Operating principles of FETs and MOSFETs (prevention of switch burnout)

Recently, many hobby products (such as electric airsoft guns) are coming equipped with FETs and MOSFETs as standard. Furthermore, aftermarket FETs and MOSFETs have become quite inexpensive, and I've been installing them more often myself.

Because I'm the type of person who likes to understand how things work, I did a little research, so please feel free to use this as a reference if you're interested.

Before discussing FETs and MOSFETs, let's review diodes.

Semiconductors and diodes

Before explaining diodes, let's first review the basics of semiconductors.

semiconductor

In short, it is an object that possesses properties intermediate between a conductor (which conducts electricity) and an insulator (which does not conduct electricity).

Adding various substances to a semiconductor results in objects with a variety of properties.

For example, applying current and voltage can lower the temperature (Peltier element) or produce light (light-emitting diode).

First, let's review the typical p-type and n-type semiconductors.

First, n-type semiconductors are materials that contain free electrons within them. Since the movement of electrons is what causes electric current, it could be said that they possess the fundamental elements of electricity.

Next, p-type semiconductors are materials that have paths (holes) through which electrons can move freely.

In high school chemistry terms, this means that the outermost electron shell is not completely full of electrons, but rather lacking them.

In other words, if there are electrons present, it can attract them, allowing electricity to flow. However, since it does not possess electrons itself, it cannot conduct electricity on its own.

For some reason, it is often represented by the character for "hole" (穴).

Diodes are basically constructed using these two types of semiconductors.

diode

Let's review some typical pn-type diodes.

Let's start by connecting them with a diagram.

Let's connect a wire to this and run some electricity through it.

First, connect the positive (+) terminal to the p-type and the negative (-) terminal to the n-type. Once connected, electrons are attracted to the positive terminal and move from one to the other, entering the holes along the way.

In other words, electricity flows. The electrons that are lost in the n-type are supplied by the power source or battery, so they don't disappear, and the electricity continues to flow.

This is called forward bias.

Next, if we connect a negative charge to the p-type and a positive charge to the n-type, electrons will be attracted to the positive charge, so no electrons will go to the p-type.

Furthermore, if all the electrons in an n-type semiconductor are attracted to the negative side, nothing will move (the negative terminal of the power supply and the electrons inside the n-type semiconductor are balanced).

In other words, electricity does not flow. This is called reverse bias.

Furthermore, a new region is created where no electrons exist at all, and this is called the depletion layer. We can calculate its width, but let's leave that aside for now.

This characteristic allows us to determine the direction in which electricity flows. Therefore, it is called a commutator (or diode).

Next, let's review the principle of FETs (MOSFETs).

The principle of FET (MOSFET)

A MOSFET is one of many types of FETs.

From now on, we will refer to FETs as MOSFETs, which are commonly used in electric airsoft guns.

First, let's look at the structure of a MOSFET.

The diagram illustrates a typical n-channel MOSFET. (Apparently, n-channel is the standard; "channel" is not a typo.)

Basically, a p-type semiconductor is used as the base, with n-type semiconductors placed where each electrode is attached. A wire called a gate is connected at the position shown in the diagram via a metal and an insulator (similar to an SBD).

As we reviewed, the electrodes are connected in an np order, so there is a reverse bias and no electricity flows within the MOSFET.

What's amazing here isWhen the same potential (voltage) as the positive side is applied to the gate, the p-type semiconductor inverts near the insulator and becomes an n-type semiconductor.

This is called an inversion layer. As someone with a mechanical engineering background, I didn't really understand the principle behind it.

However, it seems that for a semiconductor to invert, the positive voltage from the side must be greater than the negative voltage from below.

This creates an nn connection between the power supply and the MOSFET, allowing electrons to move and thus electricity to flow.

In other words, when electricity flows to the gate, electricity can pass through the MOSFET; when no electricity flows to the gate, electricity cannot pass through the MOSFET.

This characteristic is utilized to replace physical switches.

Let's review the names for each electrode.

Its characteristics include extremely fast semiconductor inversion, making it suitable for high-speed switches, and low resistance resulting in minimal losses, making it ideal for low-loss, high-speed switches.

However, for it to operate, a potential difference (voltage) must be present between the gate and the source.

In circuit notation, it is represented as follows:

It's important to note that the arrows in the circuit diagram are not incorrect.

As explained, electrons have a negative charge, so they are attracted to positive charges and move accordingly. However, in a circuit, electricity flows from positive to negative.

Actually, electrons move in the exact opposite direction. Apparently, some important person in the past made a mistake and it's been left that way ever since. (That's why I'm not good with electricity.)

Next, let's consider the challenges of attaching a MOSFET to something like an electric airsoft gun.

Operating mechanism of MOSFETs in circuits

Now let's consider how MOSFETs operate in a circuit.

The example considers a case where a MOSFET is used as an ON-OFF switch for a DC motor.

Without resistance, electricity cannot flow to the gate of the MOSFET, so first, we set up a resistor to provide a potential (voltage) between the gate and the source.

This resistance value only needs to be sufficient to create the necessary potential (voltage difference) for the MOSFET gate to open, so it varies depending on the MOSFET, but it seems to be around 10 to 30 ohms.

This is called gate resistance.

This depends on the MOSFET's specifications, so you'll need to check the catalog to decide.

This makes it seem like we could use a MOSFET as a switch in the circuit, but it appears the circuit won't work unless we can accommodate the characteristics of the MOSFET after it's switched off.

This section introduces an important characteristic of the MOSFET that occurs when that switch is turned OFF.

Even when the circuit switch is turned OFF, if there is still charge (electricity) remaining in the MOSFET, electricity will continue to flow between the DS until the charge in the inversion layer inside the MOSFET is depleted.

In other words, even when the circuit switch is turned off, electricity can still flow.

This is called the self-turn phenomenon.

Preventing this self-turn-on is simple: remove any remaining electrons in the MOSFET by adding a resistor and ground between the gate and source to eliminate the charge.

This is called gate-source resistance. It seems that a relatively large resistance is used here to quickly dissipate the charge.

It's roughly 10kΩ to 30kΩ, which is about 1000 times stronger than the first counter.

Based on these considerations, the electrical circuit required to use a MOSFET appears to be as shown in the following diagram.

This appears to make the MOSFET act as a switch in the circuit.

Summary

I now understand the principle behind MOSFETs, which are devices that utilize p-type and n-type semiconductors and can be used as switches by inverting the semiconductor's behavior.

However, since simply knowing the principles isn't enough to actually use it, I'd like to consider what's needed for implementation next time.

I want to read it together

MOSFET implementation in DC motor circuits In the previous lesson, we considered the basic mechanism of a MOSFET, so this time let's think about a simple implementation. First, based on the mechanism we learned last time, let's consider how to read MOSFET specifications. M…