Dr. Nasim Zafar

Electronics 1 - EEE 231

Fall Semester – 2012

COMSATS Institute of Information TechnologyVirtual campus

Islamabad

Current -Voltage CharacteristicsI-V Characteristics

Lecture No. 29 Contents:

Qualitative theory of operation

Quantitative ID-versus-VDS characteristics

Large-signal equivalent circuits.

 

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Nasim Zafar. 3

Lecture No. 29

Current-Voltage CharacteristicsReference:

Chapter-4.2

Microelectronic Circuits

Adel S. Sedra and Kenneth C. Smith.

Circuit Symbol (NMOS)Enhancement-Type:

G

D

S

B

ID= IS

IS

IG= 0 G-GateD-DrainS-SourceB-Substrate or Body

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Circuit Symbol (NMOS)Enhancement-Type

The spacing between the two vertical lines that represent the gate and the channel, indicates the fact the gate electrode is insulated from the body of the device.

The drain is always positive relative to the source in an n-channel FET.

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Qualitative Theory of Operation

Modes of MOSFET Operation

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Modes of MOSFET Operation

MOSFET can be categorized into three modes of operation, depending on VGS:

VGS < Vt: The cut-off Mode

VGS > Vt and VDS < (VGS − Vt): The Linear Region

VGS > Vt and VDS > VGS − Vt: The Saturation Mode

MOSFET-Structure Enhancement Type-NMOSFET

pn+n+

metal

LW

SourceS

Gate: metal or heavily doped poly-Si G

DrainD

BodyB

oxide

IG=0

ID=ISIS

x

y

(bulk or substrate)

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VGS<0

n+p n+ Structure ID ~ 0

pn+n+

n++

LW

SourceS

GateG Drain

Dbody

B

oxide

+-

VD=Vs

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VGS < Vt The Cut-off Mode:

n+-depletion-n+ structure ID ~ 0

pn+n+

n++

LW

sourceS

gateG

drainD

bodyB

oxide

+-

+++

VD=Vs

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VGS > VT The Linear Mode of Operation: n+-n-n+ structure inversion

pn+n+

n++

LW

sourceS

gateG

drainD

bodyB

oxide

+-

+++++++++

- - - - -

VD=Vs

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VGS > VT

Quantitative ID-versus-VDS Relationships

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Quantitative ID-VDS Relationships

QN = inversion layer charge

VDV

G (VG)S D (VDS)

For VG < VT, Inversion layer charge is zero (Slide11).For VG > VT, Qn(y) = QG = Cox (VG V VT) (Slide12)

Nasim Zafar.

Quantitative ID-VDS Relationships

In the MOSFET, the gate and the channel region form a parallel-plate capacitor for which the oxide layer serves as a dielectric.

If the capacitance per unit gate area is denoted Cox and

the thickness of the oxide layer is tox, then

Cox=εox/ tox (4.2)

Where εox is the permittivity of the silicon oxide

ε= 3.9 ε0= 3.9×8.854×10-12= 3.45×10-11F/m

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Quantitative ID-VDS Relationships

Current and Current Density:

In general, Jn= q n n E , for the drift current

Here, current ID is the same everywhere, but Jn (current density) can vary from position to position.

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ynqnqJJ y d

dnnnn

E sincey

yd

d)(

E

Let “ ” be the potential along the channel

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Quantitative ID-VDS Relationships

To find current, we have to multiply the above with area, but Jny,n, etc. are functions of x and z. Hence,

areaunitcharge)()(d

d

dd

dddd

nnn

nnnD

/yQyQy

Z

xqny

ZxJZzxJI yy

Integrating the above equation, and noting that ID is constant, we get

d)(n0nDDS yQ

L

ZI

VSince we know expression for Qn(y) in terms of , we can integrate this to get ID

Current and Current Density:

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Quantitative ID-VDS Relationships

2

2DS

DSTGoxn

DV

VVVCL

ZI satDS,DS0 VV TG VV ;

ID will increase as VDS is increased, but when VG – VDS = VT, pinch-off of channel occurs, and current saturates when VDS is increased further. This value of VDS is called VDS,sat. i.e., VDS,sat = VG – VT and the current when VDS= VDS,sat is called IDS,sat.

2TGox

satD, 2VV

L

CZI

satDS,D VV TG VV ;

Here, Cox is the oxide capacitance per unit area, Cox = ox / xox

Current and Current Density:

Current-Voltage Characteristics

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Current-Voltage Characteristics

A

B C DIDS

VDS

The iD-VDS Characteristics

Figure 4.11(a) shows an n-channel enhancement-type MOSFET with voltages VGS and VDS applied and with the normal directions of current flow indicated.

