MOSFET

Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) is most useful type of among all transistors. The name itself indicates that it contains metal gate terminal. The MOSFET has four terminals drain, source, gate and body or substrate (B). MOSFET has many advantages over BJT and JFET, mainly it offer high input impedance and low output impedance. It is used in low power circuits mainly in chip designing technologies.

The MOSFET transistors are available in depletion and enhancement types. Further the depletion and enhancement types are classified into N-channel and P-channel types.

A metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a field-effect transistor (FET with an insulated gate) where the voltage determines the conductivity of the device. It is used for switching or amplifying signals. The ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. MOSFETs are now even more common

A MOSFET is by far the most common transistor in digital circuits, as hundreds of thousands or millions of them may be included in a memory chip or microprocessor. Since they can be made with either p-type or n-type semiconductors, complementary pairs of MOS transistors can be used to make switching circuits with very low power consumption, in the form of CMOS logic.

Why MOSFET?

MOSFETs are particularly useful in amplifiers due to their input impedance being nearly infinite which allows the amplifier to capture almost all the incoming signal. The main advantage is that it requires almost no input current to control the load current, when compared with bipolar transistors. MOSFETs are available in two basic forms:

  • Depletion Type:The transistor requires the Gate-Source voltage (VGS) to switch the device “OFF”. The depletion mode MOSFET is equivalent to a “Normally Closed” switch.
  • Enhancement Type:The transistor requires a Gate-Source voltage (VGS) to switch the device “ON”. The enhancement mode MOSFET is equivalent to a “Normally Open” switch.

 

 

Types of Mosfet

P-Channel MOSFET

P-Channel MOSFET

The drain and source are heavily doped p+ region and the substrate is in n-type. The current flows due to the flow of positively charged holes also known as p-channel MOSFET. When we apply negative gate voltage, the electrons present beneath the oxide layer experience repulsive force and they are pushed downward in to the substrate, the depletion region is populated by the bound positive charges which are associated with the donor atoms. The negative gate voltage also attracts holes from p+ source and drain region into the channel region.

N-Channel MOSFET

N-Channel MOSFET

 

The drain and source are heavily doped n+ region and the substrate is p-type. The current flows due to the flow of negatively charged electrons, also known as n-channel MOSFET. When we apply the positive gate voltage the holes present beneath the oxide layer experience repulsive force and the holes are pushed downwards in to the bound negative charges which are associated with the acceptor atoms.

 

The positive gate voltage also attracts electrons from n+ source and drain region in to the channel thus an electron reach channel is formed

 

 

                  Comparison of n- and p-type MOSFETs

Parameter

nMOSFET

pMOSFET

Source/drain type

n-type

p-type

Channel type
(MOS capacitor)

n-type

p-type

Gate
type

Polysilicon

n+

p+

Metal

φm ~ Si conduction band

φm ~ Si valence band

Well type

p-type

n-type

Threshold voltage, Vth

·         Positive (enhancement)

·         Negative (depletion)

·         Negative (enhancement)

·         Positive (depletion)

Band-bending

Downwards

Upwards

Inversion layer carriers

Electrons

Holes

Substrate type

p-type

n-type

 

 

 

MOSFET construction

A MOSFET is based on the modulation of charge concentration by a MOS capacitance between a body electrode and a gate electrode located above the body and insulated from all other device regions by a gate dielectric layer. If dielectrics other than an oxide are employed, the device may be referred to as a metal-insulator-semiconductor FET (MISFET). Compared to the MOS capacitor, the MOSFET includes two additional terminals (source and drain), each connected to individual highly doped regions that are separated by the body region. These regions can be either p or n type, but they must both be of the same type, and of opposite type to the body region. The source and drain (unlike the body) are highly doped as signified by a “+” sign after the type of doping.

It is a four-terminal device with source(S), gate (G), drain (D) and body (B) terminals. The body is frequently connected to the source terminal, reducing the terminals to three. It works by varying the width of a channel along which charge carriers flow (electrons or holes).The charge carriers enter the channel at source and exit via the drain. The width of the channel is controlled by the voltage on an electrode is called gate which is located between source and drain. It is insulated from the channel near an extremely thin layer of metal oxide. A metal-insulator-semiconductor field-effect transistor or MISFET is a term almost synonymous with MOSFET. Another synonym is IGFET for insulated-gate field-effect-transistor.

MOSFET Usage and purpose

The applications of the MOSFET used in various electrical and electronic projects which are designed by using various electrical and electronic components. For better understanding of this concept, here we have explained some projects.

 

MOSFET Used as a Switch

Enhanced mode, an N-channel MOSFET is being used to switch the lamp for ON and OFF. The positive voltage is applied at the gate of the MOSFET and the lamp is ON (VGS =+v) or at the zero voltage level the device turns off (VGS=0).    If the resistive load of the lamp was to be replaced by an inductive load and connected to the relay or diode to protect the load. In the above circuit, it is a very simple circuit for switching a resistive load such as LEDs or lamp. But when using MOSFET to switch either inductive load or capacitive load protection is required to contain the MOSFET applications. If we are not giving the protection, then the MOSFET will be damaged. For the MOSFET to operate as an analog switching device, that needs to be switched between its cutoff region where VGS =0 and saturation region where VGS =+v.

