What Is a MOSFET?

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A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a semiconductor device that is typically used as a voltage-controlled switch.

It comprises three terminals: the gate, the drain, and the source. The main purpose of the MOSFET is to efficiently deliver power to a load.

What is a MOSFET explanation with a Gate, Drain and Source

How do MOSFETs work?

A MOSFET is a voltage controlled device composed of a gate, a source and a drain. The gate voltage is typically provided by a microcontroller or a driver IC.

By applying the gate voltage to the gate terminal, the current will start to flow from the source to the drain terminals.

To understand the efficiency of the current flow, we can take a look at the R_DS(on) rating of the device. The R_DS(on) is the resistance from the source terminal to the drain terminal when the device is on. This rating is measured in ohms or milliohms.

RDS(on) (resistance) rating of a MOSFET device

There are two different MOSFET modes, the enhancement and the depletion. Both of these modes have two channel types, the N-channel, and the P-channel. The enhancement mode means that the device is always off, and needs a gate voltage to turn on.

On the other hand, the depletion mode means that the device is always on, and needs a gate voltage to turn off. Nowadays, the most commonly used MOSFETs are N-channel enhancement-type devices.

MOSFET operation modes: enhancement and depletion

MOSFETs structure

MOSFETs can also be lateral or vertical when it comes to their structure, which means that the conducting channel of the MOSFET is either lateral or vertical.

Lateral MOSFET structure

In the lateral structure, all three terminals are on top, on the same side of the silicon die.

It is very common for planar-type MOSFETs and it may also still be used nowadays for low-power MOSFETs, such as small signal types.

Vertical MOSFET structure

In vertical-type MOSFETs, the gate and source terminals are usually on top, on the same side of the die, while the drain terminal is on the vertically opposite side, as you can see here.

This type of structure is known as a trench MOSFET, and it is the most used design by MOSFET suppliers today. For example, CoolMOS™, OptiMOS™, and StrongIRFET™ are all vertical-type trench MOSFETs.

Let‘s use the N-type enhancement mode planar MOSFET to deep dive into the MOSFET structure. The most important element is the metal oxide semiconductor interface because it is responsible for its operation. Here, you can see the detailed structure of an N-type enhancement mode planar MOSFET.

The basis of the MOSFET construction is a silicon material that is lightly doped with Boron atoms to make it slightly P-type in concentration. This is called the p-substrate. To build the source and the drain regions, high-energy and high-concentration implants of N-type atoms are used.

The gate oxide, which is built with thin oxide, is on the silicon region between the source and the drain, partially overlapping these regions. When the gate voltage is applied, the negative atoms are attracted under the gate and the N-channel is formed. At this moment, the current will begin to flow.

Please note that other manufacturing steps may vary depending on the MOSFET type. Nevertheless, these are the basic operational building blocks of a MOSFET.

Operation modes of N-channel MOSFETs

MOSFETs operate in three different modes: Depletion, Accumulation and Inversion.

These modes refer to the status of the MOSFET channel and are described below:

Depletion mode

The depletion mode refers to the MOSFET in the off state. That is when the voltage of the gate is lower than the gate threshold voltage.

The gate threshold voltage is the minimum voltage required to form the channel between the drain and the source terminals.

So, when the gate voltage is lower than the gate threshold voltage, a depletion is formed between the drain and the source and the MOSFET is in the off state. This way, no current will flow.

Accumulation mode

The accumulation mode happens when the gate voltage increases above zero but is still lower than the threshold voltage. This occurs at the beginning of the channel formation due to the positive gate voltage.

Note that there may be a very small sub-threshold or leakage current occurring in the channel! Here, an inversion layer begins to form, where the polarity of the P-substrate under the gate starts changing.

Inversion mode

The inversion mode has two types of operation:

› Triode or linear mode

› Saturation mode

Let’s start by focusing on the first one. In this operation mode, the current increases in a linear way with the applied drain voltage.

In this case, the gate voltage is higher than the gate threshold voltage. The inversion layer increases and the N-channel is formed between the drain and the source terminals, which means that the current will flow. This current is proportional to the gate and drain voltage, which is higher than zero.

On the other hand, the saturation mode occurs when the current through the device starts to level off or flatten.

This happens because the current cannot increase in a linear way for a long period of time with the applied drain voltage.

When the drain voltage reaches a value equal to the difference between the gate voltage and the threshold voltage, the conducting channel becomes “pinched off”, and the current will remain constant.

This “pinch off” occurs due to the reverse biasing effect on the drain terminal caused by the increase in the drain voltage.

Example of current-voltage curves

On the right are typical datasheet curves related to the linear and saturation modes, where we can see the relationship between voltage and the output current.

Example of MOSFET current-voltage curves

By analyzing this graph, we can see the relation between the drain-source current and the drain-source voltage, applied for different values of the gate to source voltage.

Here, the dotted line indicates the edge of the saturation boundary, until the current increases linearly with the applied drain voltage. When this limit is exceeded, the drain current remains more or less constant with the increasing drain voltage.

Relation between the drain-source current and the drain-source voltage of MOSFETs

By analyzing this graph, we can see the relation between the drain-source current and the drain-source voltage, applied for different values of the gate to source voltage.

Additionally, this graph shows the relation between the drain current and the applied gate voltage for a fixed value of the drain voltage.

As you can see, the current quickly increases once the gate voltage is higher than the threshold voltage.

Please note that there is some amount of current flow before the threshold voltage is reached. This is called subthreshold current or leakage current.

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