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ELECTRONIC DEVICES

Assist. prof. Laura-Nicoleta IVANCIU, Ph.D.

C13 – MOSFET operation

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Contents

➢ Symbols

➢ Structure and physical operation

➢ Operating principle

➢ Transfer and output characteristics

➢ Quiescent point

➢ Operating regions

➢ Examples

C13 – MOSFET operation

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Transistors

= active semiconductor devices, with three terminals

- used to amplify or switch signals

- essential components of electronic circuits

- discrete or integrated

Operating principle:

The voltage applied between two terminals (command) controls the current through the third terminal

Previously on ED (C11):

C13 – MOSFET operation

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SymbolsC13 – MOSFET operation

n-channel enhancement-type p-channel enhancement-type

general symbols

(w/ B substrate terminal)

simplified symbols

(B internally connected to S)

other symbols

Metal-oxide-semiconductor field effect transistors (MOSFETS)

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SymbolsC13 – MOSFET operation

D – drain (similar to C)

G – gate (similar to B)

S – source (similar to E)

n-channel enhancement-type p-channel enhancement-type

Metal-oxide-semiconductor field effect transistors (MOSFETS)

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Structure and physical operation

C13 – MOSFET operation

▪ creation of an n- type channel between the drain and source terminals

▪ existence of a positive potential difference between the drain and source to move the carriers

In order to have a drain-to-source current, two conditions must be fulfilled:

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Structure and physical operation

C13 – MOSFET operation

No possibility for the current to flow from drain to source

No channel

VTh – threshold voltage

0

0

=

=

D

DS

ThGS

I

V

VV

OPTIONAL

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Structure and physical operation

C13 – MOSFET operation

0

0

=

=

D

DS

ThGS

I

V

VV

OPTIONAL

Due to the electric field created by VGB > 0 (VGS > 0), the electrons in the substrate will be attracted and accumulated just under the oxide (gate region).

When VGS is increased further, the electron concentration becomes larger than the hole concentration; this process is called population inversion.

Inversion creates a conducting n-channel between the drain and the source.

The gate voltage VGS at which inversion produces an n concentration equal to the unbiased p concentration is called the threshold voltage VTh.

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Structure and physical operation

C13 – MOSFET operation

OPTIONAL

0

0

D

DSsatDS

ThGS

I

VV

VV

The current flows through the channel under the action of VDS >0. The transistor operates in the linear region.

The channel will became shallower at the drain end, because the electrons from the close vicinity of the drain region are attracted by the positive drain region.

The current depends linearly on VGS, but also depends on VDS.

( )

( ) 2

2

22

2

DSDSThGSD

DSDSThGSD

VVVVL

WKI

VVVVI

−−=

−−=

β – constructive parameter [μA/V2]K – transconductance parameter [μA/V2]W, L – physical dimensions of channel [μm]

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Structure and physical operation

C13 – MOSFET operation

OPTIONAL

The current depends non-linearly (quadratic) on VGS

0

D

DSsatDS

ThGS

I

VV

VV( ) ( )

( ) ( )22

22

21

2

1

ThGS

A

DSThGSD

ThGS

A

DSThGSD

VVL

WK

V

VVV

L

WKI

VVV

VVVI

−

+−=

−

+−=

The VDS voltage is greater than the VDSsat voltage.

All the electrons from the close vicinity of the drain region are attracted by the more positive drain region.

The channel depth becomes zero at the drain end. The transistor is now in the pinch-off region, or saturationregion or active region.

ID is almost independent of the VDS. Conduction from the source to drain still occurs with current passing through the depletion region next to the drain.

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Structure and physical operation

C13 – MOSFET operation

▪ VGS - conducting channel building (enhancement)

▪ VDS – controlled movement of the carriers (free electrons between D and S terminals)

The current depends on

),( DSGSD VVfI =

➢ The current through MOSFET

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Operating principleC13 – MOSFET operation

➢ n-channel enhancement type MOSFET

SD

G

SGD

ii

i

iii

=

=

=+

0PSDS

CoGS

Vv

vv

=

=

▪ Family of transfer characteristics

DSGSD vvi ,)(

▪ Family of output characteristics

GSDSD vvi ,)(

as parameter

as parameter

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Transfer and output characteristics

C13 – MOSFET operation

➢ Transfer characteristics

DSGSD vvi ,)( as parameter

DSsatDS Vv ])(2[2

DSDSThGSD vvVvi −−=

• Linear region (exc)

iD – linear dependence on vGS

- also influenced by the output voltage vDS

VTh= 0.58 V, β = 104 μA/V2

ThGSDSsat VvV −=

2)( ThGSD Vvi −=

• Saturation region (aF)

DSsatDS Vv

iD – quadratic dependence on vGS

drain-source saturation voltage

vDS< vDSsat

active (saturation)

region

linear region

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Transfer and output characteristics

C13 – MOSFET operation

➢ Output characteristics

GSDSD vvi ,)( as parameter

DSsatDS Vv ])(2[2

DSDSThGSD vvVvi −−=

• Linear region (exc)

iD – linear dependence on vGS

- also influenced by the output voltage vDS

2)( ThGSD Vvi −=

• Saturation region (aF)

