chapter3_handout.pdf

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DigitalICDesignandArchitecture

Goal of this chapter

Present intuitive understanding of deviceoperation

Introduction of basic device equations of PNunction

Introduction of basic device equations of FET

Analysis of secondary and deep-sub-microneffects

Review: ideal / non-ideal switch

PropertiesofSi

covalent bonding in the Si

crystal, veiwed along a direction

EnergybanddiagramofSi:

Eg=1.1eV


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Typical band structures at 0 K

A. Insulators have large band gaps which prevents electrons to jump from valence to conduction

band. B. Semiconductors have smaller band gaps such that electrons can be thermally excited tothe conduction band C. In metals, large number of electrons exist in the valence band.

Energy band model and chemical bond model of

Doping of Silicon

dopants in semiconductors: (a) donation of electronsfrom donor level to conduction band; (b) acceptance ofvalence band electrons by an acceptor level, and theresulting creation of holes; (c) donor and acceptoratoms in the covalent bonding model of a Si crystal.

Energy Band Diagram of p-type

Silicon

ln ip

A

nkT

q N F iF

E E

q

Junction between two materials

Two materials in intimate contact at equilibrium.Note: Since the net motion of electrons is zero, the equilibrium Fermi

level must be constant throughout.


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PN Junction: Energy Band

N NkT0 2

ln a

i

Vq n

1/2

2( )o a d

a d

V N NW

q N N

PN Junction Under External Bias

BandDiagramBandDiagram

Two terminal MOS Structure


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Energy Band Diagram of FET Two-Terminal MOS Structure with ExternalBias

ThreeRegionsofOperation:

1.AccumulationRegion:VG0,small

3.InversionRegion: VG>0,large

Two-Terminal MOS Structure Depletion Region Two-Terminal MOS Structure Inversion Region


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Two-Terminal MOS Structure Energy BandDiagram

Bending of the semiconductor bands at the onset of strong inversion:the surface potential fs is twice the value of fF in the neutral p material.

Two-Terminal MOS Structure Depletion Region

Two-Terminal MOS Structure Inversion Region


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CharacteristicsCharacteristics


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FET Band Diagram at Equilibrium MOS Operation: A Qualitative View

n-channel MOSFET cross-sections under different operating conditions: (a) linearregion forVG > VT and VD < (VG - VT); (b) onset of saturation at pinch-off, VG > VT andVD = (VG - VT); (c) strong saturation,VG > VT and VD > (VG - VT ).

Gradual Channel Approximation Gradual Channel Approximation


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Gradual Channel Approximation I-V Equations of NFET

I-V Equations of NFET NFET in saturation region


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NFET in saturation region: 2nd order effects NFET in saturation region: complete

NFET equations PMOS and NMOS equations


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Scaling Effects

CapacitanceCapacitance

ModelsModels


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MOSFET SaturationRegion

Capacitance Summary


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Drain/Source Capacitance Drain/Source Capacitance

Depletion Region Capacitance Depletion Region Capacitance


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Depletion Region Capacitance


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Documents

chapter3_handout.pdf