What is MOSFET and how it works

What is MOSFET and how it works | MOSFET Transistor

A MOSFET is a four-terminal device that has source (S), gate (G), drain (D) and body (B) terminals. In general, the MOSFET body is connected to the source terminal, forming a three-terminal device such as a field-effect transistor. MOSFET is generally considered as a transistor and is used in both analog and digital circuits. This is the basic introduction to MOSFET. The general structure of this device is as follows:

From the above MOSFET structure, the function of the MOSFET depends on the electrical changes that occur in the channel width along with the flow of electrons (either holes or electrons). Electric current enter the channel through the source S, terminal and exit through the drain D.

The channel width is controlled by a voltage on an electrode called the Gate G, and is located between the source and drain. It is isolated from the channel near a very thin layer of metal oxide.

When there is no voltage signal in the gate of the MOSFET, it will shows its maximum conductivity. Whereas when the voltage in the gate terminal is positive (+) or negative (-), the conductivity of the MOSFET will decrease.

When there is no voltage across the gate terminal, the device does not operate. When there is maximum voltage across the gate terminal, the device shows improved conductivity.

MOSFET Working principle

The main principle of a MOSFET is the ability to control the voltage and current between the source and drain terminals. It works almost like a transformer and the functions of the device depend on the MOS capacitor. MOS part is the important capacitor of MOSFET.

The surface of the semiconductor in the bottom oxide layer located between the source and drain terminal can be reversed from P-type to N-type by applying either positive or negative gate voltages respectively. When we apply a repulsive force to a positive gate voltage, the holes under the oxide layer are pushed down along with the substrate.

The depletion region inhabited by the negative charges associated with the acceptor atoms. When electrons are accessed, a channel is developed. The positive (+) voltage also attracts electrons (e-) from the the source S, and drain D, regions to the channel.

Now, if a voltage is applied between the drain and the source, the current flows freely between the source and drain and the gate voltage controls the electrons in the channel. Instead of a positive (+) voltage, if you apply a negative (-) voltage (V), a terminal or hole channel will be composed under the oxide layer part.

P-channel MOSFET

The P-channel MOSFET has a P-channel region located between the source and drain terminals. It is a four-pronged device with terminals such as gate, drain, source and body. The drain and source are strongly doped with the p+ region and the body or n-type substrate. Current flows in the direction of the positively charged holes.

When we apply a negative voltage with a repulsive force at the gate tip, the electrons under the oxide layer are pushed down into the substrate. The depletion region is populated by the positive charges attached to the donor atoms. The negative gate voltage also draws holes from the p+ source and drain region to the channel region.

N-channel MOSFET

The N-Channel MOSFET has an N channel region located between the source and drain terminals. It is a quadrilateral device with terminals such as gate, drain, source, and body. In this type of field-effect transistor, the drain and source are strongly doped n + region and the substrate or body is of type P.

The current flow in this type of MOSFET occurs due to the negatively charged electrons. When you apply a positive (+) voltage (V) with a repulsive force at the tip of the gate (G), the holes under the oxide layer part are pushed down into the substrate. The depletion region is filled with negative charges associated with the acceptor atoms.

When electrons arrive, a channel is formed. The positive (+) voltage also attracts electrons (e-) from the n+ of the source S, and drain D, regions to the channel. Now, if a voltage is applied between the drain and the source, the current flows freely between the source and drain and the gate voltage controls the electrons in the channel. Instead of a positive (+) voltage (V) if we apply a negative (-) voltage (V), a hole (°) channel will be formed under the oxide layer part.

MOSFET Operation Areas

For the more general scenario, the operation of this device mainly takes place in three areas which are as follows:

Cut-off area - The area where the device will be in the off state and there is no amount of current flowing through it. Here, the device acts as a primary switch and is used when they are necessary to act as switches.

Saturation region - In this region, devices will have their drain of the source current value as constant without regard to the voltage boost across the drain to the source. This happens only once when the voltage across the drain to the source terminal increases more than the value of the pressure voltage. In this scenario, the device acts as a closed switch where a saturated level of current flows through the drain to the source terminals. As a result, the saturation region is determined when the devices are supposed to switch.

Linear/ohmic region - The area where the current through the drain to the source terminal improves as the voltage across the drain to the source path increases. When MOSFETs operate in this linear region, they perform the function of an amplifier.

Let us now look at the transform characteristics of a MOSFET

Semiconductor such as a the MOSFET transistor or Bipolar Junction Transistor (BJT) also acts as switches in two scenarios, one in the on state and the other in the off state. To assume this function, let's take a look at the ideal and practical characteristics of a MOSFET transistor device.

MOSFET Perfect switching properties

When a MOSFET is supposed to function as a perfect switch, it must retain the following properties and these are

In the running state, there must be a current constraint that it carries

In the off state, the blocking of voltage levels shall not have any kind of restriction

When the machine is in the running state, the voltage drop value should be empty

The resistance in the OFF state must be unlimited

There should be no limitation on the speed of the operation

MOSFET Operation switch characteristics

Since the world is not only stuck in perfect applications, the operation of a MOSFET is viable even for practical purposes. In a practical scenario, the device should retain the following characteristics

In the operating condition, the power management capabilities must be limited which means the conduction current flow must be restricted.

In the off state, the voltage levels should not be restricted

Turning on and off for limited times limits the set speed of the device and even limits the functional frequency

If the MOSFET is turned on, there will be minimal resistance values as this results in a voltage drop in the forward bias. Also, there is a limited stop state resistance that provides reverse leakage current

When the device operates with practical characteristics, it loses power in on and off conditions. This happens even in cases of transmission as well.

