A field-effect transistor (FET) is a type of semiconductor device used in electronic circuits to control the flow of current. It belongs to the family of transistors, along with bipolar junction transistors (BJTs). FETs are particularly important because they have certain advantages over BJTs, such as high input impedance, low power consumption, and suitability for high-frequency applications.
The basic principle behind the operation of an FET is the control of current flow through a semiconductor channel by an external electric field. There are three main types of FETs:
Metal-Oxide-Semiconductor FET (MOSFET):
MOSFET is the most commonly used type of FET in modern integrated circuits (ICs) and electronic devices.
It consists of a semiconductor channel between two heavily doped regions, called the source and drain. The channel is controlled by a voltage applied to the gate terminal, which is separated from the channel by an insulating layer (oxide).
MOSFETs can be further categorized into:
a. Enhancement-mode MOSFET (E-MOSFET): In its natural state, the channel is not conducting. When a voltage is applied to the gate, it creates an electric field that induces a conducting channel, allowing current to flow between the source and drain.
b. Depletion-mode MOSFET (D-MOSFET): In its natural state, the channel is conducting. Applying a voltage to the gate reduces or depletes the channel, reducing the current flow between the source and drain.
Junction Field-Effect Transistor (JFET):
JFETs have a channel formed by a single semiconductor material, with a PN junction separating the channel from the gate terminal.
The voltage applied to the gate controls the width of the conducting channel, thus influencing the flow of current between the source and drain.
JFETs are available in two types:
a. N-channel JFET: The channel is made of N-type semiconductor material.
b. P-channel JFET: The channel is made of P-type semiconductor material.
Insulated-Gate Bipolar Transistor (IGBT):
While not a pure FET, IGBT combines features of both MOSFET and BJT.
It consists of a MOSFET-like gate controlling the current flow through a BJT-like structure.
IGBTs are used in power electronics applications, such as motor drives and power inverters, due to their ability to handle high voltage and high current.
These FET types have different characteristics and applications, making them essential components in a wide range of electronic devices, from microprocessors and memory chips to power amplifiers and voltage regulators.