ocw-ecc s13 introduction to field effect transistors...

Session 13

Introduction to Field Effect Transistors (FET)

Electronic Components and Circuits

Enrique San Millán / Celia López

Field Effect Transistors FET

OBJECTIVES• To know the structure of the device and the transistor • To know the structure of the device and the transistor

effect

• To know and distinguish different FET transistor types

• To understand the current-voltage characteristic of the device

• To identify the operating regions

Cut-off, Saturation, Ohm (triode)Cut-off, Saturation, Ohm (triode)

• To analyze FET circuits basic cases in DC

• To know small-signal model for FET transistors

2UC3M 2010 CCE - Session 13

Field Effect Transistors (FET)

Types Mode Channel

MOSFET(Metal-Oxide Semiconductor FET)

Enhancement/Accumulation

Channel N (or N-MOS)Channel P (or P-MOS)

Depletion Channel N (or N-MOS)Channel P (or P-MOS)

JFET(Joint FET)

Junction Channel NChannel P

Unipolar: only one carrier type (electrons in N-channel and holes in P-channel)

Basic characteristics

3

channel)

Control of Current (I) through Voltage (V) (BJT devices control I through I)

Three terminals:

Source (S): provides carriers

Gate (G): controls carriers flow

Drain (D): receives carriers

UC3M 2010 CCE - Session 13

Structure of Channel-n

Enhancement MOS Transistor D G S

MetalOxide (SiO2)

W

Drain (D)Gate (G) Source (S)

Metal Oxide (SiO2)

n+ n+Channelregion

Lp-typesubstrate

(Body) B

p-typesubstrate

(Body)

Drainregion

Sourceregion

n+ n+

Channelregion

L

4

D

S

BG

D

S

BG

D

S

G

D

S

G B

Body (B)


n-MOS Transistor OperationG

DS

VGS

inducedn-type

channel

p-type substrate

B

GND

e-

e- e-

e-

+u +u

G

DS

VGS

VDS

iD iG = 0 iS =iD

5

p-type substrateB

GND

inducedn-type channel

+u +u

iD


n-MOS Operation: Triode regioniD (mA)

0,4

0,3

vGS = Vt+2 V

vGS = Vt+1,5 V

G

DS

VGSVDS (small)

iD iG = 0iS =iD

vDS (mV)

0,2

0,1

0 50 100 150 200

vGS ≤ Vt

vGS = Vt+0,5 V

vGS = Vt+1 V

6

p-type substrate

B

GND

inducedn-type channel

+u +u

iD


n-MOS Operation: Saturation regioniD

SaturationTriode

vDS<vGS - vt vDS ≥ vGS - vt

G

DS

VGS

VDS

iD iG = 0iS =iD

vDS

vGS > vt

vDSsat = vGS - vt

7

p-type substrate

B

GND

n-channel

+u +u

iD


Current-Voltage Characteristic

VDS

+

-

iDiG = 0

Saturation regionTriode

region


iD (mA)

2,0 v = V +2,0

Triode region

[ ]2−−=

VGS

+

-

-

iD = iSvDS= vGS - vt

vDS (V)

1,5

2,0

1,0

0,5

0 1 2 3 4

vGS ≤ Vt (cutoff)

vGS = Vt+0,5

vGS = Vt+1,0

vGS = Vt+2,0

vGS = Vt+1,5

8

[ ]2)(2 DSDStGSD VVVVKi −−=

2)( tGSD VVKi −=

Saturation region (ID does not depend on VDS)

L

WCK oxnµ

2

1=


Output Characteristics: Saturation

iD (mA)

2,0

1,5

1,0

0,5

0 0,5 1 1,5 2 2,5 3

vDS ≥ vGS - vt

9

2)( tGSD VVKi −=

Saturation region (ID does not depend on VDS)

vGS (V)vt


Current-Voltage CharacteristicSaturation regionTriode

regioniD

vGS = Vt+2,0V

Slope = 1/ro

vDS

vGS ≤ Vt (cutoff)

vGS = Vt+0,5V

vGS = Vt+1,0V

vGS = Vt+1,5V

-VA = -1/λ

10

)1()(2

DStGSD VVVKi λ+−=

VA is the channel modulation voltage, producing a similar effect to the Early voltage in BJT

devices.

If we consider this effect then:

vGS ≤ Vt (cutoff)


p-channel MOSFET (P-MOS)

Drain (D)Gate (G) Source (S)Source (S) Gate (G) Drain (D)

Polysilicon GateOxide

Thick

In enhancement p-mos there is a

channel if: 0<≤ VV

p+ p+

p-type body

S

BG

S

BG

S

G

n+ n+

n-well

ThickSiO2

(isolation)

11

channel if:

triode region

saturation region

0<≤ tGS VV

tGSDS VVV −≥

tGSDS VVV −≤S D DS

D

G B


Depletion MOSFET

Equivalent behavior to enhancement MOSFET devices, except for

Manufactured channel:

When VGS<0 channel is reduced (e- leave Gate region)

When channel disappears (cut-off region)0<≤ VV When channel disappears (cut-off region)0<≤ tGS VV

D

S

BG

D

S

G

Saturation regionTrioderegion

vDS= vGS - vt

vDS<vGS - vt

vDS ≥ vGS - vt

iD (mA)

24

32

20

28

vGS = 2V (Vt+6)

vGS = 1V (Vt+5)

36

12

S

D

G B

vDS (V)

