Integrated Circuit Devices Professor Ali Javey Summer 2009 MS Junctions Reading: Chapter14.
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Transcript of Integrated Circuit Devices Professor Ali Javey Summer 2009 MS Junctions Reading: Chapter14.
Integrated Circuit Devices
Professor Ali Javey
Summer 2009
MS Junctions
Reading: Chapter14
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
: electron affinity
FBFCs )( EE
Metal Work Function
The work function of metals depend strongly on the “environment.”
Workfunction, M is an invariant property of metal. It is the minimum energy required to free up electrons from metal.
Question: how can you change S ?
Ideal MS Contact
Assumptions:
M and S are in intimate contact, on atomic scaleNo oxides or charges at the interfaceNo intermixing at the interface
Question: how are Vbi and Bn related?
Schottky Barrier Heights: Metal on Si
Bn tends to increase with increasing metal work function
Schottky Barrier Heights: 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 PtSi Bn (V) 0.28 0.45 0.55 0.55 0.61 0.65 0.67 0.67 0.75 0.87 Bp (V) 0.55 0.49 0.45 0.45 0.43 0.43 0.35 0.23
• Depletion Width:
Does this depletion width equation look familiar?
Question
How can you measure Bn and Bp?
Using CV Data to Determine B
Question: How should we plot the CV data to extract B?
Ev
Ev
Ec
Ef
Ef
Ec
Bn
qVbiBn
q(Vbi + V)
qV
dep
sdep W
AC
Once Vbi is known, can be determined using:
22
)(21
AqN
VV
C sd
bi
d
cBnfcBnbi N
NkTEEqV ln)(
Using CV Data to Determine B
V
1/C2
Vbi
Carrier Injection at the MS Contacts
– Thermionic emission current
– Tunneling current
– Thermally activated tunneling current
For each MS junction, 3 components contribute to the overall injection current:
What parameters affect each component?
Is the net current zero? Why?
Question:
How do p+n junctions differ from MS junctions under a forward bias?
Question
• How does the band diagram look for a MS junction with a Schottky barrier height of zero?
Applications of Schottky Diodes
• I0 of a Schottky diode is 103 to 108 times larger than a PN junction diode, depending on B . • A Schottky diode is the preferred rectifier in low voltage, high current applications.
I
V
PN junction
Schottky
B
I
V
PN junction
Schottky diode
B
diode
Ohmic MS Contacts
Two ways to achieve ohmic MS contacts:– Reduce the Schottky barrier height. How???– Reduce the Schottky barrier width (depletion
width). How????
How would each approach give you an ohmic contact?
Schottky Barriers and Fermi Level Pinning
In actual fabricated metal-Si junctions, Fermi level pinning
prevents us from ever getting zero Schottky barrier height.
Two tricks for reducing Fermi level pinning:
1. thin interfacial oxide/nitride
2. 1D semiconductors
1D nanotubeDiameter ~ 1 nm