Chapter 6 Field Effect Transistors 6.1 Transistor Operation 6.2 The Junction FET 6.3 The...

Chapter 6

Field Effect Transistors

6.1 Transistor Operation6.2 The Junction FET6.3 The Metal-Semiconductor FET6.4 The Metal-Insulator-Semiconductor FET6.5 The MOS Field-Effect Transistor

• junction FET (JFET)• metal-semiconductor FET (MESFET)• metal-insulator-semiconductor FET (MISFET)

metal-oxide-semiconductor FET (MOSFET)

Field-effect Transistor(FET)

Dominant features of p-n junction 1. The injection of minority carriers with forward bias → BJT2. A variation of the depletion width W with reverse bias → FET

FET v.s. BJT

Comparison in terms of carrier type • The FET is a majority carrier device, and is therefore often called a unipolar transistor.• The BJT, on the other hand, operated by the injection and collection of minorpty carriers.

Comparison in terms of number of terminals • Like its bipolar counterpart, the FET is a three-terminal device in which the current

through two terminal is controlled at the third.

Comparison in terms of control type • Unlike the BJT, however, field-effect devices are controlled by a voltage at the third

terminal rather than by a current.

1. FET was proposed first in 1930 by Lilienfeld.2. Bardeen and Brattain invented the first bipolar transistor.3. This major breakthrough was rapidly followed by Shockley’s extension of

the concept to the BJT.4. The first MOSFET was demonstrated in 1960 by Kahng and Atalla.

History of Transistors

We can achieve large variations in vD by making small changes in vG. (voltage-controlled amplification)

We can switch from the bottom of the load line (iD = 0) to almost the top (iD ≈ E/R) by appropriate changes in vG. (voltage-controlled switching)

• The current ID flows through an n-type channel between two p+ regions.• A reverse bias between these p+ regions and the channel causes the depletion regions to intrude into the n material.

• The effective width of the channel can be restricted.• The end of the channel from which electrons flow is called the source.• The end toward which they flow is called the drain.• The p+ regions are called gates.• The voltage VG refers to the potential from each gate region G to the source S.

6.2.1 Pinch-off and Saturation

Without Gate Control

6.2.1 Pinch-off and Saturation

With Gate Control

• The effet of a negative gate bias VG is to increase the resistance of the channel and induce pinch-off at a lower value of current.

6.2.2 Gate Control

• Pinch-off is reached at a lower value of VD (and therefore a lower ID) when a negative gate bias is applied. (see next page)

negative) ( ]2

[)( 21GD

d

GD VqN

VLxW /)(

occurs) off-(pinch )( aLxW

aqN

V

d

P 1/2]2

[

2

2d

P

NqaV

DGGDP VVVV off)-(pinch

(from Eq. 5-57)

(Vp at pinch-off: a positive number, VG is zero or negative)

• Pinch-off is reached at a lower value of VD (and therefore a lower ID) when a negative gate bias is applied.

6.2.2 Current Voltage Characteristics

dx

dVxhZI x

D )(2

])(1[])(2

[)()( 2/12/1

P

Gx

d

Gx

V

VVa

qN

VaxWaxh

• Differential volume of the neutral channel material: Z2h(x)dx• Resistivity:

)(2;

xhZ

dx

A

dldR

A

lR

IdRdV

Current I does not change with distance along the channel

The half-width of the channel at point x depends on VGx

0

0

0

G

x

Gx

V

V

V2

2d

P

NqaV

xGGx VVV

dxIdVV

VVZaDx

P

Gx

])(1[2 2/1

])(3

2)(

3

2[ 2323

0//

P

GD

P

G

P

DPD V

VV

V

V

V

VVGI

dx

dVxhZI x

D )(2

L

aZGwhere o

2

Go is the conductance of the channel for negligible W(x), i.e., with no gate voltage and low values of ID


])(3

2)(

3

2[ 2323

0//

P

GD

P

G

P

DPD V

VV

V

V

V

VVGI

]3

1)(

3

2[

]3

2)(

3

2[)(

230

230

/

/.

P

G

P

GP

P

G

P

DPD

V

V

V

VVG

V

V

V

VVGsatI

P

G

P

D

V

V

V

V1

ncensconductamutual traV

VG

V

satIsatg

P

G

G

Dm ])(1[

)()( 21

0/.

.

negative) ( ,)1().( 2G

P

GDSSD V

V

VIsatI

0th current widrain saturated : GDSS VI

pGD

G

o

VVV

V

L

aZG

0

2

Valid only up to pinch-off

The saturation current is greatest when VG is zero and becomes smaller as VG is made negative.

Experimentally


]3

1)(

3

2[).( 2/3

0 P

G

P

GPD V

V

V

VVGsatI

Short channel effect

• Effect of : Electron velocity saturation at high fields may make this assumption invalid (the assumption of Go constant is invalid)

• Effect of L: The effective channel length decreases as the drain voltage is increased beyond pinch-off.

L

aZGo

2

Therefore, the assumption of constant saturation current is not valid for vary short-channel devices.

6.3.1 The GaAs MESFET

By using GaAs instead of Si, a higher electron mobility is available. (Appendix III)

(require doping)

6.3.2 The High Electron Mobility Transistor (HEMT)

Modulation doping

To grow a thin undoped well (e.g., GaAs) bounded by wider band gap, doped barriers (e.g., AlGaAs).The donors are in the AlGaAs rather than the GaAs, there is no impurity scattering of electrons in the wwll.Modulation doped field-effect transistor (MODFET)High electron mobility transistor (HEMT) 2DEGThe donors in the AlGaAs layer are purposely separated from the interface by ~100 Å.

• The conductivity can be increased by increasing the doping in the channel and thus the carrier concentration.• However, increased doping also causes increased scattering by the ionized impurities, which leads to a degradation of mobility.

6.3.3 Short Channel Effects

s

d

v1

v

ZhqNAqnI sdsD vv

The saturated current follows the velocity saturation, and does not require a true pinch-off in the sense of depletion regions meeting at some point in the channel.

The saturated current

The transconductance gm is essentially constant:The ID-VD curves are more evenly spaced if constant saturation velocity dominates, compared with the VG-dependent spacing for the long-channel constant-mobility case.

6.3.3 Short Channel Effects

1. The saturated current follows the velocity saturation, and does not require a true pinch-off in the sense of depletion regions meeting at some point in the channel.• Most devices operate with characteristics intermediate between the constant

mobility and the constant velocity regimes. Approximation required.

2. Another important short-channel effect is the reduction in effective channel length pinch-off as the drain voltage is increased.• This effect is not significant in long-channel devices, since the change in L

due to intrusion of the depletion region is a minor fraction of the total channel length.

• In the short-channel devices, however, the effective channel length can be substantially shortened, leading to a slope in the saturated I-V characteristic that is analogous to the Early (base-width narrowing) effect in BJTs.

Chapter 6 Field Effect Transistors 6.1 Transistor Operation 6.2 The Junction FET 6.3 The...

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Transcript of Chapter 6 Field Effect Transistors 6.1 Transistor Operation 6.2 The Junction FET 6.3 The...