lecture4contactresistance slides tlm importante.pdf

8/9/2019 Lecture4ContactResistance slides tlm importante.pdf

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ECE 4813 Dr. Alan Doolittle

ECE 4813Semiconductor Device and Material

Characterization

Dr. Alan Doolitt le

School of Electrical and Computer Engineering

Georgia Institute of Technology

As with all of these lecture slides, I am indebted to Dr. Dieter Schroder from Arizona State

University for his generous contributions and freely given resources. Most of (>80%) the

figures/slides in this lecture came from Dieter. Some of these figures are copyrighted and can befound within the class text, Semiconductor Device and Materials Characterization. Every serious

microelectronics student should have a copy of this book!


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Contact Resistance

Metal-semiconductor Contacts

Specific Contact ResistanceTransfer Length

Contact String

Transfer Length MethodFour-terminal Method


3/25


Metal Semiconductor Contacts

MB

Every semiconductor device has contacts Contact resistance is a parasit ic resistance

Contacts are almost always metal-semiconductor contacts

qM q

Sq

qB

EC

qVbi

EF

EV

W

Accumulat ion Neutral Depletion

FBFCs )( EE +=


4/25


Metal Semiconductor Contacts

Energetic barriers are most often described by the Electron Affin ity Model (EAM).

EAM is a purely theoretical model and REAL CONTACTS

RARELY FOLLW THIS MODEL.

Nevertheless, EAM explains several aspects (depletion

widths, capacitance, general IV shape, etc) of ohmic andSchottky barrier diodes suff iciently that it is widely used.

Reasons for the barrier height not agreeing with theory:

Image force lowering (lower than expected barrier)

Fermi-level pinning

Interface state filling (i.e. partially pinned or voltage

dependent interface state occupation).

Lack of complete model understanding. Can you

improve on the existing model?


5/25


Carrier Motion In an ohmic contact, electrons must flow from metal to

semiconductor or f rom semiconductor to metal

Current is controlled by barrier widthrather than barrier

height

Low ND Intermediate ND High ND

Field

Emission

Thermionic/Field

Emission

Thermionic

Emission

I

V

FE

TFE

TE


6/25


Energy band diagrams for ideal MS

contacts

M >

S

M S

After the contact formation, electrons will begin to flow fromthe semiconductor to the metal.

The removal of electrons from the n-type material leavesbehind uncompensated Nd

+donors, creating a surfacedepletion layer, and hence a built-in electr ic f ield (similar top+-n junction).

Under equilibrium, the Fermi-level will be constant and noenergy transfer (current) flows

A barrierB forms blocking electron flow from M to S.

Based on the Electron Affini ty Model (EAM), the simplest ofmodels used to describe MS junctions,

B = M ...

ideal MS (n-type) contact.

B is called the barrier height . Electrons in a semiconductor wil l encounter an energy

barrier equal toM Swhile flowing from S to M.


8/25ECE 4813 Dr. Alan Doolittle

Since MS Schottky diode is a majority carrier device (i.e only majority carriers are

injected from semiconductor to the metal) and thus has no minority carrier storage, the

frequency response of the device is much higher than that of equivalent p+n diode.

The turn on voltage of a Schottky diode is typically smaller than a comparable p-njunction since the barrier to forward current flow (m-s) is typically small. This turn

on voltage can be as small as 0.3 Volts in some Si Schottky diodes.

This makes a Schottky diode the best choice for power switch protection in inductive

load applications (motors, solenoids, coils, etc) and in high frequency rectification but

not a good choice when low leakage or high breakdown voltage is required.

I-V Characteristics



I-V Characteristics

Leakage in a Schottky diode is dominated by:

1) Thermionic Emission (metal electrons emitted over the barrier not likely)

2) Thermionic Field Emission (metal electrons of higher energy tunneling through

the barrier more likely)

3) Direct tunneling (metal electrons tunneling through the barrier most likely inhigher doped semiconductors or very high electric fields).

Since generation does not require the entire bandgap energy to be surmounted, the

reverse leakage current for a Schottky diode is generally much larger than that for a

p+n diode. Likewise, breakdown (for the same reason) is generally at smaller

voltages.



MS (n-type) contact with

M > S

A forward bias will reduce the barrier

height unbalancing the electron current

flow, resulting in a huge forward current

that increases exponentially with applied

voltage

A reverse bias will increase the barrier

height resulting in a small reverse

current flow that will be dominated bytunneling currents for high doped

semiconductors and/or thermally assisted

field emission for moderate/low doped

semiconductors.