16

8

0 4 8 12

vGS ≤ -4 V (Vt)

vGS = -3V (Vt+1)

2 6 10 14

12

4vGS = -2V (Vt+2)

vGS = 0V (Vt+4)

vGS = -1V (Vt+3)


Summary

In enhancement n-MOS there is a

channel if:

in triode region

in saturation region

0>≥ tGS VV

tGSDS VVV −≤

tGSDS VVV −≥

In enhancement p-MOS there is a

channel if:

in triode region


0<≤ tGS VV

tGSDS VVV −≥

tGSDS VVV −≤in saturation regiontGSDS VVV −≥ in saturation region

tGSDS VVV −≤

iD

0 vGS

n-channel enhancement

p-channel enhancement

p-channel depletion

n-channel depletion

13

In depletion n-MOS there is a

channel if:

in triode region


0, <≥ ttGS VVV

tGSDS VVV −≤

tGSDS VVV −≥

In depletion p-MOS there is a

channel if :

in triode region


0, >≤ ttGS VVV

tGSDS VVV −≥

tGSDS VVV −≤

0 vGS


Junction Field Effect Transistor (JFET)

N channel P channelDrain

Gate

p-t

yp

e

Ga

te

n-type

Ch

an

ne

l

Drain

Gate

n-t

yp

e

Ga

te

p-type

Ch

an

ne

l

Drain

Gate

Gate

Drain

22/,)( pDSSpGSD VIKVVKi =−=

2

1

−=

p

GSDSSD

V

VIi

SourceSource

Gate

Source

Saturation regionTriode region

vDSsat= vGS - vp

vGS0 = 0

vGS1 = VGS0

Saturation

IDSIDS

Source

14

vGS4 < VP

vGS3 < VGS2

vGS2 = VGS1

Cutoff region

Saturation

region

Triode

region

vGS vp

VDSCutoff region


DC bias point:

Load line

SaturationTriodeID v = V

VDD

v1 +

vo = vDS

vGS=v1 -

RD

SaturationTriodeID

IDQ

Q

Load line slope = -1/RD

v = V

vGS = V1B

vGS < V1B

vGS > V1B

vGS = VDD

B

C

15

vGS=v1 -

vDS =vo0 VOB=VIB-VIt

vGS = …

vGS = V1Q

A

VDDVOC VOQ=VDSQ


FET transistor as amplifier:

VDD

voQ1

in triode region

Q1 in

saturation

Q1

cutoff

AVDD

Q

B

C

X

TimeVOQ=VOQ

VOB

V

vO

Slope at Q = voltage gain

16

VDD

VOC

Vt VIB=VOB+VtVIQ vivi


Small-signal model

+

vgs gmvgs

G D

+

vgs gmvgs

G D

ro

SaturationTriode

iD

vGS = Vt+2,0V

SaturationTriode

vDS= vGS - vt


iD (mA)

1,5

2,0 vGS = Vt+2,0

-

vgs gmvgs

S

-

vgs gmvgs

S

ro

17

Slope = 1/ro

vDS

vGS ≤ Vt

(cutoff)

vGS = Vt+0,5V

vGS = Vt+1,0V

vGS = Vt+2,0V

vGS = Vt+1,5V

-VA = -1/λ

vDS= vGS - vt

vDS (V)

1,5

1,0

0,5

0 1 2 3 4

vGS ≤ Vt (cutoff)

vGS = Vt+0,5

vGS = Vt+1,0

vGS = Vt+1,5


Small-signal parameters

+

-vgs gmvgs

G D

ro

)(2 tGS

VvGS

Dm VVK

V

ig

DSQds

−=∂

∂=

=

In MOSFETs

In JFETs

-

SiD

ID

An almost linear segment Slope = gm

id

tQ

18

−−=

∂

∂=

= p

GS

p

DSS

VvGS

Dm

V

V

V

I

V

ig

DSQds

12

In JFETs

0 vGSvt

vgs

vOV

t


Small-signal parameters

+v g v

G D

r

A

DD

DS

D

o V

II

v

i

r=≈

∂

∂= λ

1

+

-vgs gmvgs

S

ro

SaturationTriode

Slope = 1/r

iD

vGS = Vt+2,0V

19

D

Ao

I

Vr =

Slope = 1/ro

vDS

vGS ≤ Vt

(cutoff)

vGS = Vt+0,5V

vGS = Vt+1,0V

vGS = Vt+1,5V

-VA = -1/λ


Exercise:Operation regions. Saturation

EXAMPLE

|Vt| = 2 V

L=10um

W=100um

λ=02

/20 VuAC oxn =µ

VDD= 10 V

VD

R

ID

20

a) Design the circuit in order to obtain a ID=0.4mA

b) Calculate the value of R and voltage VD for that value of current


Exercise:Operation regions. Triode region

EXAMPLE

VDD = 5 V

K = 0.5 mA/V2

|Vt| = 1 V

VDD= 5 V

VD = 0,1 V

R

ID

21

a) Design the circuit for obtaining VD=0.1V

b) Calculate the equivalent resistance of the transistor in this case


Proposed exercise

ExampleVDD= +10V

K = 0.5 mA/V2

|Vt| = 1 V

RG2 = 10 MΩ

RG1 = 10 MΩ

RS = 6 kΩ

RD = 6 kΩ

22

a) Analyse this circuit and calculate the voltage in every node and the current in every line

b) Obtain the equivalent small-signal circuit


ocw-ecc s13 introduction to field effect transistors...

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