MOSFET - Metal Oxide Semiconductor Field Effect Transistor

- an overview or tutorial giving the basic facts about the Metal Oxide Semiconductor Field Effect Transistor (FET) or MOSFET and the basics of its operation.

Field effect transistors, FETs are widely used in electronics circuits in today's equipment. They provide advantages over other types of transistors in many circuits and applications. As a result they are used in vast quantities, as well as being incorporated into integrated circuits.

There are a number of different types of field effect transistor. The junction field effect transistor or JFET was the first to be developed, but the other major type of FET that is in widespread use today is the MOSFET. The term MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor.

MOSFET development

The concept of the MOSFET has been known for many years but they only became important in mid to late 1960s. In fact when the first bipolar transistor was invented, the researchers had initially been looking at developing field effect devices. Unfortunately they did not work and they could not discover why - it was later discovered that the problem lay in the purity of the semiconductors. However once the bipolar transistor was discovered it took the centre stage in the development arena. Nevertheless developments in processing the raw semiconductor materials enabled the field effect transistor to be developed.

One of the major problems experienced with MOSFETs themselves was associated with the insulating oxide layers and their fabrication. These were only overcome as the semiconductor materials and processing technology improved and in the late 1960s MOSFET technology became more widespread.

Now MOSFET technology is one of the most widely used semiconductor techniques, having become one of the principle elements in integrated circuit technology. Their performance has enabled power consumptions in ICs to be reduced. This has reduced amount of heat being dissipated and enabled the large ICs we take for granted today to become a reality. As a result of this the MOSFET is the most widely used form of transistor in existence today.

MOSFET Basics

Within the MOSFET family of devices there are a number of categories into which the devices can be split. The first categories depends on the type of channel use:

1. N-channel



2. P-channel

In the case of the N-channel MOSFET, the major carrier type is electrons, but for the P-channel type the major carrier is holes. Of the two types of device, the N-channel is more commonly used on its own as a discrete device, although both types are widely used in integrated circuits.

There are also two further categories into which the MOSFET family of devices can be split:

1. Depletion mode MOSFET



2. Enhancement mode MOSFET

As the name suggests the depletion mode MOSFET acts by depleting or removing the current carriers from the channel, whereas the enhancement type increases the number of carriers according to the gate voltage.