TAMU-ELEN-325
J. Silva-Martinez
Jose Silva-Martinez
Texas A&M University
Department of Electrical Engineering
College Station, TX
jsilva@ece.tamu.edu
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TAMU-ELEN-325
J. Silva-Martinez
P-MOS Transistor: 4 terminal device
Bulk
Source
P-type transistor
Polysilicon
S
G
B
Gate
N-Well
P+ Diffusion
Thin oxide
B
N+
substrate
DIODE
Drain
Thick oxide
G
S
P+
D
D
P+
N
P-channel
Polysilicon (heavily doped)
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Basic
elements:
Diffusions
+
Oxidations
+
Polysilicon
TAMU-ELEN-325
BASIC SCNA CMOS LAYERS
J. Silva-Martinez
P-channel MOSFET
Physical Layer
N-well
Drain
Silicon Nitride
CVD Oxide
Metal 1
Source
Poly Gate
Polysilicon Layer 1
p+
Polysilicon Layer 2
Gate Oxide
n-well Bulk
p+
p substrate
P+ Ion Implant
Bulk
N+ Ion Implant
Contact cut to n+/p_
N-channel MOSFET
Metal 1
Source
Via Oxide Cuts
CVD Oxide
Metal 1
Drain
Poly Gate
Metal 2
n+
Pad Contact (Overglass)
Gate Oxide
p substrate
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Bulk
n+
TAMU-ELEN-325
J. Silva-Martinez
N-MOS Transistor
N-type transistor
Inversion: channel is created
D-S current is zero
B
VG>0
VS=0
D
VD=0
G
B
P+
----------
N+
P
substrate
N+
N+
S
Inversion: Channel connects D and S
Inversion channel means that a he number of
D-S current is possible
B
VS=0
VG>0
quasi-free carriers are available and can
VD>0
move from drain to source if a proper voltage
P+
substrate
N+
P
----------
is applied
N+
IDS
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TAMU-ELEN-325
J. Silva-Martinez
MOS Transistor
Subthreshold (weak inversion)
B
P+
VT >VG>0
VS=0
N+
substrate
N-type transistor
VD>0
IDS Linear region
Saturation
--------
N+
P (NA)
Subthreshold
Mobile carriers
concentration< NA
VDS
Saturation (Strong inversion)
B
P+
substrate
VS=0
N+
VG>VT
--------
VD>0
Subthreshold (extremely low-voltage lowpower applications)
Linear region (voltage controlled resistor,
linear OTA’s, multipliers)
N+
P (NA)
Saturation region (Amplifiers)
Mobile carriers
concentration< NA
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TAMU-ELEN-325
J. Silva-Martinez
MOS Transistor
STRONG INVERSION : BODY EFFECTS
Thin oxide
B
Thick oxide
VG=VT
S
D
[
VT = VT0 + γ φ j0 − γ φ j0 − φ j0 − VChannel− B
P+
substrate
N+
N+
NA
n=NA
Polysilicon (heavily doped)
γ=
2qε si N Bulk
C OX
γ = BODY FACTOR
Threshold voltage:
Voltage needed at the gate to create an inversion layer such that the number of
carriers in the channel equals the bulk concentration (~1017-1018 electrons
/cm3)
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]
TAMU-ELEN-325
J. Silva-Martinez
MOS Transistor
Two control capacitors: CG-channel and CB-channel
Thin oxide
B
S
Thick oxide
VG=VT
D
CG-channel
Voltage independent
P+
N+
substrate
N+
Its value depends of the gate area
P
depletion region
Gate-channel resistance is extremely large
CG-channel
Channel substrate capacitance:
Voltage dependent (εεsi/tsi) and non-linear
Depends of the gate area
Depends of the square root of the bulk
doping
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TAMU-ELEN-325
J. Silva-Martinez
MOS Equations in Linear Region
VGS
GND
Drain current: Expression used in SPICE level 1
VDS
ID =
tox
N+
W
N+
ID
L
GND
VGS
VDS
W
µ n C OX (VGS −VT −0.5VDS )VDS
L
Linear approximation
IDSAT
VGS > VT
tox
N+
N+
W
VDS
VDSAT
Non-linear channel
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TAMU-ELEN-325
J. Silva-Martinez
MOS Equations in Saturation and Linear Region
VGS
GND
-
VT
+
VDS = VGS -VT
-
+
Under these conditions
ID =
N+
W
µ n C OX (VGS −VT )2
2L
N+
Strong channel
Weak channel
VGS
GND
current is highest
-
VDS > VGS -VT
+
+
-
SATURATION REGION (VDS > VGS –VT )
Carriers are attracted to the drain, and swept in the
region where channel is incomplete
Current is till given by
ID =
W
µ n C OX (VGS −VT )2
2L
or
N+
Strong channel
N+
I DSAT =
no channel
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W
µ n C OX (VDSAT )2
2L
TAMU-ELEN-325
MOS MODEL:
•Drain current, Triode region
•Drain Current, Saturation region
•Threshold voltage (zero bias)
•Threshold voltage
•KP (Spice Model)
J. Silva-Martinez
Fundamental equations
i D = µCOX
iD =
W
[VGS − VT − 0.5VDS ]VDS
Leff
µCOX
2
W
Leff
[VGS − VT ]2 [1+ λVDS ]
VT 0
[
VT = VT 0 + γ 2φF + VSB − 2φF
KP = µCOX
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]
TAMU-ELEN-325
J. Silva-Martinez
MOS MODEL:
•KP
Some values frequently used
•N-MOS 10-4 A/V
•P-MOS 0.3*10-4 A/V
•VT0
N-MOS 0.7V
P-MOS -0.9V
•Vearly
N-MOS 10 V/um
P-MOS 15 V/um
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