Example of a MOSFET as a key

In the circuit arrangement below, the enhanced mode and N-channel MOSFET are used to switch a sample lamp with the on and off conditions. Positive voltage at the gate terminal is applied to the base of the transistor and the lamp moves in the on state and here VGS = + v or at zero voltage level the device turns OFF where VGS = 0.

If the resistive load of the lamp is to be replaced by an inductive load and connected to the load-protected relay or diode. In the above circuit, it is a very simple circuit for switching a resistive load such as a lamp or an LED. But when a MOSFET transistor is used as a switch (on and off ) with either an inductive load or a capacitive load, the protection will required for the MOSFET.

If the MOSFET is not protected, it may damage the device. For a MOSFET to function as an analog switching device, it must switch between its cut-off region where VGS = 0 and its saturation region where VGS = + v.

MOSFET can also function as a transistor and is abbreviated as metallic silicon oxide field effect transistor. Here, the name itself indicates that the device can be operated as a transistor. It will have a q channel and an n channel. The device is connected in this way using four sources, gate and drain terminals, a resistive load of 24 degrees is connected in series to the ammeter, and the potentiometer is connected via the MOSFET.

In a transistor, the current flowing in the gate is in a positive direction and the source terminal is connected to ground. Whereas in bipolar junction transistors, the current flow is through the base path to the emitter. But in this device, there is no current flowing due to the presence of a capacitor at the beginning of the gate, it only requires voltage.

This can be done by continuing the simulation process and by switching ON / OFF. When the switch is in on position, there is no current (I) flowing through the circuit or MOSFET, when a resistance of 24Ω and 0.29 of the ammeter (A) voltage is connected, we find the idling voltage drop across the source due to the presence of +0.21V across this device.

The resistance (R) between drain (D) and source (S) is called RDS. Because of this RDS, voltage drop appears when there is current flowing in the circuit. RDS varies based on the type of device (it can vary between 0.001, 0.005, and 0.05 depending on the type of voltage.

How to choose MOSFET as switch?

There are a few conditions to consider while choosing a MOSFET as a switch which are as follows:

Use polarity as either a P or N channel.

Maximum rated operating voltage and current values

Increased modes of operation which means that the resistance at the drain terminal to the source when the channel is fully open

Boost operating frequency

Packing type of To-220, DPAck and many more.

What is the switching efficiency of a MOSFET?

The main limitation in the operating time of a MOSFET as a switching device is the value of the enhanced drain current that the device can be capable of. This means that the RDS in the on state is the critical parameter that determines the switching power of the MOSFET. It is represented as the ratio of the drain source voltage to the drain current. It should only count if the transistor is turned on.

Why is the MOSFET switch used in the Boost transformer?

Generally, a boost converter needs a switching transistor to power the device. Therefore, switching transistor MOSFETs are also used. These devices are used to find out the current value and voltage values. Also, considering the conversion speed and cost, they are widely used.

In the same way, a MOSFET can also be used in several ways. And these are:

MOSFET as LED switch
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MOSFET as a switch for Arduino
MOSFET switch for AC load
DC motor MOSFET switch
MOSFET negative voltage switch
MOSFET as a switch with Arduino
MOSFET as a switch with a microcontroller
MOSFET switch with hysteresis
MOSFET as switching diode and active resistor
MOSFET as a switching equation
MOSFET key for airsoft
MOSFET as gate-switching resistance
MOSFET as switching solenoid
Switching MOSFET using optocoupler
MOSFET switch with hysteresis
MOSFET application as a switch

A notable example of this device is that it is used as a switch to automatically control the brightness of street lights. These days, many of the lights we notice on highways are made up of high intensity discharge lights. But the use of HID lamps consumes increasing energy levels.

The brightness cannot be restricted based on the requirements and because of that there must be a switch for the alternate lighting method which is the LED. The use of an LED system will overcome the disadvantages of high intensity LEDs. The main concept behind building this was to control the lights directly on highways by making use of a microprocessor.

MOSFET application as a switch

This can only be achieved by adjusting the clock pulses. Depending on necessity, this device is used to switch lamps. It consists of a Raspberry Pi board which is included with a management processor. Here, LEDs can be replaced in place of HIDs and have a connection to the processor through a MOSFET. The microcontroller (MCU) offer corresponding duty cycles and then switches (SW) to the MOSFET to provide a high level of intensity (I).

MOSFET Advantages

Few of the advantages are:

It generates improved efficiency even when working with minimal effort

There is no gate current, resulting in more input impedance which increases the switching speed of the device

These devices can operate at minimum power levels and use minimum current

cons

MOSFET drawbacks are:

When these devices operate at excessive voltage levels, they create device instability

Since the devices (MOSFET transistor) have a thin oxide layer part, this may damage the device when stimulated by (ES) electrostatic charges.

Applications

MOSFET applications are

MOSFET amplifiers are widely used in wide frequency applications

Regulations for DC motors are provided by these devices

Since these speeds have enhanced switching speeds, they serve as ideal for building helicopter amplifiers

Functions as a passive component of many electronic components.

Finally, it can be concluded that transistor requires current or intensity (I) while MOSFET requires voltage (V). The driving requirements for MOSFET are much better, and much simpler compared to Bipolar Junction Transistor BJT. Also know how do I connect a Mosfet to a switch?

What is MOSFET and how it works | MOSFET Transistor