MOSFET modeling for Simulation

7/29/2019 MOSFET modeling for Simulation

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MOSFET Modeling for Simulation



Introduction

• Computer simulation is today a standard partof IC design

• Computer solves a large set of equation

describing interconnection between variouselement & device model

• Circuit under simulation can contain large no

of elements.• Model for each element can contain large no

of equations.

9/24/2013 KIIT UNIVERSITY, BHUBANESWAR 2



Introduction (cont’d)

• Behavior of entire circuit may be needed at

many points

• All device equations may be solved repeatedly

for all points.

• Thus time complexity of computer simulation

depends on complexity of device model




Types of Models

Physical model:

They are based on device physics.

Parameters in this model have physicalsignificance such as flat band voltage, doping

concentration etc.

•

They take long time to develop, varies fromprocess to process




Types of Models(cont’d)

• Empirical Models:

Based on curve fitting technique.

It can use any equation that can adequately fitdata

Parameters in this types of models are

coefficient, exponent etc. & have no physicalsignificance.





• Empirical Model Example:-





• Table models:

They are in form of table that contain a set of

o/p data for a large set of i/p parameters.

Example: contain values of MOSFET drain

current/ small signal parameters for a large no.

of combination of bias voltage




SPICE Modeling for MOS Transistor

• Goals Understand the element description for MOSFETs

Understand the meaning and significance of the

various parameters in SPICE model levels 1 through 3for MOSFETs

Understand the basic capacitance models

Have a general notion of BSIM model parameters

•

Become aware of some newer models Understand the use and shortcomings of the models

covered




The MOSFET Description Lines

Model and Element

• What does SPICE stand for?

• Simulation Program with Integrated CircuitEmphasis

• The MOSFET Model and Element DescriptionLines

• Process and circuit parameters which apply to aparticular class of MOSFETS with varyingdimensions are described for that class of MOSFETS in a single .model line in which + isused to denote line continuation.




SPICE mosfet

model levels

• Level 1 is the Schichman-Hodges model

• Level 2 is a geometry-based, analytical model

•

Level 3 is a semi-empirical, short-channelmodel

• Level 4 is the BSIM1 model

•

Level 5 is the BSIM2 model, etc




SPICE Parameters

Level 1 - 3 (Static)

Param. Parameter Description Def. Typ. Units

VTO Zero-bias Vthresh 1 1 V

KP Transconductance 2.E-05 3.E-05 A/V^2

GAMMA Body-effect par. 0.0 0.35 V^1/2PHI Surface inversion pot. 0.6 0.65 V

LAMBDA Channel-length mod. 0.0 0.02 1/V

TOX Thin oxide thickness 1.E-07 1.E-07 m

NSUB Substrate doping 0.0 1.E+15 cm^-3

NSS Surface state density 0.0 1.E+10 cm^-2

LD Lateral diffusion 0.0 8.E-05 m




SPICE Parameters

Level 1 - 3 (Static)


TPG Type of gate material* 1 1

UO Surface mobility 600 700 cm^2/V-s

IS Bulk jctn. sat. curr. 1.E-14 1.E-15 A

JS Bulk jctn. sat. curr. dens. A/m^2

PB Bulk junction potential 0.8 0.75 V

RD

Drain ohmic resistance 0 10 OhmsRS Source ohmic resistance 0 10 Ohms

RSH S/D sheet ohmic res. 0 10 Ohms/sq




SPICE Parameters

Level 1 - 3


CJ Zero-bias bulk cap./A 0 1.E-09 Fd/m^2

MJ Bulk jctn. grading coeff. 0.5 0.5

CJSW Zero-bias perimeter C/l 0 1.E-09 Fd/m

MJSW Per. C grading coeff. 0.5 0.5

FC For.-bias cap. coeff. 0.5 0.5

CGBO Gate-bulk overlap C/L 0 2.E-10 Fd/m

CGDO

Gate-drain overlap C/L 0 4.E-11 Fd/mCGSO G-S overlap C/L 0 4.E-11 Fd/m

AF Flicker-noise exp. 1 1.2

KF Flicker-noise coeff. 0.0 1.E-26




Level 1 Static Const.

For Device Equations

• Vfb = -TPG*EG/2 -Vt*ln(NSUB/ni)

- q*NSS*TOX/eOx

• VTO = as given, or

• = Vfb + PHI + GAMMA*sqrt(PHI)

• KP = as given, or

•

= UO*eOx/TOX




Level 1 Static Const.

For Device Equations

• β = KP*[W/(L-2*LD)]

• GAMMA = as given, or

•

= TOX*sqrt(2*eSi*q*NSUB)/eOx• 2*Φp = PHI = as given, or

• = 2*Vt*ln(NSUB/ni)

• ISD = as given, or = JS*AD• ISS = as given, or = JS*AS




Level 1 Static

Device Equations

• vgs < VTH, ids = 0

• VTH < vds + VTH < vgs, linear region

•

id = KP*[W/(L-2*LD)]*[vgs-VTH]*vds-vds^2/2]•

• VTH < vgs < vds + VTH, Saturation region

• id = KP/2*[W/(L-2*LD)]*(vgs - VTH)^2*(1 + LAMBDA*vds)




DC SPICE Models

Level 1 (Schichman-Hodges) DC Model

• The SPICE element description line for a MOSFET has the following

form: Mx nd ng ns <nb> mname <L=val W=val AD=val + AS=val

PD=val PS=val NRD=val OFF IC=vds, vgs, vbs TEMP=val>

• All parameter value pairs between < and > are optional.

• Equations

• VT – Equation as derived previously

• ID – Equations as derived previously with linear mode equation

times (1+λVDS) for continuity across linear-saturation

boundary. Both use Leff in place of L where:• Leff = L – 2 LD

• Key Parameters: What do they represent?

KP – process trans conductance k‘




DC SPICE Models

Level 1 (Continued)

• VTO (note O, not 0!) – zero substrate-bias threshold voltage

VT0

• GAMMA – substrate-bias or body-effect coefficient γ

•

PHI – twice the Fermi potential 2ΦF• LAMBDA – channel length modulation λ

• Additional Parameters: What do they represent?

• LD – Lateral diffusion (If not present, may need to find Leff

manually!)

• TPG – Type of gate material: 0 – A1, +1 – opposite to

substrate, -1 – same as substrate. Default +1 For the

typical CMOS process, TPG = 1 for NMOS and – 1 for PMOS




DC SPICE Models

Level 1 (Continued)

• NSUB – substrate impurity concentration NA (NMOS) ND (PMOS)

• NSS – Surface state density – Used to define surface component

of VT0.

• TOX – Oxide thickness tox

• U0 (note 0, not O) – Surface mobility µ0

• RD, RS – Drain resistance, Source resistance

• RSH – Drain and Source sheet resistance (Ω/□)

• Derived Parameters. Note that if some parameters missing, others,

if present, can be used to derive them. E. g. NSUB to derive PHI, andTOX and U0 to derive KP. Question: What parameters to derive

GAMMA? If the derivable parameters are present in the model,

they will be used; if not, derived if possible from other parameters

(and defaults), else, defined.9/24/2013 KIIT UNIVERSITY, BHUBANESWAR 19



SPICE simulation of MOS V-I

Characteristics• *drain characteristics of MOSFET

vds 1 0 dc 5

vgs 2 0 dc 1

vnull 1 3 dc 0

m1 3 2 0 0 nmos w=10u l=1u.model nmos nmos()

.dc vds 0 5 .1 vgs 0 5 1

.print dc i(vnull)

.plot dc i(vnull)

.end




To be continued……….


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MOSFET modeling for Simulation