Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

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Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1

Transcript of Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Page 1: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Lecture Metal-Oxide-Semiconductor (MOS)

Field-Effect Transistors (FET)MOSFET

Introduction

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Page 2: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Goals

• Describe operation of MOSFETs.

• Define MOSFET characteristics in operation regions of 1. cutoff, 2. triode and 3. saturation.

• Develop mathematical models for i-v characteristics of MOSFETs.

• Introduce graphical representations for output and transfer characteristic descriptions of electron devices.

• Define and contrast characteristics of enhancement-mode and depletion-mode FETs.

• Define symbols to represent MOSFETs in circuit schematics.

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Page 3: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

MOS Field-Effect Transistors

– Primary component in high-density VLSI chips such as memories and microprocessors

Cha

nnel

I

I

Vcontrol

The control voltage determines the value of the current in the

channel I

Drain

Source

Gate

Symbolic View

How do we create the channel?

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Page 4: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Enhancement Type NMOS Transistor: Structure

• 4 device terminals: Gate(G), Drain(D), Source(S) and Body(B).

• Source and drain regions form pn junctions with substrate.

• vSB, vDS and vGS always positive during normal operation.

• vSB always < vDS and vGS to reverse bias pn junctions

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Page 5: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Definitions

• Vt: Threshold Voltage for MOS Transistor in general. The gate voltage required to form the channel between the source and drain

• VTN: Threshold Voltage for N-Channel Transistor.

• VTP: Threshold Voltage for P-Channel Transistor.

• The body (or bulk) current (iB) is always zero

• The gate current (iG) is always zero

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Page 6: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

NMOS Transistor: Qualitative I-V Behavior

• VGS<<VTN & VGS<<0 : No Channel for current-conduction (2 back-back diodes), Only small leakage current flows, iD ≈ 0 .

• 0<VGS<VTN: Depletion region formed under gate merges with source and drain depletion regions. No current flows between source and drain.

• VGS>VTN: Channel formed between source and drain. If vDS>0,, finite iD flows from drain to source.

OFF

OFF

ON

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Page 7: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

For vGS > VTN-- Triode Region Characteristics

Di

DSv

TNV

GSv

L

WnK

Di

DSv

DSv

2DS

vTNV

GSv

L

WnKDi

where, Kn= Kn’W/L

Kn’=μnCox’’ (A/V2)

Cox’’=εox/Tox

εox=oxide permittivity

(F/cm)

Tox=oxide thickness (cm)

for

TNV

GSV

DSV

TNV

GSV &

•A channel is induced between the source and drain, and current will flow•Electrons flow from the source to drainCurrent flows from drain to source

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Page 8: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

NMOS Transistor: Triode Region Characteristics (contd.)

• Output characteristics appear to be linear.

• FET behaves like a (gate-source) voltage-controlled resistor between source and drain with

TNV

GSV

LW

nKD

iDS

vonRchannel

'1)( resistance

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Page 9: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

MOSFET as Voltage-Controlled Resistor

Example 1: Voltage-Controlled Attenuator

TNVGG

VRnKRonRonR

svov

11

If Kn=500μA/V2, VTN=1V, R=2kΩ and VGG=1.5V, then,

667.0V15.12000

2V

μA5001

1

sv

ov

To maintain triode region operation,

V)15.1(667.0 S

v or V750.0S

v

TNVGG

Vov orTNVGS

vDS

v

If Kn=500μA/V2, VTN=1V, R=2kΩ and VGG=1.5V, then,

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Page 10: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

NMOS Transistor: Saturation Region

• If vDS increases above triode region limit, channel region disappears, also said to be pinched-off.

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Page 11: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

VGS > VTN, and VGD < VTN

VDS > VGS-VTN

Channel pinches off iD is independent on VDS

Saturation region

2))(('2

1TNGSnD VV

L

WKi

• If vDS increases above triode region limit, channel region disappears ( pinched-off).

• Current saturates at constant value, independent of vDS.

• Saturation region operation mostly used for analog amplification.

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Page 12: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

NMOS Transistor: Saturation Region (contd.)

2

2

TNVGS

vL

WnKDi

for TNVGS

vDS

v

TNVGS

vDSAT

v is also called saturation or pinch-off voltage

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Page 13: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Circuit Symbols for N-Channel MOSFET (Enhancement type)

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Page 14: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

I-V Characteristics for N-Channel MOSFET (Enhancement type)

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Page 15: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

iD

VT

Slope = KnVDSW/L

vGS

Small vds: triode region Higher vds; saturation region

iD-VGS characteristics

DSv

TNV

GSv

L

WnK

Di

2

2

TNVGS

vL

WnKDi

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Page 16: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Terminal Voltage Levels

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Page 17: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Depletion-Mode MOSFETS

• NMOS transistors with

• Ion implantation process used to form a built-in n-type channel in device to connect source and drain by a resistive channel

• Non-zero drain current for vGS=0, negative vGS required to turn device off.

0TNV

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Page 18: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Transfer Characteristics of MOSFETS

• Plots drain current versus gate-source voltage for a fixed drain-source voltage

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Page 19: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Enhancement-Mode PMOS Transistors: Structure

• P-type source and drain regions in n-type substrate.

• vGS<0 required to create p-type inversion layer in channel region

• For current flow, vGS< vTP

• To maintain reverse bias on source-substrate and drain-substrate junctions, vSB <0 and vDB <0

• Positive bulk-source potential causes VTP to become more negative

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Page 20: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Enhancement-Mode PMOS Transistors

Direction of current is opposite to n-channel

In figures (c) and (d) the body is connected to the source

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Page 21: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

Enhancement-Mode PMOS Transistors: Output Characteristics

• For , transistor is off.

• For more negative vGS, drain current increases in magnitude.

• PMOS is in triode region for small values of VDS and in saturation for larger values.

TPVGS

V

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Page 22: Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1.

DC –Analysis of n-channel MOSFET

])(2)[('2

1DSTNGSnD VVV

L

WKi

Check VGS

VGS< VTN

Cutoff region

iD=0

VGS> Vt

VDS < VGS –VTN

Triode regionVDS > VGS –VTN

Sat. region2))(('

2

1TNGSD VV

L

WKi

we start the analysis by assuming certain operating region

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