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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
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.
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