Metal  Oxide  Semiconductor  Field-­‐Effect    Transistors  (MOSFETs)  :  Basics  

 

ESc201A  :    IntroducAon  to  Electronics  L16

rhegde Dept. of Electrical Engineering

IIT Kanpur

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Introduction

Classification of MOSFET

Widely used in IC circuits

§ P channel ü Enhancement type ü Depletion type

§ N channel ü Enhancement type ü Depletion type

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MOSFET

Metal Oxide Semiconductor Field Effect Transistor

An NMOSFET

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Device Structure of Enhancement-Type NMOS

L: 1 to 10 µm W: 2 to 500 µm Thickness of oxide layer: 0.02 to 0.1 µm

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Device Structure of Enhancement-Type NMOS

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Symbols

NMOSFET

(a)  Circuit symbol for the n-channel enhancement-type MOSFET.

(b)  Modified circuit symbol with an arrowhead on the source terminal to distinguish it from the drain and to indicate device polarity (i.e., n channel).

(c) Simplified circuit symbol to be used when the source is connected to the body or when the effect of the body on device operation is unimportant.

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Symbols

NMOSFET

PMOSFET

Mostly Used

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Physical Operation

•  Creating an n channel

•  Drain current controlled by vDS

•  Drain current controlled by vGS

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Creating a Channel for Current Flow

Ø The enhancement-type NMOS transistor with a positive voltage applied to the gate.

Ø  An n channel is induced at the top of the substrate beneath the gate.

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Drain Current Controlled by Small Voltage vDS

Ø An NMOS transistor with vGS > Vt and with a small vDS applied.

Ø The channel depth is uniform.

Ø The device acts as a resistance.

Ø The channel conductance is proportional to effective voltage.

Ø Drain current is proportional to (vGS – Vt) vDS.

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vDS is increased

Ø Operation of the enhancement NMOS transistor as vDS is increased.

Ø The induced channel acquires a tapered shape.

Ø Channel resistance increases as vDS is increased.

Ø Drain current is controlled by both of the two voltages.

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Channel Pinch- Off

•  Channel is pinched off Ø Inversion layer disappeared at the drain point Ø Drain current is n’t disappeared

•  Drain current is saturated and only controlled by the vGS

•  Triode region and saturation region

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Drain Current Controlled by vGS

•  vGS creates the channel.

•  Increasing vGS will increase the conductance of the channel.

•  At saturation region only the vGS controls the drain current.

•  At subthreshold region, drain current has the exponential relationship with vGS

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Transfer Characteristics

VGS(V)

ID(mA)

Vt: Threshold Voltage

For VDS ≥ VGS – VT

N-MOSFET

Cut Off

‘On’

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I-V Characteristics

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Regions of Transistor ‘Operation’

•  Cut off region (vGS < Vt ) –  Input voltage less than threshold voltage

vGS(V)

iD(mA)

VT: Threshold Voltage

N-MOSFET

Cut Off

‘On’

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Regions of Transistor ‘Operation’

•  Triode region (vGS > VT and vDS < vGS – VT) –  Linear relationship between iDS and vDS reflects resistive

behaviour for small vDS

vGS(V)

iD(mA)

VT: Threshold Voltage

N-MOSFET

Cut Off

‘On’

GS T

DS GS T

v Vv v V

>

< −

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Regions of Transistor ‘Operation’ •  Saturation region (vGS > VT and vDS ≥ vGS – VT)

–  Transistor is ‘on’ –  Drain bias is above saturation voltage –  Amplifier should operate in this region

iD(mA)

VT: Threshold Voltage

N-MOSFET

Cut Off

‘On’

GS T

DS GS T

v Vv v V

>

> −

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Saturation region (vGS > VT and vDS > vGS – VT)

The current iDS begins to saturate as vDS approaches the value of (vGS − VT).

The saturation region of MOSFET Operation

MOSFET operates in saturation region when following two conditions are met:

GS T

DS GS T

v Vv v V

>

> −

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Different values of vGS (> Vt ) provides different iDSand vDS Characteristics

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The Switch Current Source MOSFET Model

MOS Device Open State Closed State

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The Switch Current Source MOSFET Model When vGS > VT and vDS > vGS – VT the amount of current provided by the source is

( )22D GS TKi v V= −

'where n n oxWK k CL

µ= = W: gate width; L: gate Length

Unit of K: A/V2

k’n: Constant related to MOSFET properties (A/V2) µn: Electron mobility in channel Cox: Capacitance per unit area of parallel plate capacitor by gate electrode and channels

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Example-1: Determine the current iDS for the circuit shown below. Assume: K= 1mA/V2 and VT = 1V

( )22D GS TKi v V= −

( )21 2 12Di = − 0.5 mADi =

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Example-2: Assume: K= 1mA/V2 and VT = 1V

What should be the minimum value of the drain to source vDS for which MOSFET will operate in saturation region. (Assume VGS is 2V)

+ _

For the MOSFET to operate in saturation:

GS T

DS GS T

v Vv v V

>

> − 1 VDSv >

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Example-3: Assume: K= 1mA/V2 and VT = 1V

What is maximum value of vGS for which MOSFET will operate in saturation region

+ _

For the MOSFET to operate in saturation:

GS T

DS GS T

v Vv v V

>

> −5 1

6 VGS

GS

vv> −

<

1 VGSv > 1 V 6 VGSv< <

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p Channel Device

•  Structure of p channel device Ø The substrate is n type and the inversion layer

is p type. Ø Carrier is hole. Ø Threshold voltage is negative. Ø All the voltages and currents are opposite to

the ones of n channel device. Ø Physical operation is similar to that of n channel

device.

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Complementary MOS or CMOS

Ø The PMOS transistor is formed in n well.

Ø Another arrangement is also possible in which an n-type body is used and the n device is formed in a p well.

Ø CMOS is the most widely used of all the analog and digital IC circuits.

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Large Signal Equivalent Circuit Model for NMOS

Large Signal Equivalent circuit model of the n-channel MOSFET in saturation, incorporating the output resistance ro. The output resistance models the linear dependence of iD on vDS