MOSFET (Construction, Working and Characteristic)

Hello everyone, Hope you all are doing good:) The topic for today is “MOSFET”. In today’s article, we are all gonna see every basic bit related to the MOSFET and its features. However, the first MOSFET was invented by Mohamed M.Atalla and Dawon Khang at the Bells lab in 1959 and was represented in 1960.

What is a MOSFET?

MOSFET stands for a Metal Oxide Semiconductor Field Effect Transistor. It is a Field Effect transistor where the voltage determines the conductivity of the device (i.e. the gate to source voltages controls the gate current). Besides this, it is also known as Metal Oxide Silicon Transistor.

It is the basic building block of modern electronics and the most consequently developing device in history to date. IT is certainly used for the switching purpose and for the amplification of the electronic or digital signal.

TYPES Of MOSFET

There are certainly two types of MOSFET:-

  • Enhancement Mosfet
    • P Type enhancement mosfet
    • N Type enhancement mosfet
  • Depletion Mosfet

CONSTRUCTION

Although the basic construction of a N channel enhancement MOSFET is

The MOSFET mainly consists of three parts Drain, Gate, and Source. in the construction what mainly happens is that lightly doped region i.e. Substrate is doped with a highly doped region. Depending on the type of substrate the MOSFET is evidently classified as P-Type or N-Type substrate.

WORKING

Whereas the connection of the MOSFET is;

When Vds voltage is applied between drain and source without a gate to source voltage, zero current flows through the drain to source.

If we increase the Vgs voltage in a positive direction the concentration of electrons near the SiO2 layer increases, Thus at a particular value of Vgs there is a considerable / Measurable amount of current flowing between drain and source. This value of Vgs is called Threshold Voltage. It is represented by Vt.

The amplitude of the current flowing between drain and source terminal increases with the increasing value of gate to source voltage(i.e. Id is directly proportional to the Vgs) this proportionality is because of the increase in the number of electrons in the channel.

Note:- For any value below the threshold value there is no channel therefore, there is no current flowing through the MOSFET

SYMBOLS

CHARACTERISTICS

Basically, there are two main characteristics of a MOSFET

Transfer characteristic:-

Certainly, it gives the relation between the drain current and the gate to source voltage.

Drain characteristic:-

However, it gives the relation between the drain current and the drain to source voltage.

BASIC MOSFET AMPLIFIER CONFIGURATIONS

  1. Firstly Common Source Configuration.
  2. Secondly Configuration with common gate.
  3. And lastly Common Drain Configuration.

TYPES OF BIASING In MOSFET

Fixed biasing:-

Certainly Fix the dc voltage Vgs to specify the saturation current of the MOSFET.

Circuit diagram Fixed biasing in a mosfet.

Id = (1÷2) Kn * ( Vgs – Vt)2

= (1÷2) Kn * (Vg – Vt)2

2. Self Bias Circuit:-

However this bias condition is specified by Vgs = Vg = IdRs

Vg = Vgs + (Kn÷2)( Vgs- Vt )2 (Rs)

circuit diagram Self biasing in a mosfet.

Certainly, the drain current has a better tolerance to variation in the device.

3. Feedback Current:-

Basically, a single power supply is needed to ensure that the MOSFET is in saturation region Vgs = Vds operating point

Id = (Kn÷2)*(Vgs – Vt)2

Circuit diagram of feedback biasing of a mosfet.

but Id from the figure is;

Id = (Vdd – Vgs)÷Rd

∴(Vdd – Vgs)÷Rd = (Kn÷2)*(Vgs – Vtn)2

However, the value of feedback resistor Rg affects the small-signal gain.

4. Voltage Divider Bias Circuit:-

voltage divider biasing of a mosfet

Vgs = (R2÷(R1+R2))÷Vdd

Vds = Vdd – IdRd

5. Constant Current Biasing:-

Constant Current biasing circuit diagram of mosfet

In this type of biasing instead of source resistor, we are using constant current source Rd is chosen in such a way that the MOSFET operates in the active region

Id = (K’W÷2L)*( Vgs – Vt)2

so the voltage at source ; Vgs + Vs = 0

∴Vs = -Vgs

Consequently the drain current through drain resistor;

Id = (Vdd – Vd)÷Rd

So the drain to source voltage;

Vds = Vd – Vs

NON-IDEAL PARAMETER

1.Finite output impedance:

So when a MOSFET is biased in the saturation region, the drain current Id is independent Of the Vds(drain to source voltage). Therefore, the output resistance or the drain resistance is given by the formula;

Ro = Vds/ Id

But, much of the worth of Id isn’t utterly constant it changes slightly with increasing or decreasing Vds voltage.

