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BASICS -1

Hi guys welcome to basics part 2 today we are going to see about MOSFET. Before going to know about MOSFET you must know about transistor.

What is a Transistor?

A transistor is a semiconductor device which has 3 terminals. It is used for switching as well as amplification.It is a current controlled device

The three terminals of the transistor are:

  • Base
  • Emitter
  • Collector

Symbol of a Transistor

transistors

What is MOSFET?

MOSFET stands for Metal Oxide Silicon Field Effect Transistor. It is also a semiconductor device consists of 4terminals.It is also used for switching as well as amplification purposes. It is a voltage controlled device.

The four terminals of the MOSFET are

  • Gate
  • Source
  • Drain
  • Body

In General the fourth terminal substarte or body is shorted to source so we consider only three terminals Base,Source,Drain.

Here you might get one doubt why we go for MOSFET even the Transistor also doing the same work the answer is there are numerous reasons are there some of them are less size, less power,high input impedence,less noise etc.

MOSFET’S are classified into two types

  • Depletion Mode
  • Enhancement Mode

Again each mode consists of two types of MOSFET’s

  • PMOS
  • NMOS

Depletion Mode MOSFET

The Depletion Mode MOSFET has channel was constructed during manufacturing process. Beacuse of this channel it conduct between its drain to source when the gate voltage Vgs is 0 volts. Hence its called as normally ON Transistor

Enhancement Mode MOSFET

The Enhancement Mode MOSFET works as quite opposite to Depletion Mode when there is no voltage across the gate terminal then it doesn’t conduct. It conducts only when the gate terminal has maximum voltage applied

PMOS (Enhancement Mode)

In PMOS the source and drain terminals are P -type and substrate is N -type. PMOS will turned ON when the input is 0. Symbol of PMOS is shown below

pmos

NMOS (Enhancement Mode)

In NMOS the source and drain terminals are N -type and substrate is P -type. NMOS will turned ON when the input is 1. Symbol of NMOS is shown below

nmos

Both PMOS and NMOS works as opposite to each other.

NOTE: The terminals of MOSFET are interchangeable there is no fixed terminal for Source and Drain. The terminal which has higher potential acts as source & which has low potential acts as drain

Operating Regions of MOSFET

The MOSFETS has three operating regions.They are

  • Cut-off Region
  • Ohmic or Triode Region
  • Saturation Region

Let us discuss about each region in detail taking NMOS as example

Cut-off Region

As it name suggests that Cut-off it means it wont turned ON. Consider a NMOS which has gate voltage of 0 volts. In this condition it wont turned ON because Vgs ≥ Vth.

Don’t get scared while seeing those Vgs& Vth names I will don’t take you into derivations. Honestly speaking I’m also not like those derivation parts. Coming to topic

Vgs – Gate to Source Voltage

Vth – Threshold voltage

Threshold Voltage (Vth) is nothing but a minimum voltage required to turn ON the MOSFET. Every MOSFET has Threshold voltage if the Gate to Source Voltage (Vgs) is greater than or equal to threshold voltage then only the device will turned ON otherwise its in OFF state

Let us take one example

Consider the NMOS has Vthof 1v  VDD 5V Now apply (0V& 5V) to gate and see when NMOS is turned ON

Case I :

Vg =0V

Vgs = Vg -Vs     => 0 – 0 = 0V

Here, Vs is nothing but Voltage across at Source terminal i.e VSS = 0v

Vg is nothing but voltage across Gate terminal

Now check the condition

Vgs> Vth   => 0V ≥1V   (Condition fail )

Hence it is in Cut-off region

Case II:

Vg = 5V

Vgs = Vg -Vs     => 5 – 0 = 5V

Now check the condition

Vgs> Vth   => 5V ≥ 1V   ( Condition true )

Hence its not in Cut-off region

Ohmic or Triode region

The ohmic region is a region where the current (IDS)increases with an increase in the value of Vds. In this region the channel was created due tothe voltage between gate & source(Vgs) is greater than the threshold voltage( Vth)

When the Vgs> Vth the device is in triode region or saturation depending upon the Vdsvalue

Vds < Vgs– Vth

Then the device is in Triode region

Saturation region

When Vgs > Vth

Vds > Vgs– Vth

Then the MOSFET is in saturation region.In this region the current no more depends on Vds. In saturation region the current Id doesn’t increase with increase in Vds. But current can increase if you increase the gate to source voltage Vgs. We use this region for amplifiers.

Equations

                           PMOS                            NMOS
     Vsg < |Vt|            OFF        Vgs < Vt            OFF
  Vsd < Vsg – |Vt|          LINEAR     Vds < Vgs  -Vt          LINEAR
  Vsd > Vsg – |Vt|      SATURATION     Vds > Vgs – Vt       SATURATION

 

Transfer Characteristics of MOSFET

transfer_characteristics

Drain Characteristics of MOSFET

drain_characteristics

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