Metal-Semiconductor Junction Basic MESFET Operation · Metal-Semiconductor Junction Basic MESFET Operation . Field Effect Transistors 2014-01-28 Lecture 6, High Speed Devices 2014

Lecture 6: III-V FET DC I - MESFETs

2014-01-28 1 Lecture 6, High Speed Devices 2014

Metal-Semiconductor Junction

Basic MESFET Operation

Field Effect Transistors


N+ N+

Source Drain

• The gate electrode controls the carrier concentration in the channel • Source/Drain set the potential at the source/drain side • Electrons flow from source to drain IDS and n(x,y) depend on geometry

and transport properties. • 2D problem (in x and y)

W

Lg

VDS

Vg

x

y

Gate

Field Effect Transistors


p-type

n+ n+

Bulk MOSFET

n n+ n+

p-type, S.I. Insulating

MESFET

Metal

Oxide

n- n+ n+


SOI, Quantum Well MOSFET Metal

Oxide

Oxide or wide bandgap Semicondudctor

Metal Depletion region

Vg=1V

VD=1V

Vg=-1V

n- n+ n+


HEMT Metal

Wide band semiconductor

Wide band semiconductor

Si vs. III-V Field Effect Transistors (FETs)


Si: µn≤ 1300 Vs/cm2. vsat ≈ 8×106 cm/s

InGaAs µn ≈ 14000 cm2/Vs! vsat ≈ 2×107 cm/s

SiO2-Si excellent interface

InGaAs- GaOxInOxAsOx – poor interface

Si p-type

n+ n+

SiO2/HfSiO2

An Si MOSFET uses an oxide (SiO2, SiHfO2) to isolate the gate

n+ n+

Schottky Barrier

GaAs n

GaAs Semi-Insulating

A III-V Metal-Semiconductor FET (MESFET)

• III-V MOSFETs are difficult to fabricate

• Alternative: Semiconductor to isolate the gate from the channel

• Simple: MESFETs • Better: HEMTs or III-V

MOSFETs

Metal-Semiconductor Junction


qfm

qcs

qfn Fb fbi

q

qqq

nbbi

smb

F

ff

cffSchottky barrier height Build in potential

However, now we ignored that we terminated the crystal and created a lot of surface states....

Too simplistic!

Fb

Similar to a p+N junction!

Metal-Semiconductor Junction II


q

nbbi

F ff

Experiments show only a very weak dependence of fb as a function of metal work function

Surface reconstruction and surface defects create a large number of states in the bandgap, which “pins” the Fermi-energy The energy position of these surface states sets the Schottky barrier height. For GaAs, fb≈0.7-0.8V InP fb ≈ 0.3V, InAs fb ≈ -0.1V, In0.53Ga0.47As fb ≈ 0.1V

bqf

fb is a material parameter!

fm

fb

MESFET Structure – simplest FET transistor


Semi-insulating

Vgs (negative) Source Gate Drain

x

y

a Nd

•Schottky depletion under gate modulates channel thickness b

•VDS causes current to flow from source to drain, which can be

modulated by Vgs

b

Depletion Region: n≈0

Metal-Semiconductor Junction III


Depletion thickness, maximum Xdep=a

2

2yyXN

qy

yXNq

y

Nq

dy

d

depd

s

s

depd

s

d

s

f

0)0( sf

abi

d

sdep V

qNX f

2

2

002

aqN

s

d

f

Potential needed to fully deplete down to a

y

Xd

ep

a

r

0)()( depdeps XX

dy

d

f

Ref. potential

2 minute exercise – part 1


Vgs=0 Vgs=-1V A B

C D

Black – Vg = 0V Which green plot corresponds to Vgs=-1 V?

Remember: negative bias increases the potential energy of an electron!!

Depletion Edge

2 minute exercise – part 2


-

+ Vsub

Vgs Vgs=-1V Vsub=1V

Black plot – Vg and Vsub = 0V Which red plot corresponds to Vgs=-1 V and Vsub = 1V?

