• Fet And Mosfet In Hindi
• Compare Fet And Mosfet
• Fet And Mosfet Symbol
• Fet And Mosfet Characteristics
• Fet And Mosfet Difference
• Basic Mosfet Operation

In 1949, it took ENIAC (Electronic Numerical Integrator And Computer) 70 hours to calculate the value of Pi up to 2037 digits. Now, the smartphone in your hand can do the same task in 0.01 Seconds.

MOSFET

Il MOSFET (acronimo del termine inglese metal-oxide-semiconductor field-effect transistor, ovvero semiconduttore metallo-ossido- transistor ad effetto di campo), scritto anche MOS-FET o MOS FET e spesso conosciuto come transistor MOS, in elettronica indica una tipologia di transistor a effetto di campo largamente usata nel campo dell'elettronica digitale, ma diffusa anche nell'elettronica. My son wants to know more about MOSFET. I recommend trying to create a Long Duration Time Delay circuit. He said is easier than transistors can output power and good for switching and amplifier. OFF After Delay circuit using MOSFET. ON After Delay using MOSFET. The Metal Oxide Semiconductor FET abbreviated as MOSFET. Jul 20, 2020 My son wants to know more about MOSFET. I recommend trying to create a Long Duration Time Delay circuit. He said is easier than transistors can output power and good for switching and amplifier. OFF After Delay circuit using MOSFET. ON After Delay using MOSFET. The Metal Oxide Semiconductor FET abbreviated as MOSFET.

This miraculous growth in speed was made possible by a tiny device inside electronic gadgets called a transistor. More specifically a type of transistor called MOSFET. MOSFET is an electrically driven switch, which allows and prevents a flow of current, without any mechanical moving parts.

The MOSFET stands for METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR(Fig 1). In MOSFET, the MOS part is related to the structure of the transistor, while the FET part is related to how it works. It is also known as IGFET (Insulated Gate Field Effect Transistor). The following image we have shown is a practical MOSFET. But in the digital world, the size of MOSFET is too small (in nm) that billions of them can be fabricated on a single chip.

There are two basic types of MOSFET:

1.Enhancement-MOSFET

2.Depletion-MOSFET

Here we are explaining most popular type, Enhancement-MOSFET or E-MOSFET.

Structure of MOSFET

Like any other conventional transistor, A MOSFET is also made from a semiconductor material such as silicon. In its pure form, a semiconductor has very low electrical conductivity. However, when you introduce a controlled amount of impurities into the semiconductor material, its conductivity increases sharply. This procedure of adding impurities is called doping and the impurity is called dopant.

Pure silicon does not have any free electrons (Fig:2A ), and because of this its conductivity is very low; however, when you inject an impurity, which has extra electrons, into the silicon, the conductivity of the resultant material increases dramatically. This is known as N-type doping (Fig:2B). We can also add impurities with fewer electrons, which will also increase the conductivity of pure silicon. This is known as P-type doping (Fig:2C).

Fet And Mosfet In Hindi

When the concentration of the impurity is lower (approx. one dopant atom is added per 100 million atoms), the doping is said to be low or light. On the other hand, if it is higher, the doping is referred to as high or heavy. Now, let's get back to the workings of MOSFETs. If you dope a silicon wafer with two highly doped n region as shown in the figure, you will get the basic structure of a MOSFET (Fig:3). It is interesting to note that, even in the P region, there are very few free electrons that are capable of conducting electricity. We call them minority carriers. Later we will see why the minority carriers are significant in the MOSFET.

P-N junction

Whenever a P-N junction is formed, the excess electrons in the N region have a tendency to occupy the holes in the P region. This means that the PN junction boundary naturally becomes free of holes or free electrons. This region is called a depletion region. The same phenomenon also happens in the P-N junction of the MOSFET (Fig:4).

When simplified MOSFET is connected to power source

Now let's connect a power cell across the MOSFET as shown in the figure (Fig:5). On the right-hand side P-N junction, the electrons are attracted to the positive side of the cell and the holes are moved away. In short, the depletion region width on the right-hand side is increased due to the power source. This means that there won’t be any electron flow through the MOSFET.

In short with this simple arrangement the MOSFET will not work. Let’s see how it is possible to have an electron flow in the MOSFET using a simple technique. To do this we first need to understand the workings of the capacitor.

Working of capacitor

Inside the capacitor, you can see two parallel metal plates separated by an insulator (Fig:6). When you apply a DC power source across these, the positive terminal of the cell attracts electrons in the metal plate and these electrons are accumulated on the other metal plate. This accumulation of charge creates an electric field between the plates.

Working of MOSFET

Let’s replace one plate of the capacitor with the P type substrate of the MOSFET. If you connect a power source across the MOSFET as shown, just as in a capacitor the electrons will leave the metal plate. In a MOSFET these electrons will be dispersed into the P-substrate (Fig:7).