Fig. 4.11 (a): An n-channel enhancement type MOSFET

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The iD-VDS Characteristics

Figure 4.11 (b) shows a typical set of iD-VDS Characteristics.

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The iD–vDS Characteristics for a MOSFET Device with k’n(W/L) = 1.0 mA/V2.

The iD-VDS Characteristics

Current-Voltage characteristics of Fig. 4.11 (b) show that there are three distinct regions of operation:

The Cutoff Region,

The Triode Region, and

The Saturation Region.

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The iD–vDS Characteristics for a MOSFET Device.

The iD-VDS Characteristics

The iD-VDS Characteristics

Saturation Region: The saturation region is used if the MOSFET is to operate as

an amplifier.

Cutoff and Triode Regions: For operation as a switch, the cut-off and triode regions are

utilized.

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Operation in the Triode Region

To operate the MOSFET in the triode region we must first induce a channel:

VGS Vt (Induced channel) ≧

VDS＜ VGS – Vt (Continuous Channel)

The n-channel enhancement-type MOSFET operates in the triode region when VGS is greater than Vt and the drain voltage is lower than the gate voltage by at least Vt volts.

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The iD-VDS Characteristics

The Triode Mode:

In the triode region, the iD-VDS characteristics can be described by the following equation:

ID = kn’(W/L)[(VGS-VT)VDS - 1/2VDS

2] (4.11)

Where kn’= μnCox is the process transcondctance parameter, its value is determined by the fabrication technology

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The iD-VDS Characteristics

The Triode Mode:

• If VDS is sufficiently small

• ID = kn’(W/L)[(VGS-VT)VDS] (4.12)

This linear relationship represents the operation of the MOSFET as a linear resistance rDS whose value is controlled by VGS.

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Operation in the Saturation Region

To operate the MOSFET in the Saturation Region we must first induce a channel.

vGS ≧ Vt (Induced channel) (4.16)

vGD Vt≦ (Pinched-off channel) (4.17)

vDS v≧ GS-Vt (Pinched-off channel) (4.18)

The n-channel enhancement-type MOSFET operates in the saturation region when vGS is greater than Vt and the drain voltage does not fall below the gate voltage by more than Vt.

The boundary between the triode region and the saturation region is characterized by

vDS= vGS-Vt (Boundary) (4.19)

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The iD-VDS Relationship

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Saturation Mode

In the Saturation region, the iD-VDS characteristics can be described by eq. (4. 20):

Nasim Zafar.

The iD–vGS characteristic

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The iD–vGS Characteristic for an NMOS Transistor in Saturation

Summary: MOSFET I-V Equations

The Cut-off Region: VGS< VT

ID = IS = 0

The Triode Region: VGS>VT and VDS < VGS-VT

ID = kn’(W/L)[(VGS-VT)VDS - 1/2VDS

2]

The Saturation Region: VGS>VT and VDS > VGS-VT

ID = 1/2kn’(W/L)(VGS-VT)2

Output Characteristics of MOSFET

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Large-Signal Equivalent-Circuit Model

In saturate mode, MOSFET provides a drain current whose value is independent of the drain-voltage VDS and is determined by the gate-voltage VGS

Thus, the Saturated MOSFET behaves as an ideal current source whose value is controlled by VGS according to the nonlinear relationship in Eq. (4.20).

Figure 4.13 shows a circuit representation of this view of MOSFET operation in the saturation region. Note that this is a large-signal equivalent-circuit model.

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Large-signal equivalent-circuit model of an n-channel MOSFET operating in the saturation region.

MOSFET Summary

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I-V Characteristics of MOSFET

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A majority-carrier device: fast switching speed

Typical switching frequencies: tens and hundreds of kHz

On-resistance increases rapidly with rated blocking voltage

The device of choice for blocking voltages less than 500V

1000V devices are available, but are useful only at low power

levels (100W)

MOSFET: Summary

MOSFET Summary

Importance for LSI/VLSI– Low fabrication cost– Small size– Low power consumption

Applications– Microprocessors– Memories– Power Devices

Basic Properties– Unipolar device– Very high input impedance– Capable of power gain– 3/4 terminal device, G, S, D, B– Two possible channel types: n-channel; p-channel

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MOSFET: Merits/ Demerits

Advantages• Voltage controlled device• Low gate losses• Parameters are less sensitive to junction temperature• No need for negative voltage during turnoff Limitations• One disadvantage of MOSFET devices is their extreme sensitivity to

electrostatic discharge (ESD) due to their insulated gate-source regions.

• The SiO2 insulating layer is extremely thin and can be easily punctured by an electrostatic discharge.

• High-on-state drop as high as 10V• Lower off-state voltage capability• Unipolar voltage device.

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