basic mosfet structure
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Government Engineering College, Bharuch
VLSI Presentation
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Basic MOSFET Structure
Prepared by:
Xyz
6th Sem E.C.(A)
Electronics and Comm. Dept.
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MOSFET Structure
p-Si
n+L
Source Gate Drain
Field Oxide
Gate Oxide
Bulk (Substrate)
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• Importance for LSI/VLSI
–Low fabrication cost
–Small size
–Low power consumption
• Applications
–Microprocessors
–Memories
–Power Devices
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i. Basic Properties
i. Unipolar device
ii. Very high input impedance
iii. Capable of power gain
iv. Two possible device types:
enhancement mode; depletion mode
v. Two possible channel types: n-channel;
p-channel
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Symbols
G
D
S
BG
D
S
B
p Channel MOSFET n Channel MOSFET
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Current-Voltage Characteristic
A
B CDIDS
VDS
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Channel Formation
p-Si
B
VG +VDS
n-Channel
S D
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Increased VDS
p-Si
B
VG +VDS
n-Channel
S D
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Triode Region A voltage-controlled resistor @small VDS
G
pn+n+
metal
S DB
oxide
+-
+++
+++
- - - -
VGS1>Vt
pn+n+
metal
S DB
oxide
+-
+++
++++++
- - - - - -
VGS2>VGS1
pn+n+
metal
S DB
oxide
+-
+++
++++++
- - - - - - - - -
VGS3>VGS2+++
ID
VDS
0.1 v
increasingVGS
Increasing VGS puts more charge in the channel, allowing more drain current to flow
cut-off
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Saturation Regionoccurs at large VDS
p
n+n+
metal
source
S
gate
Gdrain
D
body
B
oxide
+-
+++
++++++
VDS large
As the drain voltage increases, the difference in voltage between the drain and the gate becomes smaller. At some point, the difference is too small to maintain the channel near the drain pinch-off
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Simplified MOSFET I-V Equations
Cut-off: VGS< VT
ID = IS = 0Triode: VGS>VT and VDS < VGS-VT
ID = kn’(W/L)[(VGS-VT)VDS -
1/2VDS2]
Saturation: VGS>VT and VDS > VGS-VT
ID = 1/2kn’(W/L)(VGS-VT)2
where kn’= (electron mobility)x(gate capacitance)
= mn(eox/tox) …electron velocity = mnE
and VT depends on the doping concentration and gate material used (…more details later)
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Thank You