The positive charge generated on the metal plate, due to the electron displacement, will generate an electric field as shown. Due to the presence of electric field the MOSFET possess FET; Field Effect Transistor in its name.

Remember, there are some free electrons even in the P-type region. The electric field produced by the capacitive action will attract the electrons to the top. We will assume the electric field generated is quite strong. Some electrons were recombined with the holes, and the top region becomes overcrowded with electrons after all the holes there are filled. Just below this region, all the holes were filled but there were no free electrons either. This region has become a new depletion region. This process essentially breaks the depletion region barrier and a channel for the flow of electrons is created (Fig:8).

If we apply a second power source as we did at the beginning the electrons easily flow towards the metal plate. This is the way a MOSFET turns to the ON state (Fig:9).

You can easily correlate the naming of the transistor terminals; Source, Drain and Gate with the nature of the electron flow

If the applied voltage is not sufficient enough or less than the threshold voltage, the electric field will be weak and there won’t be a channel formation and hence no electron flows. Thus just by controlling the GATE voltage, we will be able to turn the MOSFET ON and OFF. Due to this ability to change conductivity with the amount of applied voltage at the gate, the MOSFET is also known as Voltage Controlled Device. The threshold voltage of MOSFET mainly depends on the thickness of the oxide layer.

Why source has been always connected to substrate?

In MOSFET both the source and drain must be at higher or equal potential than the substrate to stop an unwanted electron flow. Since drain voltage is always greater than the substrate voltage, so we don't consider the drain-substrate side. Whereas in the source side, this electron flow is stopped by keeping source and substrate at the same potential. That's why in MOSFET, the source is always connected to the substrate.

Example:

Consider the heat-based fire alarm circuit as shown in the figure (Fig:11). This circuitry consists of a Thermistor, a buzzer, a MOSFET and some other passive components. The thermistor in the circuit decreases its resistance with an increase in temperature. Initially, at room temperature, the voltage at the GATE is low due to the high thermistor resistance, and that is not sufficient to turn ON the MOSFET. If the temperature increases, the thermistor’s resistance decreases, this will lead to a high GATE voltage, which then turns ON the MOSFET and the alarm.

MOSFET used in digital electronics

• MOSFETs open the door to digital memory and digital processing.
• MOSFETs combine together to form the basic memory element of a static RAM.
• At the lowest level MOSFETs are interconnected to form logic gates.
• At the next level, the gates are combined to form processing units that perform thousands of logical and arithmetical operations.

Advantages of MOSFET over BJT

• Unlike BJTs, MOSFET have a scalable nature, so that millions of MOSFET can be fabricated on the single wafer.
• A BJT wastes a small part of its main current when it’s switched ON; such power wastage is not there in MOSFETs.
• The other advantage of a MOSFET is that it is a unipolar device means; it only operates with one type of charge carrier, be it a hole or an electron, so it is less noisy.

FET stands for the Field-effect transistor. These transistors are designed to overcome the drawbacks of the bipolar junction transistors. As the basic transistors have the junction of the emitter in the mode of forwarding bias this makes the device to operate at low impedance levels. This introduces considerable amounts of noise levels. FETs possess all the characteristics which can overcome the drawbacks of bipolar junction transistors and can be a good replacement for the vacuum tubes as well as BJTs. It also consists of three terminals. But these terminals are referred to as the source, drain, and gate.

These FETs are known for their unipolar characteristics. The reason behind its characteristics is that the functioning of this transistor depends upon the concentration of either holes or electron carriers. Field effect transistors can also be used in switching circuits, buffer amplification circuits and in integrated circuits.

What is a FET?

The transistor that is capable of transferring the signals from the high resistance to the low resistance values same like bipolar junction transistors but by overcoming the disadvantages of it in its uni-polar way is defined as a field-effect transistor (FET).

FET is designed in such a way there exist three terminals that are known as source, gate and the drain. These terminals are responsible for the influencing of the majority carriers by supplying it with possible supplies of voltages. This leads to the generation of the current. The flow of current can be controlled by the applying the voltage justifying the characteristics that it is a voltage-controlled device.

Types of FET

Based on its construction FET’s are classified as

(1) Junction Field Effect Transistor (JFET)

These JFET’s working is based on the channels formed between the terminals. The channel can be of either n-type or the p-type. Because of the n-type channel, it is referred to as n- channel JFET and due to a p-type channel formed it is referred to as p-channel JFET.

Symbol of N-Channel JFET

JFET Working

The construction of the JFET is as similar to that of BJT it can be formed by using n-type and p-type materials. N-type is placed in between p-types or the p-types is placed in between n-types. As like N-P-N and P-N-P transistors formed in BJT, these are also formed in FET. These JFET’s consists of the channel that may be n or p-type.