Auto Intensity Control of Street Lights using MOSFET

Now-a-days most of lights placed on the highways are done through High Intensity Discharge lamps (HID), whose energy consumption is high. Its intensity cannot be controlled according to the requirement, so there is a need to switch on to an alternative method of lighting system, i.e., to use LEDs. This system is built to overcome the present day drawbacks of HID lamps.

This project is designed to control the lights automatically on the highways using microprocessor by variants of the clock pulses. In this project, MOSFET plays major role that is used to switch the lamps as per the requirement. The proposed system using a Raspberry Pi board that is a new development board consist a processor to control it. Here we can replace the LEDs in place of HID lamps which are connected to the processor with the help of the MOSFET. The microcontroller release the respective duty cycles, then switch the MOSFET to illuminate the light with bright intensity

 

 

Marx Generator Based High Voltage Using MOSFETs

The main concept of this project is to develop a circuit that delivers the output approximately triple to that of the input voltage by Marx generator principle. It is designed to generate high-voltage pulses using a number of capacitors in parallel to charge during the on time, and then connected in series to develop a higher voltage during the off period. If the input voltage applied is around 12v volts DC, then the output voltage is around 36 volts DC.

This system utilizes a 555 timer in astable mode, which delivers the clock pulses to charge the parallel capacitors during on time and the capacitors are brought in a series during the off time through MOSFET switches; and thus, develops a voltage approximately triple to the input voltage but little less, instead of exact 36v due to the voltage drop in the circuit. The output voltage can be measured with the help of the multimeter.

 

EEPROM based Preset Speed Control of BLDC Motor

The speed control of the BLDC motor is very essential in industries as it is important for many applications such as drilling, spinning and elevator systems. This project is enhanced to control the speed of the BLDC motor by varying the duty cycle.

EEPROM based Preset Speed Control of BLDC Motor

The main intention of this project is to operate a BLDC motor at a particular speed with a predefined voltage. Therefore, the motor remains in an operational state or restarted to operate at the same speed as before by using stored data from an EEPROM.

The speed control of the DC motor is achieved by varying the duty cycles (PWM Pulses) from the microcontroller as per the program. The microcontroller receives the percentage of duty cycles stored in the EEPROM from inbuilt switch commands and delivers the desired output to switch the driver IC in order to control the speed of the DC motor. If the power supply is interrupted, the EEPROM retains that information to operate the motor at the same speed as before while the power supply was available.

 

LDR Based Power Saver for Intensity Controlled Street Light

In the present system, mostly the lightning-up of highways is done through High Intensity Discharge lamps (HID), whose energy consumption is high and there is no specialized mechanism to turn on the Highway light in the evening and switch off in the morning.

Its intensity cannot be controlled according to the requirement, so there is a need to switch to an alternative method of lighting system, i.e., by using LEDs. This system is built to overcome the present day, drawback of HID lamps.

This system demonstrates the usage of LEDs (light emitting diodes) as light source and its variable intensity control, according to the requirement. LEDs consume less power and its life is more, as compared to conventional HID lamps.

The most important and interesting feature is its intensity that can be controlled according to requirement during non-peak hours, which is not feasible in HID lamps. A light sensing device LDR (Light Dependent Resistance) is used to sense the light. Its resistance reduces drastically according to the daylight, which forms as an input signal to the controller.
A cluster of LEDs is used to form a street light. The microcontroller contains programmable instructions that controls the intensity of lights based on the PWM (Pulse width modulation) signals generated.

The intensity of light is kept high during the peak hours, and as the traffic on the roads tend to decrease in late nights; the intensity also decreases progressively till morning. Finally the lights get completely shut down at morning 6 am, to resume again at 6pm in the evening. The process thus repeats.

 

SVPWM (Space Vector Pulse Width Modulation)

The Space Vector PWM is a sophisticated technique for controlling AC motors by generating a fundamental sine wave that provides a pure voltage to the motor with lower total harmonic distortion. This method overcomes the old technique SPWM to control an AC motor that has high-harmonic distortion due to the asymmetrical nature of the PWM switching characteristics

In this system, DC supply is produced from the single-phase AC after rectification, and then is fed to the 3-phase inverter with 6 numbers of MOSFETs. For each phase, a pair of MOSFETare used, and, therefore, three pairs of MOSFETs are switched at certain intervals of time for producing three-phase supply to control the speed of the motor. This circuit also gives light indication of any fault that occurs in the control circuit

Therefore, this is all about types of MOSFET applications, finally, we will conclude that, the MOSFET requires high voltage whereas transistor requires low voltage and current. As compared to a BJT, the driving requirement for the MOSFET is much better. Furthermore, any queries regarding this article you can comment us by commenting in the comment section below.

Reference

https://www.typesnuses.com/types-of-mosfet-applications/

https://www.electronicsforu.com/resources/learn-electronics/mosfet-basics-working-applications

https://en.wikipedia.org/wiki/MOSFET

https://www.electronicshub.org/transistors-classification-and-types/

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