Depletion egdes

MESFET Operation


DVgs

-

+ VDS Vgs

DVgs

VDS

DVgs

The potential under the gate is set by the channel-gate potential Vcs(x)

Id

Vcs=Vds Vcs=0

Id

Resistive voltage drop along channel

Vcs(x)

Calculation of the current


-

+ VDS Vgs

Id

𝐽𝑛 = 𝑞𝜇𝑛𝑛𝛻𝑉 𝛻 ∙ 𝐽𝑛 = 0

𝜕2𝜙

𝜕𝑥2+𝜕2𝜙

𝜕𝑦2= ∆𝑉 =

−𝑞

𝜀𝑠𝑁𝑑 − 𝑛

y x

Complicated 2D problem!

GCA – Gradual Channel approximation: d2f/dx2<<d2f/dy2

fs(x,0)=0

𝜕2𝜙

𝜕𝑦2=−𝑞

𝜀𝑠𝑁𝑑

𝐽𝑛 = 𝑞𝜇𝑛𝑛𝜕𝜙(𝑥, 𝑏)

𝜕𝑥

𝑑𝐽𝑛(𝑥)/𝑑𝑥 = 0

Xdep(x) varies slowly with x Electric field in x-direction is ‘small’

Drift Current


Dchch

ndch

Ixix

i

xbxqWNxi

0

x

xx s

f

00f

f xaX s

dep

00

1f

f xaXaxb s

dep

)(xVVx csgsbis ff

Gradual channel approximation b(x) varies slowly with x b(x) is determined by solving fs(y)

Drift current

Continuity

Depletion

E-field

L

xs

snd

L

x

L

x

ssndD daqWNdx

x

xxaqWNdxI

f

ff

f

f

f

f

f

0000

11

Saturation Voltage, Pinch-off


Vds=0V Vds>0>VDS,sat Vds=VDS,sat Vds>VDS,sat

At pinch-off, the depletion region reaches the S.I region Our 1D-decoupled model breaks down: (d2f/dx2>>0) we need to solve 2D possion equation, Dfs(x,y) (numerical solutions needed) Result: Channel of finite thickness forms at channel edge. Increased Vds drops inside this region, or between channel-drain. Current remains independent of Vds after Vds,sat

gsbisatDS VV ff00,

0 0.5 1 1.5 2 2.5 30

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

Vds

Ids -

arb

units

Current-Voltage Characteristics

2014-01-28 15 Lecture 6 High Speed Devices 2014

000000

0)(

f

f

f

f

f

f Lds

xxu sss

2/32/300

3

2

3

2sdsd

L

aqWNI nd

D

f00f

f gsbi Vs

satDSDS

satDSgsbi

satDSDS

DSgsbi

VVVV

d

VVVV

d

,

00

,

,

00

1

f

f

f

f

2

002

aqN

s

d

f

00

00,

ff

ff

biT

gsbisatDS

V

VV Pinch-off voltage Threshold Voltage

Vgs=0

Vgs<0

Vgs=-|VT|

MESFET limitations


2/300

,3

2

3

1ss

L

aqWNI nd

satD

f

00

,1

f

f gsbind

gs

satD

m

V

L

aqWN

dV

dIg

• We want a high gm – but:

• Positive gate-voltage – very high gate leakage!

• From electrostatics: a<L/3 • Increase ND – but this lowers µn due to impurity scattering and increases

gate leakage! • Increase µn by material choice: Need to have a high Schottky barrier!

(InGaAs, InAs can’t be used!)

• We can do better using heterostructures or MOSFETs!

Documents

Metal-Semiconductor Junction Basic MESFET Operation · Metal-Semiconductor Junction Basic MESFET Operation . Field Effect Transistors 2014-01-28 Lecture 6, High Speed Devices 2014