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ESE 568: Mixed Signal Circuit Design and Modeling
Lec 2: September 4th, 2019 MOS Models: Devices and Large Signal
Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
Lecture Outline
! MOSFET (Large Signal) " Physical Device " Device Models
" Examples
" 2nd order effects
2 Penn ESE 568 Fall 2019 - Khanna
MOSFET
Metal Oxide Semiconductor Field Effect Transistor: Device and Models
Penn ESE 568 Fall 2019 - Khanna 3
The Operational Basis of a FET
4 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
Metal
Oxide (Insulator)
P-type Semiconductor
The Operational Basis of a FET
5 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
The Operational Basis of a FET
6 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
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The Operational Basis of a FET
7 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
MOSFET Physical Structure
8 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
nMOS IV Characteristics
9 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
nMOS IV Characteristics
10 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
nMOS IV Characteristics
11 Only valid for vds near 0 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F.
Najmabadi, UCSD)
nMOS IV Characteristics
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3
nMOS IV Characteristics
13 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
nMOS IV Characteristics
14 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
Transistor IV curves
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Channel Length Modulation
16 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
Channel-length Modulation
Channel Length Modulation
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Channel-length Modulation
Body Effect
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The Operational Basis of a FET
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Not exactly Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
The Operational Basis of a FET
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Not exactly
Weak Inversion Strong Inversion
Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
Weak Inversion (or Cut-off or Subthreshold)
! Transition from insulating to conducting is non-linear, but not abrupt
! Current does flow " But exponentially dependent on VGS
21 Penn ESE 568 Fall 2019 – Khanna
Weak Inversion (or Cut-off or Subthreshold)
If VGS <Vth,
IDS = ISWL
!
"#
$
%&e
VGS−VthnkT /q
!
"#
$
%&
1− e−VDSkT /q!
"#
$
%&!
"
##
$
%
&&
1+λVDS( )
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! Current is from the parasitic NPN BJT transistor when gate is unbiased and there is no conducting channel
Penn ESE 568 Fall 2019 – Khanna
MOS IV Characteristic Equations
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Simplified for hand analysis
Not exactly, subT current flows
[1+λ(vDS−VOV)]
[1+λ(vDS−VOV)]
Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
pMOS Device
24 Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
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MOS IV Characteristic Equations
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Simplified for hand analysis
Not exactly, subT current flows
[1+λ(vDS−VOV)]
[1+λ(vSD−VOV)]
Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
nMOS Exercise
! http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html
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Example: Operating Regions
! Assume VTHn = -VTHp = 0.5V, µnCox = µpCox = 50µA/V2, λ = 0, and M1 and M2 have the same value of W and L (are same size)
! Determine operating region for M1 and M2 assuming: " Vbias = 1.2 " Vbias = 0.2 " Vbias = 0.65
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2nd Order Effects
! Mobility Degradation with Normal Field " Vertical field " Triode and saturation region
! Velocity Saturation " Lateral field " Saturation region
! Short Channel Effects
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Mobility Degradation with Normal Field
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! Usually modeled empirically ! Affects both saturation and triode regions
" Strong inversion only
(VDS small)
Mobility Degradation with Normal Field
! High gate-to-source voltage
! θ= mobility modulation factor (empirical)
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µn (eff ) ≈µn0
1+θ(VGS −VT )
Penn ESE 568 Fall 2019 – Khanna
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Velocity Saturation
! Affects saturation region in strong inversion ! Once velocity saturates:
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Mobility degradation due to lateral electric field (VDS/Leff)
Penn ESE 568 Fall 2019 – Khanna
Velocity Saturation
! Long Channel
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Velocity Saturation
! Long Channel ! Short Channel
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Short Channel Effects – VT Reduction
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n+ n+
pn+ depletion
region
pn+ depletion
region VGS induced depletion
region
S G
D
n+ n+
S G
D
QB0 QB0(sc)
xj
Leff Leff
VT0 (short channel) = VT0 - ΔVT0
Penn ESE 568 Fall 2019 – Khanna
Short Channel Effects – VT Reduction
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n+ n+
pn+ depletion
region
pn+ depletion
region VGS induced depletion
region
S G
D
n+ n+
S G
D
QB0 QB0(sc)
xj
Leff Leff
VT0 (short channel) = VT0 - ΔVT0
Penn ESE 568 Fall 2019 – Khanna
Short Channel Effects - DIBL
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! Drain Induced Barrier Lowering " VT Reduction with Drain Bias
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MOS IV Characteristic Equations
37
Simplified for hand analysis
Not exactly, subT current flows
[1+λ(vDS−VOV)]
[1+λ(vDS−VOV)]
Penn ESE 568 Fall 2019 – Khanna (Slides adapted from F. Najmabadi, UCSD)
Lecture Outline
! MOSFET has modes of operation " Subthreshold " Triode " Saturation " Have simplified model for hand analysis
! Second order effects " Mobility Degradation with Normal Field " Short Channel Effects
" Velocity Saturation " VT reduction " DIBL
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Admin
! HW 1 due Sunday at midnight in Canvas " Allow time for submitting " Make sure your submission is the correct file
" Check!
! Diagnostic Quiz extended " Complete by Friday midnight or get a 0
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