Download VLSI DEsign Methodology

The Devices:
MOS Transistor
[Adapted from Rabaey’s Digital Integrated Circuits, ©2002, J. Rabaey et al.]
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The MOS Transistor
Polysilicon
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Aluminum
The MOS Transistor
Gate Oxyde
Gate
Source
Polysilicon
n+
Drain
n+
p-substrate
Bulk Contact
CROSS-SECTION of NMOS Transistor
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Field-Oxyde
(SiO2)
p+ stopper
Switch Model of NMOS Transistor
| VGS |
Source
(of carriers)
Open (off) (Gate = ‘0’)
Gate
Drain
(of
carriers)
Closed (on) (Gate = ‘1’)
Ron
| VGS | < | VT |
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| VGS | > | VT |
Switch Model of PMOS Transistor
| VGS |
Source
(of carriers)
Open (off) (Gate = ‘1’)
Gate
Drain
(of carriers)
Closed (on) (Gate = ‘0’)
Ron
| VGS | > | VDD – | VT | |
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| VGS | < | VDD – |VT| |
MOS transistors Symbols
D
D
G
G
S
S
NMOS Enhancement NMOS Depletion
D
G
G
S
PMOS Enhancement
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D
B
S
NMOS with
Bulk Contact
Channel
MOSFET Static Behavior
Positive voltage applied to the gate (VGS > 0)
•The gate and substrate form the plates of a capacitor.
•Negative charges accumulate on the substrate side (repels mobile holes)
•A depletion region is formed under the gate (like pn junction diode)
+
S
VGS
-
D
G
n+
n+
n-channel
Depletion
Region
p-substrate
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B
Current-Voltage Relations
Assume VGS > VT
•A voltage difference VDS will cause ID to flow from drain to source
•At a point x along the channel, the voltage is V(x), and the gate-tochannel voltage is VGS - V(x)
•For channel to be present from drain to source, VGS - V(x) > VT,
i.e. VGS - VDS > VT for channel to exist from drain to source
VGS
VDS
S
G
n+
–
V(x)
ID
D
n+
+
L
x
p-substrate
B
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MOS transistor and its bias conditions
Linear (triode) Region
•When VGS - VDS > VT , the channel exists from drain to source
•Transistor behaves like voltage controlled resistor
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Saturation Region
•When VGS - VDS  VT , the channel is pinched off
•Electrons are injected into depletion region and accelerated
towards drain by electric field
•Transistor behaves like voltage-controlled current source
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Pinch-off
Current-Voltage Relations
Long-Channel Device
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Current-Voltage Relations
Long Channel transistor
6
x 10
-4
VGS= 2.5 V
5
VDS = VGS - VT
Resistive
Saturation
4
ID (A)
VGS= 2.0 V
3
Quadratic
Relationship
VDS = VGS - VT
2
VGS= 1.5 V
cut-off
1
0
VGS= 1.0 V
0
0.5
1
1.5
2
2.5
VDS (V)
NMOS transistor, 0.25um, Ld = 10um, W/L = 1.5, VDD = 2.5V, VT = 0.4V
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