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Generalization of Metal Semiconductor

Contact Energy Relationships

n-type p-type

M> S rectifying ohmic

M< S ohmic rectifying



Carrier Motion

n-type and p-type substrates are analogs of oneanother

n - Substrate p - Substrate



Specific Contact Resistivity

The specific contact resistivity, c, is an area-independentparameter

Thermionic emission

For B= 0.6 V,A* = 110 A/cm2

K2

, T= 300 K

1

0

=

=

V

cdV

dJ

1* //2

kTqVkTq eeTAJ B

kTqkTq

cBB ee

TqA

k /1

/

*

=

c= 1600 -cm2 Rc= 1.6x10

11

(A= 1

mx1

m)




Thermionic emission

Thermionic-field emission

Field emission

oB Eq

c eC/

11

=

ooB Eq

c eC

/

21

=

10-3

10-2

10-1

100

1016 1017 1018 1019 1020 1021

E00,

kT(eV)

Doping Density (cm-3)

TE TFE FE

E00

kT

kTq

cBe /1

=

kTEEE ooooo /coth

*4 tunnos

Doo

mK

NqhE

=




cis very sensitive to doping density

10-8

10-7

10-6

10-5

10-4

10-3

1016 1017 1018 1019 1020 1021

c(ohm-cm

2)

ND(cm-3)

n-Si10-8

10-7

10-6

10-5

10-4

10-3

1016 1017 1018 1019 1020 1021

c(ohm-cm

2)

NA(cm-3)

p-Si



Contact Resistance

Contacts are characterized by:Contact resistance, Rc()

Specific contact resistivi ty,c(-cm

2)

LWA

R c

c

cc

p

n-Type

Current Flow

Direction

WLA

RT

c

eff

cc

Ac =actual contact area Aeff =effective contact area

Aeffcan be lessthanAc

p

n-Type

shcT RL /

tor.semiconductheof)(outintometalthe

(from)toansferscurrent trthe(1/e)most""over whichdistancetheisLengthTransferT

L

Not derived in

notes. See text

page 141-142for discussion



Contact - Current Crowding

Current flowing into/out of a contact can crowd into aportion of the contact

Current has the choice to flow throughcor Rsh

sh

cT

T

Tcsh

RL

LLZ

LxLRIxV

==

;/sinh

cosh)(

n-type

p-type

I

Rsh

c

I

0 L x



Contact - Current Crowding

LT: transfer length

0

0.2

0.4

0.6

0.8

1

0 2 4 6 8 10

V(x)

x (m)

c=10-5 cm210-6

10-7

10-8

L=10 m, Z=50 mRsh=10 /square

10-6

10-5

10-4

10-3

10-2

10-8 10-7 10-6 10-5 10-4 10-3 10-2

LT(c

m)

Rsh=10 /square

c( cm2)

30 100300

1000

sh

cT

T

Tcsh

RL

LLZ

LxLRIxV

=

=

/sinh

cosh)(

n-type

p-type

I

Rsh

c

I

0 L x



Contact String (Chain)

2Nseries-connectedcontacts aremeasured(N100-1000 ormore)

Does not allowseparation of Rm,Rs, and Rc

Suitable forprocess control,but not for

detailed contactcharacterization

Must know Rsfromindependentmeasure of sheetresistance in order

to extract RC

)2()2( cscsm RRNRRRNR

Rc

pn

n pI

I

V

V

Rm

Rc

RS



Contact String (Chain) Contact string consists of pnjunctions

The junctions are reversebiased

When the junction voltage exceeds VBD, it breaks down

Can have the case where poor contacts, i.e., high Rc, cannot be

detected because junctions break down

Measurement depends on substrate grounded or not

Rm

Rs

Rc

Grounded?

I VV1 V2

n pI

V



Transfer Length Method (TLM)

Consists of identical contacts with varying spacings d

W

dR

WLRRR

sh

T

c

scT ++= 2

2

W

L d1 L d2 L d3 L d4 L

n-typep-substrate

2RcSlope =Rsh/W

0 d2LT

c

RT

( ) ( )W

LdR

W

dR

WL

LRR

Tshsh

T

Tsh

T

222

+=+

shcTRL /


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Four Terminal (Kelvin) Method

Determines the contact resistanceindependent of the sheet

resistance

WLRI

VR ccC ;

12

I I1

p 2

n

p

II

n2

1 V1

V2

V1I

Rc R

s

Rc

Rc

Rs

V2

I

Heed warnings on accuracy (too complex to discuss in class) summarized in our text on pages 150-156.


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Via Contact Resistance

Level

1

Level 2

Current

Voltage

Rc

Current

Current Current

Level 1

Level 2

Rc


25/25

Review Questions

What is the most important parameter to givelow contact resistance?

What are the three metal-semiconductor

conduction mechanisms?

What is Fermi level pinning? Does the contact chain give detailed contact

characterization? Why or why not?

What is the transfer length method?

Why is the Kelvin contact test structure best for

contact resistance measurements?

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