETs can act in depletion or enhancement mode, as will be explained below. Consider the N channel Depletion MOSFET illustrated below. The device consists of a block of lightly doped P type semiconductor. Within this there are two heavily doped N Type regions to which the drain and the source connections are made. These two regions are joined by a channel of N type material. The gate is connected to an aluminium layer that lies over the N channel, but there is a layer of metal oxide between them which insulates the gate from the conducting channel.

The layers of conductive gate, insulating oxide layer and conducting N channel form a capacitor (i.e. two conductors separated by an insulator).

When a voltage is applied between drain and source and VGS is 0, then the N channel enables a current ID to flow. However when VGS is applied, with a polarity that makes the gate negative with respect to the conducting channel, then electrons will be transferred to the gate (exactly as occurs on the negative plate when charging a capacitor). The electric field produced by these electrons, extends across the oxide layer and repels the electrons in the N channel, pushing them into the P substrate. This will reduce the number density of charge carriers in the N channel and so reduce ID. If the magnitude of VGS is increased, then eventually the N channel will be completely depleted of electrons and no current will flow.

Field Effect Transistor
Field Effect Transistor

Enhancement mode.

If we reverse the polarity of VGS, then the positive potential of the gate attracts more electrons into the conduction channel and enhances its conduction properties.

Field Effect Transistor

The diagrams below show the circuit symbol for an N channel and a P channel depletion MOSFET.

N Channel Depletion MOSFET
P Channel Depletion MOSFET

Enhancement type MOSFET.

In these devices there is no conducting channel between the source and drain. In an N channel enhancement MOSFET, when the gate is made positive it attracts electrons. When VGS exceeds a threshold value, the electrons form a conducting channel between the Drain and Source. We refer to this as an induced channel. As VGS is increased more electrons a drawn into the induced conduction channel and so ID increases. Below the threshold value there is no channel and so this device can only work in enhancement mode.

Field Effect Transistor
Field Effect Transistor

The diagrams below show the circuit symbol for an N channel and a P channel enhancement MOSFET.

N Channel Enhancement MOSFET
P Channel Enhancement MOSFET