Basics of Mosfet

8/4/2019 Basics of Mosfet

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MOS Transistor (MOSFET)

n+ n+

p-

SOURCE DRAIN

GATE

SUBSTRATE

GATE OXIDE

CHANNEL


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MOS Transistor Operation

Channel is normally non-conducting (open-circuit):No bias voltage (or small voltage) applied to Gate

Source and Drain junctions are reverse-biasedrelative to the substrate

Depletion regions surround the Source and Drain

Channel is depletedof mobile charge carriers

Channel starts to conduct when:Gate voltage VGS exceeds the Threshold VoltageVTMobile charge is present in the channel


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p Substrate

S DG

VDS +

_

Source Depletion

Region

Drain Depletion

Region

mA

IDS = 0

n+ n+


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p Substrate

S DG

VDS +

_

mA

VGS < VT

+_

Channel

in Depletion

IDS = 0

n+ n+


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MOSFET Operation 0 < VGS < VT

Channel in DepletionMajority carriers in p-type substrate (holes) are

repelled by positive charge on the Gate

No current flows in the channel VGS VT

Channel in Inversion

Electrons (minority carriers in substrate) are drawninto channel by positive charge on the Gate

Current flows in the channel


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p Substrate

S DG

VDS +

_

mA

VGS < VT

+_

Channel

in Depletion

IDS = 0

n+ n+


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p Substrate

S DG

VDS +

_

mA

VGS VT

+_

Channel

in Inversion

IDS 0

n+ n+


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Mobile charge concentration in the channel isproportional to that part of the gate-source voltagewhich exceeds the threshold voltage VT, that is by

(VGS - VT). In this way the gate-source voltagecontrols the drain-source current.

MOS TRANSISTOR MODEL

Capacitance of gate-oxide-substrate capacitor is CG

Mobile charge (electrons) in the channel is given by:

( )TGSG VVCQ =


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Formation of the Inversion Layer

GATE OXIDE SILICON

+ve

Charge

-ve

Charge

NA

z

Charge Density

GATE OXIDE SILICON

+veCharge

-ve

Charge

NA

z

Charge Density

Inversion

Layer

(Electrons)


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is mean transit timefor electrons to cross the channel

All mobile charge in the channel is swept through the

drain once per transit time; hence drain current is:

( )

=

= TGSGDSVVCQ

I

average drift velocity of the electrons in the channelunder the influence of an electric field E is:

Ev =

is the mobilityor velocity per unit field strength


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Let the length of the channel (distance from source todrain) be L; then if the velocity v is constantthroughout the channel we can write:

=Lv

Assume E is uniform along the entire length of the

channel, and that it is created by a voltage VDSbetween drain and source, then:

L

V

EDS

=

[Note: It will be explained later that this condition is only

fulfilled if VDS


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The transit time is

DS

2

V

L

=

Substitute into previous equation for IDS:

( ) DSTGS2G

DS VVV

L

CI

=

Gate capacitance CG = Cox.WL, where Cox is the gateoxide capacitance per unit area and WL is the area of

the channel. Therefore:

( ) DSTGSoxDS VVVL

WCI =


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MOS Transistor

n+ n+

L

W


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For VDS


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Triode Region 0 < VDS < (VGS - VT)

( )

= 2

VVVVL

WCI

2

DSDSTGSoxDS

The voltage in the channel varies between Source andDrain now that VDS is getting bigger; as a result the

charge concentration in the channel also varies withdistance from the Source

This equation reduces to the earlier one if VDS is small.


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Saturation Region VDS (VGS - VT)

( )2

TGSoxDS VVL

WC2

1I =

When VDS reaches (VGS - VT) the end of the channelclosest to the drain pinches off and the drain currentsaturates.

The amount of charge induced in the channel cannotbe increased beyond this level.


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MOS Channel Pinch-Off

n+ n+

Electron

Density

Depletion

Region

Inversion

Layer

S

G

D


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MOSFET Transfer Characteristic

0

20

40

60

80

100

0 1 2 3 4 5

VGS (V)

IDS

(A)

VT


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MOSFET Drain Characteristics

VDS (V)

0

ID

S

(A)

2 4 6 8 10 12

200

400

600

(VGS VT) = 4V

3V

2V

1V

Triode

RegionSaturation

Region

800


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MOSFET Transfer Characteristic

VDS = 10V

T

Gate-Source Voltage (V)

0.00 2.50 5.00 7.50 10.00

DrainCurre

nt(A)

0.00

5.43

10.86


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MOSFET Drain CharacteristicsT

Input voltage (V)

0.00 2.50 5.00 7.50 10.00

Current

(A)

0.00

6.00

12.00VGS = 10V

VGS = 8V

VGS = 6V

VGS = 4V


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MOSFET Circuit SymbolsD

G

S

Substrate

n-channel

Enhancement Mode

MOSFET

D

G

S

Substrate

p-channel

Enhancement Mode

MOSFET


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MOSFET TransconductanceThe transconductance parameter gm is defined

as:

GS

DSm

V

Ig

=

Enhancement MOSFET in saturation:

( ) ( )2TGS2TGSoxDS VV2VVLWC21I ==

L

W

Cox=Where


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MOSFET Transconductance

( )TGSGSDS

m VVV

I

g =

=

( ) =DS

TGS I2VV

DSm I2g =


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Basics of Mosfet

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