Spartan Semiconductor Services
EE/MatE 129
MOS-1
David W. Parent (SJSU)
Fall 2001
Lecture 3
1
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MOSFET Overview
• What is a MOSFET?
• Why use MOSFETs?
• MOSFET Physics
–
–
–
–
Energy Bands
Diode
MOS Diode
MOFET
Lecture 3
2
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What is a MOSFET?
• Metal (or poly-silicon doped heavily to act
like a metal)
• Oxide (SiO2, Acts as an insulator.)
• Semiconductor (One can selectively change
the carrier type to n-type or p-type.)
• Field Effect (Device is controlled by an
electric field as opposed to current.)
• Transistor (Three terminal device)
Lecture 3
3
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MOSFET Cross Section
Al
D
G
S
n+
n+
p
SiO2
Si
Lecture 3
4
2
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D
MOSFET Operation
G
S
n+
n+
Channel
Depletion
p
ID
4
Pinch-off
VD (Sat.)
3
2
VG =
Lecture 3
1v
VD
5
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Why use MOSFETS?
• The controlling terminal (the gate) uses
minimal power to maintain its state (on or
off)
– The gate is a MOS structure with the oxide
acting as an insulator. The only power used is
to charge and discharge the gate capacitance.
– Power is dissipated as heat to the lattice
• Slows down device below specified clock speed.
• This allows more transistors per die.
Lecture 3
6
3
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Why use MOSFETS?
• Other types of transistor logic are inherently
faster, but these logic gates (ECL) use too
much power to integrate the same
complexity on a die as one can achieve with
MOSFETS.
• CMOS logic goes ‘rail to rail’ with out
regeneration circuitry.
Lecture 3
7
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MOSFET Physics
• Solitary atoms of silicon have discrete energy
levels that electrons can occupy.
• As Si atoms become closer, these levels start to
interact and in according to the Pauli Exclusion
Principle the discreet levels turn into bands.
– The Conduction Band: Electrons promoted to this level
are free to move under drift or diffusion.
– The Valence Band: Electrons that are promoted to a
trap in the forbidden region leave behind holes that are
free to move under drift or diffusion
Lecture 3
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4
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Energy Bands
p
n
Ec
Ec
EF
EF
Ev
Ev
Lecture 3
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Energy Bands
Ec
qVo
EF
Ev
Energy Bands
Particle flow
Current flow
Hole Diffusion
Hole Drift
Electron Diffusion
Electron Drift
Lecture 3
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5
I
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Diode
• A Diode allows current flow in one
direction (forward bias), but not in the other
(reverse bias).
– Forward Bias: The applied bias lowers the
built in potential allowing diffusion or carriers
across the junction.
– Reverse Bias: The applied bias increases the
barrier blocking further diffusion of carriers.
Lecture 3
11
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MOS Diode
• Metal Gate, Oxide insulator, Semiconductor
• Operation:
– Apply a voltage to the gate and it attracts carriers of
opposite polarity to the applied voltage.
– One can attract carriers that are opposite to the
semiconductor type.
– VT For a large enough voltage one can change the
material from n-type to p-type or p-type to n-type near
the oxide gate. The voltage at which this inverted
region has the same carrier concentration as the
substrate is VT.
Lecture 3
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MOSFET
• Two diodes, biased back to back, one diode
is all ways reversed biased so no current can
flow.
• If a MOS diode is placed in between, a
conducting channel can be created thus
allowing current to flow based on a gate
voltage
Lecture 3
13
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MOSFET
-V
D
S
p+
p+
N
G
-V
D
S
p+
p+
N
Lecture 3
14
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Next Time
• VT calculation
• IV characteristics
• PMOS Inverter
Lecture 3
15
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