A Field-Effect Transistor (FET) is a type of transistor that uses an electric field to control the flow of current between its source and drain terminals. FETs are a fundamental component of modern electronics and are used in a wide range of applications, including amplification, switching, signal modulation, and more.
The basic principle behind FET operation is the modulation of the conductive channel between the source and drain terminals by an external electric field. There are three main types of FETs:
MOSFET (Metal-Oxide-Semiconductor FET):
MOSFETs are the most commonly used type of FETs. They have a metal gate separated from the semiconductor channel by an insulating oxide layer, typically made of silicon dioxide. MOSFETs can be further divided into two subtypes:
n-channel MOSFET (nMOS): In an nMOS, the majority carriers (electrons) flow through an n-type semiconductor channel. A positive voltage applied to the gate terminal relative to the source terminal creates an electric field that attracts electrons, forming a conductive channel between the source and drain. This turns the transistor on, allowing current to flow.
p-channel MOSFET (pMOS): In a pMOS, the majority carriers (holes) flow through a p-type semiconductor channel. A negative voltage applied to the gate terminal relative to the source terminal creates an electric field that repels holes, thus forming a conductive channel between the source and drain. This turns the transistor on for pMOS.
JFET (Junction Field-Effect Transistor):
JFETs have a single semiconductor channel, either n-type or p-type, and they rely on the reverse-biased junction between the gate and channel regions to control the current flow. JFETs are categorized into two types:
n-channel JFET: In an n-channel JFET, a negative voltage applied to the gate terminal relative to the source terminal creates a depletion region, reducing the width of the conductive channel and limiting current flow between the source and drain.
p-channel JFET: In a p-channel JFET, a positive voltage applied to the gate terminal relative to the source terminal creates a depletion region, similarly controlling current flow.
IGFET (Insulated Gate Field-Effect Transistor):
IGFET is a more general term that encompasses both MOSFETs and JFETs. The term highlights the common feature of having an insulated gate to control the current flow through a semiconductor channel.
FETs offer advantages such as high input impedance, low power consumption, and fast switching speeds. They play a critical role in digital integrated circuits (ICs), analog signal processing, power amplification, and many other electronic applications. The choice between different types of FETs depends on the specific requirements of the circuit and the desired performance characteristics.