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Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-1
Chapter 9 Metal-Semiconductor Contacts
Two kinds of metal-semiconductor contacts:
• metal on lightly doped silicon –• rectifying Schottky diodes• metal on heavily doped silicon –• low-resistance ohmic contacts
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-2
9.1 Schottky BarriersEnergy Band Diagram of Schottky Contact
• Schottky barrier height, φB , is a function of the metal material.
• φB is the single most important parameter. The sum of qφBn and qφBp is equal to Eg .
Metal Depletionlayer Neutral region
qφBn
Ec
EcEf
Ef
Ev
EvqφBp
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-3
Schottky barrier heights for electrons and holes
φBn increases with increasing metal work function
Metal Mg Ti Cr W Mo Pd Au Ptφ Bn (V) 0.4 0.5 0.61 0.67 0.68 0.77 0.8 0.9φ Bp (V) 0.61 0.5 0.42 0.3
WorkFunction 3.7 4.3 4.5 4.6 4.6 5.1 5.1 5.7ψ m (V)
φBn + φBp ≈ 1.1 V
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-4
φBn Increases with Increasing Metal Work Function
qφBn Ec
Ev
Ef
E0
qψM
χSi = 4.05 eV
Vacuum level,
Ideally, qφBn= qψM – χSi
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-5
φBn is typically 0.4 to 0.9 V
• A high density of energy states in the bandgap at the metal-semiconductor interface pins Ef to a range of 0.4 eV to 0.9 eV below Ec
• Question: What is the typical range of φBp?
qφBn Ec
Ev
Ef
E0
qψM
χSi = 4.05 eV
Vacuum level,
+ −
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-6
Schottky barrier heights of metal silicide on Si
Silicide-Si interfaces are more stable than metal-silicon interfaces. After metal is deposited on Si, an annealing step is applied to form a silicide-Si contact. The term metal-silicon contact includes silicide-Si contacts.
Silicide ErSi1.7 HfSi MoSi2 ZrSi2 TiSi2 CoSi2 WSi2 NiSi2 Pd2Si PtSiφ Bn (V) 0.28 0.45 0.55 0.55 0.61 0.65 0.67 0.67 0.75 0.87φ Bp (V) 0.55 0.49 0.45 0.45 0.43 0.43 0.35 0.23
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-7
Using CV Data to Determine φB
AW
C
qNVW
NNkTq
EEqq
dep
s
d
bisdep
d
cBn
fcBnbi
ε
φε
φ
φφ
=
+=
−=
−−=
)(2
ln
)(
Question: How should we plot the CV data to extract φbi?
Ev
Ev
Ec
Ef
Ef
Ec
qφBn
qφbiqφBn
q(φbi + V)
qV
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-8
Once φbi is known, φΒ can be determined using
22)(21
AqNV
C sd
bi
εφ +
=
d
cBnfcBnbi N
NkTqEEqq ln)( −=−−= φφφ
Using CV Data to Determine φB
V
1/C2
−φbi
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-9
9.2 Thermionic Emission Theory
2//0
//23
2
/)(2/3
2/)(
A/cm 100 where,
421
/2 /3
22
kTqo
kTqV
kTqVkTqnthxMS
nthxnth
kTVqnkTVqc
B
B
BB
eJeJ
eeTh
kqmqnvJ
mkTvmkTv
eh
kTmeNn
φ
φ
φφ
π
π
π
−
−→
−−−−
≈=
=−=
−==
==
Efn
-q(φB − V)
qφBqVMetal
N-typeSiliconV Efm
Ev
Ec
x
vthx
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-10
9.3 Schottky Diode
Ef
qφB
Efn
> qφB
qV
- - E − Ef = qφB qφB
(a) V = 0. IS M→ IM S→ = = Ι0
Efn
- - < qφB qφB
(b) Forward bias. Metal is positive wrt Si. IS M→ IM S→>> = Ι0
qV
(c) Reverse bias. Metal is negative wrt Si.
IS M→ IM S→<< = Ι0
(d) Schottky diode IV.
V
I
Reverse bias Forward bias
IS M→
IM S→
= −Ι0
≈ 0
IM S→ = −Ι0 IS M→ = Ι0 eqV/kT
-I M S→ = −Ι0
I S M→ = Ι0
Efn
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-11
)1(
)KA/(cm 1004
/00
/0
223
2
/20
−=−=+=
⋅≈=
=
→→
−
kTqVkTqVSMMS
n
kTq
eIIeIIIIh
kqmK
eAKTI B
π
φ
Ef
Efn
- - < qφB qφB
qV
IM S→ = −Ι0 IS M→ = Ι0 eqV/kT
9.3 Schottky Diode
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-12
9.4 Applications of Schottky Diodes
• I0 of a Schottky diode is 103 to 108 times larger than a PN junction diode, depending on φB . A larger I0 means a smaller forward drop V. • A Schottky diode is the preferred rectifier in low voltage, high current applications.
I
V
PN junction
Schottky
φB
I
V
PN junction
Schottky diode
φBdiode
kTq
kTqV
BeAKTI
eII/2
0
/0 )1(
φ−=
−=
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-13
Switching Power Supply
Question: What sets the lower limit in a Schottky diode’s forward drop?
Synchronous Rectifier: For an even lower forward drop, replace the diode with a wide-W MOSFET which is not bound by the tradeoff between diode V and I0 : I = I0eqV/kT
ACDC AC AC DC
utilitypower
110V/220V
PN Junctionrectifier
Hi-voltage
MOSFETinverter
100kHzHi-voltage
TransformerSchottkyrectifier
Lo-voltage 50A1.8V
feedback to modulate the pulse width to keep Vout = 1.8 V
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-14
There is no minority carrier injection at the Schottky junction. Thus, the CMOS latch-up problem can be eliminated by replacing the source/drain of the NFET with Schottky junctions.
In addition, the Schottky S/D MOSFET would have shallow junctions and low series resistance. So far, Schottky S/D MOSFETs have lower performance.
No excess carrier storage.What application may benefit from that?
9.4 Applications of Schottky Diodes
P-body
gatesilicidesource
silicide drain
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-15
GaAs MESFET
The MESFET has similar IV characteristics as the MOSFET, but does not require a gate oxide.
N-channelN+ N +
metal
gatesource drain
GaAsSemi-insulating substrate
Question: What is the advantage of GaAs over Si?
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-16
9.5 Ohmic Contacts
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-17
SALICIDE (Self-Aligned Silicide) Source/Drain
Rs RdS D
Ggate
oxide
dielectric spacercontact metal
channel
N+ source or drainCoSi2 or TiSi2
After the spacer is formed, a Ti or Mo film is deposited. Annealing causes the silicide to be formed over the source, drain, and gate. Unreacted metal (over the spacer) is removed by wet etching.
Question:• What is the purpose of siliciding the source/drain/gate?• What is self-aligned to what?
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-18
dBn NVHndthxdMS
onns
emkTqNPvqNJ
mmhmH
/)(
13/29
2/21
Vcm /104.5/4
−−→
−−
=≈
×==
φπ
επ
9.5 Ohmic Contacts
- -
x
Silicide N+ Si
- -
x
Vd
Bnsdep qN
W φε2=
dBn NHeP φ−=Ev
Ec , Ef Efm
Ev
Ec , Ef
φBn – VφBn
Tunneling probability:
Semiconductor Devices for Integrated Circuits (C. Hu) Slide 9-19
2//1
cmΩ ⋅∝=
≡
−→ dBn
dBnNH
dthx
NHMS
c eNHqv
edV
dJR φφ
9.5 Ohmic Contacts
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