9.2.2.2. Field-Effect Transistors

Table of Contents

Basic idea

A field effect transistor, or FET, is a transistor in which an electric field controls the flow of charge through a material channel. Unlike a bipolar junction transistor, which uses both electrons and holes in its operation, a FET is usually described as a voltage controlled device. A voltage applied at one terminal changes how easily current can pass between two other terminals.

The three main terminals of a FET are called the source, the drain, and the gate. Charge carriers enter the channel from the source and leave through the drain. The gate controls the channel conductivity. In many cases, the gate draws very little current, which is one reason FETs are so useful in electronic circuits.

Structure and terminals

The channel is a narrow region of semiconductor connecting source and drain. The gate is placed so that its electric field can change the number of charge carriers in this channel. If the channel becomes more conductive, the drain current increases. If the channel becomes less conductive, the drain current decreases.

In circuit symbols and descriptions, two voltages are especially important. The gate to source voltage is written as $V_{GS}$, and the drain to source voltage is written as $V_{DS}$. The current flowing from drain to source is written as $I_D$.

The essential control idea of a FET is:
A change in $V_{GS}$ changes the channel conductivity, which changes the drain current $I_D$.

Rendering scientific sketch…

Basic FET concept

Main types of field effect transistors

There are two very important families of FETs, the junction FET, or JFET, and the metal oxide semiconductor FET, or MOSFET. Both use an electric field to control current, but they do so in different ways.

JFET

In a JFET, the gate forms a reverse biased junction with the channel. This reverse bias changes the width of a depletion region inside the semiconductor. As the depletion region grows, the channel becomes narrower, so current decreases.

A JFET can be thought of as having a channel that already exists when no gate voltage is applied. The gate voltage then squeezes the channel and reduces current.

MOSFET

In a MOSFET, the gate is insulated from the semiconductor by a very thin oxide layer. Because of this insulation, gate current is extremely small. The electric field from the gate can create or modify a conducting channel between source and drain.

MOSFETs are the most common transistors in modern electronics. They are used in digital logic, amplifiers, switching circuits, and integrated circuits.

n-channel and p-channel devices

FETs can be made as n-channel or p-channel devices. The names tell us which type of charge carrier mainly moves through the channel.

For an n-channel FET, the current is mainly carried by electrons. For a p-channel FET, the current is mainly carried by holes. In practice, n-channel devices are often preferred because electrons usually move more easily through the semiconductor than holes, giving higher performance.

Type	Main carriers	Typical control tendency
n-channel	Electrons	Positive $V_{GS}$ usually increases conduction in enhancement MOSFETs
p-channel	Holes	Negative $V_{GS}$ usually increases conduction in enhancement MOSFETs

Enhancement and depletion behavior

A useful way to classify FETs, especially MOSFETs, is by asking what happens when the gate voltage is zero.

In an enhancement mode MOSFET, there is no strong conducting channel at $V_{GS}=0$. The gate voltage must first create the channel. For an n-channel enhancement MOSFET, making the gate positive relative to the source attracts electrons and forms a conductive path.

In a depletion mode device, a channel already exists at zero gate voltage. Applying the proper gate voltage reduces the channel conductivity and may eventually turn the device off.

Enhancement mode means the gate voltage creates or strengthens the channel.
Depletion mode means the channel already exists and the gate voltage can weaken it.

MOSFET operation

Because MOSFETs are so important, it is helpful to understand their operation at a basic level. Consider an n-channel enhancement MOSFET. When $V_{GS}$ is small, the device is off and almost no drain current flows. As $V_{GS}$ increases, electrons gather near the semiconductor surface under the gate. Once a certain gate voltage is reached, a conducting channel forms. This special voltage is called the threshold voltage, written $V_T$.

If $V_{GS} > V_T$, the device can conduct. Then the amount of current also depends on $V_{DS}$.

