A FET is a three-terminal device that uses an electric field to control the current flowing through the device, and it also has a high input impedance, which is useful in many circuits. Field effect transistors are key active devices in the electronics industry. The FETs used in many circuits are constructed from discrete components, from RF technology to power control and switching to general amplification.

However, the primary use of FETs is within integrated circuits. In this application, FET circuits can only consume very little power, which enables huge VLSIs to work fast. If bipolar technology were used, the power consumption would be orders of magnitude higher, and the resulting power would be too large to fit in a single integrated circuit.

FET History

The concept had been around for many years before batch FETs hit the market. But there are many difficulties in implementing the device and making it work.

In the 1940s, Bell Labs formed the Next Foundation, where a semiconductor research group was established. The group has studied a number of fields related to semiconductors and semiconductor technology, one of which is devices that modulate the current flowing in semiconductor channels.

In these early experiments, the researchers couldn't get the idea to work, turning their idea into another idea, eventually inventing another form of semiconductor electronics: the bipolar transistor.

After this, most of the semiconductor research was focused on improving bipolar transistors, and the idea of FETs was not fully studied for some time. Field effect transistors are now widely used, providing the main active components in many integrated circuits. Without them, electronic technology would be very different from what it is today.

FET Basics

FETs are based on the concept that charges on nearby objects can attract charges within a semiconductor channel. A field effect transistor consists of a semiconductor channel with electrodes at both ends called drain and source. A control electrode called the gate is placed very close to the channel so that its charge can affect the channel. The gate of the FET controls the flow of carriers (electrons or holes) from the source to the drain in this way. It does this by controlling the size and shape of the conductive channels.

The semiconductor channel in which the current occurs can be either P-type or N-type. This creates two types or classes of FETs, called P-channel and N-channel FETs.

Among other things, increasing the voltage on the gate can deplete or increase the number of charge carriers available in the channel.

Junction FET circuit symbol

Because only the electric field controls the current flowing in the channel, the device is said to be voltage-operated and it has a high input impedance, usually many megohms. This can be a distinct advantage over current-operated bipolar transistors and has a much lower input impedance.

FET circuit

Field effect transistors are widely used in all forms of circuits, from those used in circuits with discrete components to those used in integrated circuits.

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