11.6. Field Effect Transistors (FET)

A Field Effect Transistor (FET) is a three-terminal semiconductor device that uses an electric field to control the flow of current. Unlike BJTs, FETs are voltage-controlled devices. The most common type of FET is the Metal-Oxide-Semiconductor FET (MOSFET).

A Field Effect Transistor, or FET, is an electronic component with three terminals: the source, drain, and gate. FETs differ from Bipolar Junction Transistors (BJTs) because they are controlled by voltage rather than current. This means that applying a voltage to the gate terminal influences the flow of current between the source and drain terminals. The most prevalent type of FET in electronic circuits today is the Metal-Oxide-Semiconductor FET (MOSFET), which is often used in various applications due to its efficiency and effectiveness.

Think of a FET as a water faucet. The gate is like the faucet handle; when you turn it (apply voltage), you allow water (current) to flow from the source (water supply) to the drain (where the water goes out). If you keep the faucet closed, the water remains in the pipe, illustrating how the FET controls current with voltage.

The flow of current in an FET is controlled by the voltage applied to the gate, which creates an electric field that modulates the conductivity of the channel between the source and drain. FETs have high input impedance, making them ideal for high-precision applications.

In a FET, the gate voltage creates an electric field in a channel that connects the source to the drain. Depending on the gate voltage, this electric field can allow or restrict the flow of electrical current. If the voltage is increased above a certain threshold, it enhances conductivity, allowing more current to flow. This characteristic allows FETs to operate at very high input impedances, which is beneficial in sensitive electronics like sensors or amplifiers where precision is key.

Imagine a narrow path in a park where a gate is used to allow or restrict people from passing through. The gate is controlled by a lever (the gate voltage). When the lever is down (low voltage), fewer people can enter, making it less crowded. But when the lever is raised (high voltage), many more can pass through freely. This setup shows how the FET allows current to change based on the gate voltage.

FETs are widely used in amplifier circuits due to their high input impedance. FETs are used as switches in logic circuits and power supplies. FETs are used in analog signal processing, such as in audio and RF circuits.

FETs are versatile components utilized in numerous applications. Their high input impedance makes them particularly suitable for use in amplifier circuits, where they can amplify weak signals without significantly affecting the input signal. FETs also function as electronic switches in logic circuits, enabling functions within computers and various digital devices. Furthermore, they are essential in analog signal processing, helping to manage audio signals in sound equipment and radio frequency signals in communication devices.

You can think of FETs in amplifiers like a megaphone. When you speak into the megaphone, your voice is weak, but the megaphone amplifies it so that many people can hear you. Similarly, FETs boost weak electrical signals to usable levels. In the case of an FET acting as a switch, imagine it as a traffic light controlling the flow of cars; it either allows many cars to go through (turning on) or stops them (turning off), much like a FET turning current on and off.

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