Rendering scientific sketch…

n-channel enhancement MOSFET idea

Operating regions

For a MOSFET, the drain current changes differently in different operating regions. At a beginner level, the most important regions are cutoff, linear region, and saturation region.

In cutoff, the gate voltage is too small to form a channel, so the transistor is effectively off.

In the linear region, also called the ohmic region, the channel exists and the transistor behaves somewhat like a controlled resistor.

In saturation, the current becomes much less dependent on $V_{DS}$ and is controlled mainly by $V_{GS}$. This region is very important for amplification.

For an ideal n-channel enhancement MOSFET, the regions are often summarized as follows:

Region	Condition	Behavior
Cutoff	$V_{GS} < V_T$	Very small current
Linear	$V_{GS} > V_T$ and $V_{DS} < V_{GS} - V_T$	Current rises with $V_{DS}$
Saturation	$V_{GS} > V_T$ and $V_{DS} \ge V_{GS} - V_T$	Current mainly set by $V_{GS}$

For an ideal n-channel enhancement MOSFET:
Cutoff: $V_{GS} < V_T$
Saturation begins when: $V_{DS} = V_{GS} - V_T$

Current equations

A full treatment belongs in more advanced electronics, but a simple model is very useful. In saturation, the drain current of an ideal MOSFET is often written as

$$
I_D = k(V_{GS} - V_T)^2
$$

where $k$ is a constant that depends on the device.

This equation shows an important point. The current depends strongly on how far the gate voltage is above threshold. A small change in gate voltage can cause a large change in drain current.

In the linear region, a simple model is

$$
I_D = k\left[2(V_{GS} - V_T)V_{DS} - V_{DS}^2\right]
$$

These formulas are idealized, but they capture the basic behavior.

Why FETs are useful

FETs are popular because they can be controlled by voltage and usually require very little input current. This gives them high input resistance, especially MOSFETs. Because the gate current is tiny, the control circuit does not need to supply much power.

This makes FETs excellent for integrated circuits, where millions or billions of transistors may be placed on one chip. They are also useful as electronic switches. A MOSFET can be driven into cutoff for an off state or into strong conduction for an on state.

FETs as switches and amplifiers

A FET can work in two very common ways. In switching applications, the transistor is driven either off or on. This is the basis of digital electronics. In amplifier applications, the transistor is biased in a region where small changes in gate voltage cause changes in drain current. That changing current can then produce a larger voltage change elsewhere in the circuit.

The details of amplifier circuits belong to other chapters, but the key transistor idea is simple. The FET allows a small controlling voltage at the gate to regulate a larger current through the drain source path.

Comparison with bipolar junction transistors

A short comparison helps place FETs in context.

Feature	FET	BJT
Control type	Voltage controlled	Current controlled
Input current	Very small	Larger than FET
Main carriers	Usually one type	Both electrons and holes
Input resistance	High	Lower than FET

This is why MOSFETs dominate digital electronics, while BJTs are still important in many analog and special purpose circuits.

Practical ideas

Real FETs are not perfectly ideal. Threshold voltage varies from one device to another. The current in saturation is not completely constant with changing $V_{DS}$. Also, MOSFET gates can be damaged by static electric charge because the oxide layer is very thin. So in practice, careful handling is needed.

Another practical point is that the names source and drain are defined by how the device is used in a circuit, but in many MOSFET structures they are physically similar. The applied voltages determine the actual current direction.

Important practical facts:
MOSFET gates draw very little current.
MOSFET gates can be sensitive to electrostatic discharge.
Threshold voltage $V_T$ is a key parameter that determines when conduction begins.

Summary

A field effect transistor controls current by using an electric field to change the conductivity of a semiconductor channel. Its main terminals are source, drain, and gate. JFETs control a pre existing channel by reverse biasing a junction, while MOSFETs use an insulated gate to create or control a channel. FETs may be n-channel or p-channel, and MOSFETs may be enhancement mode or depletion mode. Their high input resistance and easy voltage control make them fundamental building blocks of modern electronics.

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