Transcript Today`s Slides

The Threshold Voltage
The voltage applied between the gate and the source
which causes the beginning of the channel surface
strong inversion.
Threshold voltage Vt is a function of :
» Vfb = flatband voltage; depends on difference in work function
between gate and substrate and on fixed surface charge.
» Fs = surface potential (FD).
» Gate oxide thickness.
» Charge in the channel area.
» Additional ion implantation.
n Typical
values: 0.2V to 1.0V for NMOS and -0.2 to 1.0V for PMOS
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Threshold Adjust
An effective mean to adjust the
threshold is to change the doping
concentration through an ion
implantation dose.
Ion Implantation (dopant)
S
NMOS transistors implanted with
n-type dopant results in a decrease
in threshold voltage
NMOS transistors implanted with
p-type dopant results in an increase
in the threshold voltage.
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D
channel
p Substrate
VTO’=VTO + (q.DI/Cox)
DI = dose of dopant in the channel
area(atoms/cm2)
Cox = gate oxide capacitance per unit
area
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Threshold Adjust
D
VT = VTO   
–  S + VS B –
G
 
B
S
 Threshold voltage is a function of source to
substrate voltage VSB.
 Body factor  is the coefficient for the VSB
dependence factor.
VSB
S
Fs is the surface potential ~ -0.6V for NMOS
 is the body factor ~ 0.6 to 1.2 V1/2
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TOH’S MODEL
In the Saturation region,
In the Linear region,
for V
I D  kVsat CoxW (V gs  Vt )
I D  nCox
2
W
V
[Vgs  ds ]
L
2
ds
for V
ds
 Vdsat
 Vdsat
In the Saturation region, Vdsat = (1-K )(Vgs-Vt )
1
1

Esat
L
1
1  Esat
E
(Vgs  Vt )
2V
E sat  sat
n
K
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Models for manual analysis
NMOS Transistor
2

1
IDS N = KN  VGSN – VTN VDSN – ---VDSN 
2
2
1
IDSN = ---KN VGSN – VTN  1 +  VDSN 
2
VDSN <VGSN-VTN
VDSN >VGSN-VTN
KN=(W/L)K’N
PMOS Transistor
2

1
I D SP = –K P  VGS P – V TPV D SP – --- VD SP 
2
VDSP > VGSP -VTP
2
1
ID SP = – -- Kp  VGS P – VTP    1 – VDS P
2
VDSP < VGSP-VTP
KP=(W/L)K’P
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Dynamic Behavior of MOS Transistor
G
CGS
CGD
D
S
CGB
CSB
CDB
B
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Prentice Hall/Rabaey
Diffusion Capacitance
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Prentice Hall/Rabaey
SPICE MODELS
Level 1: Long Channel Equations - Very Simple
Level 2: Physical Model - Includes Velocity
Saturation and Threshold Variations
Level 3: Semi-Emperical - Based on curve fitting
to measured devices
Level 4 (BSIM): Emperical - Simple and Popular
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MAIN MOS SPICE PARAMETERS
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Prentice Hall/Rabaey
SPICE Parameters for Parasitics
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Prentice Hall/Rabaey
SPICE Transistors Parameters
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Prentice Hall/Rabaey