Symbol for P-Channel JFET

• Based on the channel it is known as n-channel JFET or the p-channel JFET.
• For the n-channel JFET, the positive side is connected to the source terminal.
• The drain terminal receives the highest potential in comparison with the gate in this n-channel JFET.
• The junction formed due to the interaction of the drain and the gate terminal will be in reverse bias.
• Due to this reason, the width of the depletion region that is present near to the drain is greater in comparison with the source.
• Because of this condition, the majority of the charge carriers that are electrons flow can be seen from the terminals drain to the source.
• As this potential at drain tends to increase the flow of carriers increases the flow of current also increases.
• But certain increment at the voltages at drain and source the flow of current is stopped.
• The JFET is generally known for the characteristics of controlling the current by the application of the input voltages.
• The value of the input impedance is at a peak in this transistor.
• When the JFET is in its ideal mode there is no current evidence at the gate terminal.

That how a n-channel JFET works. Only the variation at the polarities of the supplies makes the FET work as p-channel JFET.

(2) Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

Compare Fet And Mosfet

In MOSFETs, the functioning is based on the channels which already exist or formed when a voltage is applied. Based on these modes of operations MOSFETs are further classified into Depletion mode and the enhancement mode. In the enhanacement mode the channel is induced due to the application of the voltage at the gate but in the depletion mode, the MOSFET is operated due to the already existing channel in it.

MOSFET

MOSFET Types:

The depletion model of MOSFET is also classified into ntype and ptype. The only difference between this is the deposition of the substrates. Due to the concentration of the carriers that are preferably the majority is responsible for the formation of the region called depletion. This width of the depletion is responsible for the effect of conductivity.

In the enhancement mode when a voltage applied to the gate terminal beyound a threshold voltage a channel is formed. It may be n-type for P type susbstrate and p- type for an N type substate. Based on the channel formation enhancement mode is classified as N type Enhancement MOSFET and P type Enhancement MOSFET. Enhancement type MOSFET are most widely used than the depletion type.

Biasing

The FET biasing is also done like transistor biasing. It can be fixed bias, self-bias, and the potential- Divider biasing.

Fet And Mosfet Symbol

(1) Fixed Bias

Fixed bias in the FET can be obtained by supplying with the battery voltage. The terminal gate must connect with the negative supply of the battery and no current flow is evident through the resistor.

(2) Self Bias

Fet And Mosfet Characteristics

As the name suggests if the external supply is not provided for the circuitry. This type of bias is known as self-bias. Any changes in the transconductance values that reflect the distortion of the operating point. These parameters can be controlled and they are not easily affected in the self-bias.

(3) Potential Divider Bias

The circuit is provided with the supply at the input but the two resistors are connected in such a way that the voltage at the input is divided with the help of resistors. Hence this circuitry is referred to as potential divider.
These biasing techniques are chosen based on the necessity and the increment of the conductance values.

Fet And Mosfet Difference

Characteristics

The FET characteristics mainly depend on the various operating regions. The regions are ohmic, saturation, cut-off and the breakdown region.

(1) Ohmic Region

The region at which the transconductance shows the linear response and the current at the terminal gate is opposed by the resistance is known as ohmic region.

(2) Saturation Region

At this region, the device is at fully ON mode. During this condition the maximum amount of current flow through the transistor at a steady state.

(3) Cut- off Region

There is no flow of current can be evident at the transistor during this region. Hence it is referred to as the device in the OFF condition.

(4) Breakdown Region

Basic Mosfet Operation

When the applied voltage exceeds the condition of the maximum value of the voltage then the transistor enters into the breakdown condition indicating that the transistor resists the flow of current.

Applications

The applications of the FET are as follows

1. For applications like low noise, these types of transistors are preferred.
2. FET’s have a preferred utilization during the applications of it as a buffer.
3. These are used in the cascade amplifiers.
4. The main feature behind this is that its input capacitance is low.
5. For analog switching, the FET is preferred.
6. It is preferred during oscillation circuits.
7. For the current limiting circuits JFET’s are preferred.

In this way, the field-effect transistors have many applications. It may be JFET or the MOSFET both have many applications based on its highly unified characteristics. Each of it is preferred as switches and can be used in amplifications etc…

FET as a Switch

FET can be used as one of the applications of switching because it can be operated as either fully ON and the fully OFF. Similar to BJTs a FET also consists of the active region, cut-off region, and the saturation region as mentioned above.

As the voltage applied at the junction of gate and source is zero then the working condition of FET is in saturation because the maximum amount of the current flows through it. When the applied voltage that is lesser than the cut-in voltage or more negative.

Then the operating region of the FET is considered to be in cut-off mode. During its working in the cut-off region, there is no evident flow of the current through the circuit. These are the reasons due to which the FET operates as a switch. When the FET is connected with the load in a parallel manner then it acts as an analog switch. FET’s can also be connected in the series to act as a series switch.

In this way, the basic operation of the FET along with its types and the biasing techniques have been discussed above. The junction field-effect transistor is the first classification of the FET that is classified based on the junctions formed of –type or p-type. These FETs classification based on the channels formed are done known as MOSFETs.

After analyzing the types of FETs can you describe which one is better and mostly the preferable one among JFET and MOSFET?