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Models for Hand Analysis. NMOS Transistor. V DSN V GSN -V TN. V DSN V GSN -V TN. K N =(W/L)K’ N. PMOS Transistor. V DSP V GSP -V TP. V DSP V GSP -V TP. K P =(W/L)K’ P. pMOS Current model. VDSP>VGSP -VTP. VDSP
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CONCORDIAVLSI DESIGN LAB
1
Models for Hand Analysis
NMOS Transistor
IDS N KN VG SN VTN– VD SN12---VD SN
2–
=
ID SN12---KN VG SN VTN– 2 1 VD SN+ =
PMOS TransistorI D SP KP– VGS P VTP– VD SP
12---VD SP
2–
=
ID SP12---– Kp VGS P VTP– 2 1 VDS P– =
VDSN VGSN-VTN
VDSN VGSN-VTN
VDSP VGSP -VTP
VDSP VGSP-VTP
KN=(W/L)K’N
KP=(W/L)K’P
CONCORDIAVLSI DESIGN LAB
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pMOS Current model
I D SP KP– VGS P VTP– VD SP12---VD SP
2–
=
ID SP12---– Kp VGS P VTP– 2 1 VDS P– =
VDSP>VGSP -VTP
VDSP <VGSP-
VTP
CONCORDIAVLSI DESIGN LAB
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Channel Resistance
)(
1
tgsn VVB 2][
2
tgsn VV R= R=
CONCORDIAVLSI DESIGN LAB
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Variation of resistance with Vgs
CONCORDIAVLSI DESIGN LAB
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LENGTH L S L/S
WIDTH W S W/S
THIN OXIDE tox S tox/S
DIFFUSION DOPING ND 1/S ND . S
SUBSTRATE DOPING NA 1/S NA . S
SUPPLY VOLTAGE VDD 1/S VDD/S
Linear Scaling
CONCORDIAVLSI DESIGN LAB
Scalling Effects
6
CONCORDIAVLSI DESIGN LAB
7
Velocity Saturation and mobility Degradation
CONCORDIAVLSI DESIGN LAB
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TOH’s Model for Short Channel
]2
[2
dsgsoxnD
VV
L
WCI dsatds VV fo
r)( tgsoxsatD VVWCkVI dsatds VV for
))(1( tgsdsat VVkV
)(1
1
1
1
tgssat
sat
VV
LE
E
EK
n
satsat
VE
2
CONCORDIAVLSI DESIGN LAB
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Secondary Effects
Subthreshold current: is the small current that flows from drain at Vgs < Vt
Punch through: If a large voltage is applied to Vds, then the depletion region of the drain can extend to the source, a punch through occurs and under these condition a large current can flow from the drain to source.
Hot carrier: As a results of scaling, device dimensions are reduced while, doping concentrations are increased, while voltages are not reduced to the same proportion, as a consequence there is an increase in electric field in the channel region while, the thickness of the gate insulating layer is thinner. Due to the acceleration of electrons by the Vds, electrons and holes gaining high speed can penetrate the gate insulator and change its characteristics.
Channel hot electrons: If the Vds is increased, then the lateral electric field is increased and the electric field accelerates the electrons near the drain with high kinetic energy they are injected into the oxide near the drain.
CONCORDIAVLSI DESIGN LAB
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Semiconductor Resistors
Resistance
R= (l /A) = (/t). (l /w) = Rsh. (l /w)
Rsh = sheet resistance Ω/
For 0.5u process: N+ diffusion : 70 Ω/ M1: 0.06 Ω/ P+ diffusion : 140 Ω/ M2: 0.06 Ω/ Polysilicon : 12 Ω/ M3: 0.03 Ω/ Polycide:2-3 Ω/ P-well: 2.5K Ω/ N-well: 1K Ω/
w
current
l t
(A)
1
n n q p p q + ------------------------------------------------=
CONCORDIAVLSI DESIGN LAB
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Semiconductor Resistors
AlAl
n+
Diffusionn+
Field oxide
polysilicon
Polysilicon Resistor Diffusion Resistor
SiO2
CONCORDIAVLSI DESIGN LAB
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Variations in Width and Length
Weff
Wdrawn
WD WD
1. Width Oxide encroachment Weff = Wdrawn- 2WD
2. Length Lateral diffusion LD = 0.7Xj Leff = Ldrawn- 2LD
Ldrawn
LD Leff LD
polysilicon
polysilicon
CONCORDIAVLSI DESIGN LAB
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Semiconductor Capacitors
1. Poly Capacitor: a. Poly to substrate b. Poly1 to Poly2
2. Diffusion Capacitor
n+ (ND)
depletion region
substrate (NA)bottomwallcapacitance
sidewallcapacitances
CONCORDIAVLSI DESIGN LAB
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Transistor Resistance
:
Two Components:
Drain/ Sources Resistance:
RD(S) = Rsh x no. of squares+ contact resistance.
Channel Resistance: Depends on the region of operation:
L
W
(D) (S) n+ n+
(G)
RS Rch RD
Linear
RCH2
K' WL----- V
GSVT– 2
----------------------------------------------------= Saturation
RCH1
K'WL----- VGS VT– VDS–
---------------------------------------------------------------- '=
CONCORDIAVLSI DESIGN LAB
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Dynamic Behavior of MOS Transistor
DS
G
B
CGDCGS
CSB CDBCGB
Prentice Hall/Rabaey
CONCORDIAVLSI DESIGN LAB
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The Gate Capacitance
Prentice Hall/Rabaey
CONCORDIAVLSI DESIGN LAB
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Average Gate Capacitance
Most important regions in digital design: saturation and cut-off
Different distributions of gate capacitance for varying
operating conditions
Prentice Hall/Rabaey
CONCORDIAVLSI DESIGN LAB
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Diffusion Capacitance
Prentice Hall/Rabaey
CONCORDIAVLSI DESIGN LAB
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Diffusion Capacitance
MJSW
B
BD
DJSW
MJ
B
BD
DJD
P
VP
C
P
VA
CC]1[]1[
)22(*** WLCWLCC JSWJD
CONCORDIAVLSI DESIGN LAB
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SPICE TRANSISTOR MODEL
CONCORDIAVLSI DESIGN LAB
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SPICE MODELS
Level 1: Long Channel Equations - Very Simple
Level 2: Physical Model - Includes VelocitySaturation and Threshold Variations
Level 3: Semi-Emperical - Based on curve fittingto measured devices
Level 4 (BSIM): Emperical - Simple and Popular
CONCORDIAVLSI DESIGN LAB
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MAIN MOS SPICE PARAMETERS
Prentice Hall/Rabaey
CONCORDIAVLSI DESIGN LAB
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SPICE Parameters for Parasitics
Prentice Hall/Rabaey
CONCORDIAVLSI DESIGN LAB
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SPICE Transistors Parameters
Prentice Hall/Rabaey
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Example
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