ESE370:
Circuit-Level
Modeling, Design, and Optimization
for Digital Systems
Day 9: September 26, 2011
MOS Model
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Penn ESE370 Fall2011 -- DeHon
Today
• MOS Structure
• Basic Idea
• Semiconductor Physics
– Metals, insulators
– Silicon lattice
– Band Gaps
– Doping
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Penn ESE370 Fall2011 -- DeHon
MOS
• Metal Oxide Semiconductor
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MOS
gate
drain
src
channel
• Metal – gate
• Oxide – insulator separating gate from
channel
– Ideally: no conduction from gate to channel
• Semiconductor – between source and
drain
• See why input capacitive?
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Penn ESE370 Fall2011 -- DeHon
Capacitor
gate
drain
src
channel
• Charge distribution and field?
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Penn ESE370 Fall2011 -- DeHon
gate
Idea
drain
src
channel
• Semiconductor – can behave as metal
or insulator
• Voltage on gate creates an electrical
field
• Field pulls (repels) charge from channel
– Causing semiconductor to switch
conduction
– Hence “Field-Effect” Transistor
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Source/Drain Contacts
• Contacts: Conductors  metalic
– Connect to metal wires that connect
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Fabrication
•
•
•
•
•
Start with Silicon wafer
Dope
Grow Oxide (SiO2)
Deposit Metal
Mask/Etch to define
where features go
http://www.youtube.com/watch?v=35jWSQXku74
Penn ESE370 Fall2011 -- DeHon
Cartoon fab seq.: t=2min—4min
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Dimensions
• Channel Length (L)
• Channel Width (W)
• Oxide Thickness (Tox)
• Process named by
minimum length
– 45nm  L=45nm
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Semiconductor Physics
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Conduction
• Metal – conducts
• Insulator – does not conduct
• Semiconductor – can act as either
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Penn ESE370 Fall2011 -- DeHon
Why metal conduct?
• (periodic table)
http://chemistry.about.com/od/imagesclipartstructures/ig/Science-Pictures/Periodic-Table-of-the-Elements.htm
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Conduction
• Electrons move
• Must be able to “remove” electron from
atom or molecule
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Atomic States
• Quantized Energy Levels
• Must have enough energy to change
level (state)
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Penn ESE370 Fall2011 -- DeHon
Thermal Energy
• Except at absolute 0
– There is always free energy
– Causes electrons to hop around
• ….when enough energy to change states
– Energy gap between states determines
energy required
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Silicon Atom
• How many valence electrons?
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Silicon
• 4 valence electrons
– Inner shells filled
– Only outer shells contribute to chemical
interactions
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Penn ESE370 Fall2011 -- DeHon
Silicon-Silicon Bonding
• Can form covalent bonds with 4 other
silicon atoms
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Penn ESE370 Fall2011 -- DeHon
Silicon Lattice
• Forms into crystal lattice
http://www.webelements.com/silicon/crystal_structure.html
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Silicon Lattice
• Cartoon two-dimensional view
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Outer Orbital?
• What happens to outer shell in Silicon
lattice?
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Energy?
• What does this say about energy to
move electron?
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Energy
State View
Valance Band – all states filled
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State View
Energy
Conduction Band– all states empty
Valance Band – all states filled
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Band Gap and Conduction
Insulator
Metal
Ec
8ev
Ec
Ev
OR
Ev
Ev
Ec
Semiconductor
1.1ev
Ec
Ev
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Doping
• Add impurities to Silicon Lattice
– Replace a Si atom at a lattice site with
another
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Penn ESE370 Fall2011 -- DeHon
Doping
• Add impurities to Silicon Lattice
– Replace a Si atom at a lattice site with
another
• E.g. add a Group 15 element
– E.g. P (Phosphorus)
– How many valence electrons?
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Doping with P
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Doping with P
• End up with extra electrons
– Donor electrons
• Not tightly bound to atom
– Low energy to displace
– Easy for these electrons
to move
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Penn ESE370 Fall2011 -- DeHon
Doped Band Gaps
• Addition of donor electrons makes more
metallic
– Easier to conduct
Semiconductor
0.045ev
1.1ev
Ec
ED
Ev
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Localized
• Electron is localized
• Won’t go far if no low energy states
nearby
• Increase doping concentration
– Fraction of P’s to Si’s
– Decreases energy to conduct
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Electron Conduction
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Electron Conduction
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Capacitor Charge
• Remember capacitor charge
++++++++
- - - - - - - - -
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MOS Field?
• What does “capacitor” field do to the
doped semiconductor channel?
Vgs=0
No field
- - -
=
+
+++++
+ + +
- - -
Vcap>0
Vgs>0
Conducts
-----35
Penn ESE370 Fall2011 -- DeHon
MOS Field Effect
• Charge on capacitor
– Attract or repel charge in channel
– Change the donors in the channel
– Modulates conduction
+++++
– Positive
• Attracts carriers
– Enables conduction
– Negative?
• Repel carriers
– Disable conduction
Penn ESE370 Fall2011 -- DeHon
------
------
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Group 13
• What happens if we replace Si atoms
with group 13 atom instead?
– E.g. B (Boron)
– Valance band electrons?
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Penn ESE370 Fall2011 -- DeHon
Doping with B
• End up with electron vacancies -- Holes
– Acceptor electron sites
• Easy for electrons to shift into these
sites
– Low energy to displace
– Easy for the electrons to move
• Movement of an electron best viewed as
movement of hole
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Hole Conduction
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Doped Band Gaps
• Addition of acceptor sites makes more
metallic
– Easier to conduct
Semiconductor
Ec
0.045ev
1.1ev
EvEA
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Penn ESE370 Fall2011 -- DeHon
Field Effect?
• Effect of postivive field on Acceptordoped Silicon?
Vgs=0
No field
+ + +
+
+ + +
+ + +
- - -
Vcap>0
Vgs>0
No conduction
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Penn ESE370 Fall2011 -- DeHon
Field Effect?
• Effect of negative field on Acceptordoped Silicon?
Vgs=0
No field
+ + +
+
- - +++++
Penn ESE370 Fall2011 -- DeHon
- - + + +
Vcap<0
Vgs<0
Conduction
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MOSFETs
• Donor doping
– Excess electrons
– Negative or N-type material
– NFET
• Acceptor doping
– Excess holes
– Positive or P-type material
– PFET
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Penn ESE370 Fall2011 -- DeHon
Admin
• Intel Fabrication Talk Wednesday (28th)
at 4pm (Raisler)
• HW3
– Largely based on Wed. lecture
– Should be able to do SPICE parts now
– Will want to read text 3.3.2
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Penn ESE370 Fall2011 -- DeHon
MOSFET
• Semiconductor can act like metal or
insulator
• Use field to modulate conduction state
of semiconductor
-----+++++
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Penn ESE370 Fall2011 -- DeHon