Working of FET as a Switch

Working of FET as a Switch
Working of FET as a Switch

FET (Field Effect Transistor) transistors use electric fields instead of current. Its capacity to control current makes it a popular switch. FET switch structure, operation principles, advantages, limitations, and applications will be covered in this part.

FET overview

Three-terminal FETs have a gate, drain, and source. It modulates channel conductivity between the source and drain using the gate’s electric field. The FET type determines the channel type: n- or p-type. Understanding FET structure and operation is crucial to understanding its switch function.

FET switch purpose

FET switches control circuit current. The channel conductivity between the source and drain can be changed by applying voltage to the gate terminal. In “on” mode, the FET channel lets current to flow; in “off” mode, it blocks it. FETs are essential in electronic devices that need precise current flow control due to their ability to transition between states.Do you want to know how FETs switch? You’ve found it. As an electronics hobbyist, I’ve played with these essential parts for hours. From smartphones to computers to cars, FETs exist in many everyday gadgets. They simplify circuit current regulation. This article will explain FETs’ structure and how they work as on/off switches. You’ll study FET switching principles and why FETs are better than bipolar junction transistors in many situations. You’ll comprehend one of electronics’ most crucial and extensively used components at the end. So relax, take a coffee, and let’s begin!

Working of FET as a Switch
Working of FET as a Switch

FET structure and operation

To understand how a FET switches, you must first grasp its structure and operation.

FET structure

FETs have source, drain, and gate terminals. A semiconductor channel has a gate on top that controls conductivity and a source and drain at its ends. Voltage on the gate provides an electric field that depletes or strengthens the channel, allowing current to flow between source and drain.

Cut-off Region FET

Depleting the gate channel makes it an insulator without voltage. In the cut-off region, no current flows between source and drain. FET is “switched off” in this state.

In-Saturation FET

With enough gate voltage, the channel is strengthened and allows maximum current between source and drain. In saturation, the FET is totally “switched on”. Gate voltage and FET parameters determine current.Switching a FET on and off requires managing the gate voltage. FETs switch on when the gate voltage is high enough to allow current to pass through their channels. FETs turn off when the gate voltage is removed, stopping current. In power electronics and digital circuits, the FET may switch quickly by pulsing the gate voltage on and off.

FETs have advantages over bipolar junction transistors due to their working principles. FETs operate at high frequencies, have high input impedance, and no forward biasing current. However, FETs have larger on-resistances and overheat more easily. Learning how FETs switch helps improve them for diverse applications.

Regional FET Cut-Off and Saturation

FETs are electronic switches. Cut-off and saturation are its functioning zones. FETs conduct like closed switches in saturation and non-conducting like open switches in cut-off.

FET Cutoff Area

No channel connects source and drain in the cut-off region. Because the gate-source voltage (VGS) is smaller than the FET threshold voltage (VTH). Zero drain current (ID) exists since there is no channel. As an open switch, the FET inhibits current passage between source and drain.

FET Saturation Zone

Gate-source voltage (VGS) exceeds FET threshold voltage (VTH) in saturation. An inversion layer or channel between the source and drain allows current to flow. FET becomes closed switch. The gate-source voltage sets the maximum drain current (ID).

Due to its two functioning areas, the FET is an efficient electronic switch. Increase or decrease the gate-source voltage to quickly turn the FET on and off. FETs are utilized in power electronics, digital circuits, and other applications that need quick and efficient switching due to their high switching speed and high input impedance.

To use a FET as an electrical switch, push the gate-source voltage over and below its threshold voltage. This switches it between the cut-off region, where no current flows, and the saturation region, where maximum current flows. Learn to move between these two operating areas, and you’ll be using FETs in no time!

Switching with FETs: Pros and Cons

There are positives and cons to using FETs as circuit switches. FETs are ideal for high-frequency applications like RF circuits because they switch at nanosecond speeds. Due to their high input impedance, they don’t load the input signal. FETs have drawbacks as switches.


Due to their small size and absence of minority charge carriers, FETs flip quickly. This lets them function at gigahertz frequencies. Given its high input impedance, they require little input current to operate. This helps switch small signals without distortion. Power electronics switches using FETs can withstand high power levels.


As switches, FETs need accurate biasing, which is difficult. Their efficiency is lower than bipolar junction transistors due to increased conduction losses. Parasite capacitances can impair FET high-frequency performance, especially at high temperatures. FETs cost more than bipolar devices.

Based on your application, examine these pros and downsides before choosing a FET for switching. Speed and input characteristics make FETs ideal for high-frequency and high-impedance applications. For lower-frequency power switching, bipolar junction transistors may be more efficient and cost-effective. Understanding how FETs switch lets you choose the ideal one for your next circuit.

Applications of FET Switching

Power electronics and digital circuits use FET switches. The FET switches between cut-off and saturation regions. Like an open switch, the FET prevents current in the cut-off zone. In saturation, the FET enables maximal current flow, behaving as a closed switch.

Cutoff Region

When VGS is below Vth, the FET operates in the cut-off zone. Here, the channel is drained of charge carriers, thus no current flows between drain and source. The FET blocks current like an open switch.

Saturation Zone

A conductive channel forms when VGS exceeds Vth, allowing current to flow between drain and source. FET functions as a closed switch. The drain-source current (IDS) is highest for a given VGS in saturation. Increasing VGS further does not change IDS. The FET is at full current.

Advantages of FET Switching

Switching applications benefit from FETs:

High input impedance allows FETs to switch on and off quickly. High-frequency operation is useful for power electronics and digital circuitry. Due to their high input impedance, FETs switch on with little power. This boosts efficiency.FETs are compact, integrated devices that take up little space on circuit boards.

Solid-state FETs have no moving parts, making them reliable. Long operating lives are possible.Fast switching and high input impedance reduce electromagnetic interference and noise in FETs. It produces a “clean” switching signal.Many electronic devices and systems use FETs. Power and digital applications that demand high frequency operation benefit from their fast, efficient switching. Understanding FETs as switches can illuminate many modern technology.

Power Electronics and Digital Circuits with FETs

FETs manage current in power electronics and digital circuits as switches. The FET switches between cut-off and saturation regions.In the cut-off zone, the FET conducts no current. Open switch-like. Gate-source voltage (Vgs) is below FET threshold voltage (Vth). Despite applying voltage between the drain and source terminals, no current flows across the channel.

At saturation, the FET conducts current entirely. Like a switch closed. Vgs surpasses Vth, generating a channel inversion layer that allows drain-to-source current. Max drain current (Id) is controlled by drain-source voltage (Vds). FET channels short circuit.Power electronics uses FETs to move between cut-off and saturation areas quickly. This helps power converters generate high-frequency regulated waveforms effectively. Digital circuits use FETs as solid-state on/off switches for logic gates and memory.

Fast switching, high input impedance, and low power consumption are FET switching’s key benefits. FETs are more expensive than BJTs, have limited power handling capacity, and are susceptible to overvoltage transients that can destroy the thin oxide layer.FETs transformed power and digital electronics. Their low power loss and high frequency operation enable smaller, lighter, and more efficient devices. FETs will boost power density and enable electric vehicles, renewable energy, and 5G networks.


That concludes a brief explanation of FET switches. You now understand how FETs serve as solid-state switches in power electronics and digital logic circuits by knowing their structure and operation. A tiny control voltage can turn a FET on and off, allowing or blocking current. FET switches are more complicated than mechanical switches, but their fast speed, low loss, and compact size have permitted tremendous technological improvements that affect our daily lives. Next time you use an electrical device, consider how FETs make it possible.

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