These changes happen as a result of inversion charge carriers that match to zero and moves off from the drain terminal, and thence, the effective channel length decreases and hence, as a result, the slope of the drain characteristic within the saturation region exits. however, its slope can also tend at the interval the terms of Id and Vds as;

Id= K[ (Vgs – Vt)2 *(1+(λVds)) ]

whereas; λ= channel length modulation

extrapolated V-I Characteristic of mosfet

if we extrapolated the V-I characteristic curve to intercept the negative voltage axis at a point consider it to be as A thus, Va = -Vds

However, the channel length modulation variables λ and Va are interconnected to each other. At Va, Id=0 as a result putting this value in the above equation;

1+ λVds = 0

λ = -1/Vds

λ = -1/-Va

Va = 1/λ

Although we know that;

ro = ∂Vds/ ∂Id____________(Vgs- Constant)

Ro = Vds-(-Va) / K[(Vgs-Vt)2(1+Vds)]

= Vds+(1/λ) / K[(Vgs-Vt)2(1+Vds)]

= λVds+1 / λK[(Vgs-Vt)2(1+Vds)]

ro = 1 / λK[(Vgs-Vt)2]

ro = Va / Ids

2. Body Effect:-

Body effect refers to the change in the threshold voltage as a result of the voltage variation between the transistor source and body.

For better understanding assume that both the source and body are at the same potential initially, now go on increasing the body potential in a negative direction as a result more electrons will be attracted towards the gate of the MOSFET and away from the main body. As we know that the threshold voltage is the gate voltage at which the number of charges in the carrier is equal to the gate voltage hence, as a result, the value of the threshold voltage increases which consequently decreases the flow of the current through the device. So this effect in total is called as Body Effect.

3. Subthreshold Voltage:-

When Mosfet is conducting in saturation region Id is given as;

Id = K [Vgs – Vt]2

That is to say, taking the square root of the above equation

√Id = √K Vgs – √K Vt

from the above equation we can say in short that the drain current(√Id) is a linear function of Vgs

If we compare the practical and the ideal result of the MOSFET it shows that when Vgs is slightly less than the threshold voltage a very small amount of current flows the circuit. This current is called a Subthreshold current whereas the voltage at which it flows is called a Subthreshold Voltage.

This effect is not significant in a single device but when thousand or millions of MOSFETs on an integrated circuit are biased just before the threshold voltage, then the power of supply current will not be equal to zero it may contribute to significant power dissipation in the integrated circuit.

4. Breakdown Effect:-

Avalanche Breakdown:-

The drain to substrate P N Junction may break because the applied drain voltage is too high.

Punch through Breakdown:-

This break occurs when the drain voltage is large enough for the depletion region around the drain to the external breakthrough channel completely.

Near Avalanche:-

However, the source-Substrate-Drain structure is equivalent to that of BJT. As device size sinks because of that parasitic bipolar transistor action with an increase in drain voltage. this parasitic action enhances the breakdown effect.

5. Temperature Effect:-

However, Vth and Kn are functions of temperature. The magnitude of Vth decreases as the temperature decreases and Id increases for the applied Vgs.

Kn is directly proportional to the mobility of the charge carrier in the inversion layer. which in turn increases the temperature and the electrons start vibrating but on the other hand its mobility decreases and as a result the total drain current decreases.

Temperature dependence of Kn is more as compared to Vth which in all sum ups to reduce Id for the applied Vgs voltage.

LOAD LINE AND MODES OF OPERATION

However, with the help of the load line, we can visualize the region in which the MOSFET is biased.

Q Point graph with DC load line of a mosfet

Basically writing Kirchoff’s voltage law for output loop

Vds = Vdd – IdRd

IdRd = Vdd – Vds

Id = (1/Rd)Vdd – (1/Rd)Vds

Whereas the two ends of the points of the line are determined by 2 conditions

  1. Vds = VdD; Id = 0
  2. Vds = 0; Id = (Vdd/Rd)

so Any point on the DC load line is called a Q Point.

If the gate to source voltage Vgs is less than Vt then, as a result, the drain current gets equal to zero and consequently, the MOSFET operates in the “Cutoff Region”.

If the gate to source voltage Vgs is slightly greater than Vt then, as a result, the drain to source current is not equal to zero and consequently, MOSFET is operating in the “Non-Saturation Region”.

Note:- whenever Vds = Vds(sat) = Vgs – Vt is called the “Transition Point”

CONCLUSION

I hope that you liked the article and it has helped you to clear your visuals regarding the topic. and if yes then, please make sure to share it with others and comment on the pat which you liked the most. besides, also feel free to comment down below if there’s any doubt. and above all suggest the next topic on which you would like to read next on.

Have a nice day 🙂

Regards.

Read more

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  2. Logic Families and its Features| TTL Vs CMOS
  3. What is a DC Machines?- DC Motors OR DC Generator.
  4. ALL ABOUT COMMUNICATION SYSTEM.
  5. MOSFET (Construction, Working and Characteristic)
  6. Control System(Features, Types etc)

4 Comments

  • Ashwin February 20, 2021 at 3:33 am Reply

    Nice 👍🏻

  • Ashwin February 20, 2021 at 3:33 am Reply

    Nice

  • Aayushi Singh March 1, 2021 at 11:12 am Reply

    Detailed and waiting for next

  • Yash March 1, 2021 at 11:18 am Reply

    Great work..

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