DSsatDS Vv

iD – quadratic dependence on vGS

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Quiescent pointC13 – MOSFET operation

IDex

Quiescent point Q = a point on the output characteristic iD(vDS) of MOSFET

▪ Q is defined by VDS and ID

▪ Q(VDS, ID) is at the intersection between the load line and the output characteristic corresponding to vGS

Load line:

vDS = VPS - RD iD

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Operating regionsC13 – MOSFET operation

Modes of useswitching mode: (off) / (exc)

as an amplifier: (aF)

voltage-controlled linear resistance (small vDS)

T – (aF): VTh < VGS < VGS3

T – (off): VGS < VTh

T – (exc): VGS > VGS3

])(2[2

DSDSThGSD vvVvi −−=

2)( ThGSD Vvi −=

0=Di

IDex

Boundary (aF) - (exc): vDS,sat = vGS - VTh

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Operating regionsC13 – MOSFET operation

Linear region (exc):

])(2[2

DSDSThGSD vvVvi −−=

Saturation (active) region (aF):

2)( ThGSD Vvi −=

L

WK

2= ( )2

2ThGSD Vv

L

WKi −=

For integrated transistors:

In the active region

K – transconductance parameter [µA/V2]W - the width of the channel [µm]L - the length of the channel [µm]

β – parameter of the MOSFET - beta factor

- measured in µA/V2, mA/V2, A/V2

- constructive parameter

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➢ Example - 1

Compute the values of VGS, ID, VDS, VGD, VDSsat for:i) vCo = 0.5 V; ii) vCo = 2.5 V; iii) vCo= 2.75 V.

Place the quiescent points Q(VDS, ID) in the plan of output characteristics.

Find the operating regions of T.

ExamplesC13 – MOSFET operation

V5

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➢ Example - 2

a) What is the operating region of T for:i) vGS = 0.8 V; ii) vGS = 2.5 V; iii) vGS= 4 V.

b) What is the minimum value of vGS so that T stays in (exc)?

ExamplesC13 – MOSFET operation

VPS

Β = 2 mA/V2

VTh = 1 V

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➢ Example - 2

a)

i) Because vGS < VTh => T - (off)

ii) vGS > VTh T is on, in (aF) or in (exc)

▪ Assume T in (aF), so that iD = β(vGS – VTh)2

▪ Compute vDS

▪ Compare vDS with vDSsat :

If vDS > vDSsat, the assumption was true, and T is in (aF)

If vDS < vDSsat, the assumption was false, and T is in (exc)

ExamplesC13 – MOSFET operation

VPS

Β = 2 mA/V2

VTh = 1 V

Solution:

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➢ Example - 2

a) ii)

ExamplesC13 – MOSFET operation

VPS

Β = 2 mA/V2

VTh = 1 V

Solution:

iD = 2(2.5 -1)2= 4.5mA

vDS = VPS - RD iD

vDS = 20 - 3·4.5 = 6.5 V

vDS sat= vGS – VTh = 2.5 - 1= 1.5 V

vDS > vDssat, T is in (aF)

vGD = vGS – vDS = 2.5 - 6.5 = -4V < VTh

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➢ Example - 2

a) iii) vGS > VTh T is on, in (aF) or in (exc)

▪ Assume T in (aF), so that iD = β(vGS – VTh)2

▪ Compute vDS

▪ Compare vDS with vDSsat :

ExamplesC13 – MOSFET operation

VPS

Β = 2 mA/V2

VTh = 1 V

Solution:

vDS = 20 - 3·18 = -34 V

vDS sat= vGS – VTh = 4 - 1= 3 V vDS < vDssat, T is in (exc)

iD = 2(4 -1)2= 18 mA

Alternative method: compare the value of iD in (aF) with iDex

mA67.63

20,==

−=

R

V

R

vVi PSexDSPS

Dex iD = 18 mA > iDex = 6.67 mA T - (exc)

What is the actual value of iD?

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➢ Example - 2

b) Minimum value of vGS so that T stays in (exc)

ExamplesC13 – MOSFET operation

VPS

Β = 2 mA/V2

VTh = 1 V

Solution:

The operating point of T is placed on the vDSsat curve:

vDSsat = VPS – RiD

vDSsat = vGSmin - VTh

R β(vGSmin – VTh)2+ (vGSmin - VTh) - VPS= 0

Two solutions for vGSmin, choose the convenient one: vGSmin >VTh

VGSmin = 2.744 V

VPS – RiD = vGSmin – VTh

iD = β(vGSmin - VTh)2

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➢ Example - 3

ExamplesC13 – MOSFET operation

a) Determine the operating region of the transistor for:i) R1 = 4.5 MΩ; R2 = 0.5 MΩ; ii) R1 = 12 MΩ; R2 = 6 MΩ;

b) What can be the range for R1 with R2 = 0.5 MΩ so that the transistor is (on)?

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SummaryThe MOSFET is no longer a random acronym, after digging into:

➢ Symbols➢ Structure and physical operation➢ Operating principle➢ Transfer and output characteristics➢ Quiescent point ➢ Operating regions➢ Examples

Next week: Recap. Preparation for exam.

To do: Homework 10

C13 – MOSFET operation