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Tutorial 7. Derek Wright Monday, March 7 th , 2005. Silicon MOSFETs. Introduction MOS Capacitors MOSFET Structure MOSFET Scaling Gate Dielectrics Gates Junctions and Contacts Alternate MOSFET Structures. Introduction. MOSFETs are a kind of Field Effect Transistor used in digital ICs

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Tutorial 7

Tutorial 7

Derek Wright

Monday, March 7th, 2005

Silicon mosfets
Silicon MOSFETs

  • Introduction

  • MOS Capacitors

  • MOSFET Structure

  • MOSFET Scaling

  • Gate Dielectrics

  • Gates

  • Junctions and Contacts

  • Alternate MOSFET Structures


  • MOSFETs are a kind of Field Effect Transistor used in digital ICs

    • Use a FET because gate voltage uses less current than BJT’s base current

  • BJT was developed first, and FET was theorized, but impractical

    • Couldn’t make the Field Effect work due to technology constraints at the time

Mos capacitor
MOS Capacitor

  • It’s important to understand how a MOS capacitor works:

    • Capacitance is a limiting factor in IC performance

  • Mobile charges in gate

  • Mobile charges in channel (between drain and source)

  • Separated by dielectric (gate oxide)

    = capacitor

Mos capacitor2
MOS Capacitor

  • http://jas.eng.buffalo.edu/education/mos/mosCap/biasBand10.html

    • Shows how a depletion layer forms

    • The blue charge is what lets current go from source to drain

    • Other good applets on the site

Mos capacitor4
MOS Capacitor

  • Capacitance changes with applied voltage

  • Leads to complicated CMOS simulations

  • Can be exploited in some kinds of VCOs (MOS Varactor)

Mosfet structure
MOSFET Structure

  • We use a MOS capacitor in inversion mode

  • The minority carriers form the “channel”

  • Ions are implanted on either side of the gate to act as sources of carriers

  • Contacts are put on the diffusions to form the source and drain

  • Carriers go from the source to the drain

Mosfet scaling
MOSFET Scaling

  • Reducing the size of MOSFETs in ICs has many benefits:

    • Higher density

    • Higher speed

    • Lower Power

  • It also introduces many problems:

    • Thin gate oxides

    • Short channel effects

    • Higher leakage current

Gate dielectrics
Gate Dielectrics

  • Gate thickness scales by 1/ with decreasing device dimensions

  • We’re fast approaching the practical limit of how thin SiO2 gates can get

  • Tunneling can occur causing gate leakage

  • Other problems like hot carriers start to become problematic

Gate dielectrics1
Gate Dielectrics

  • We can use a thicker dielectric if it has a higher r

  • These “high-k” dielectrics mean that a given gate voltage will produce a higher E-field

  • Or, a given gate voltage will produce the same E-field with a thicker dielectric layer

Gate dielectrics2
Gate Dielectrics

  • Problems with a thin gate:

    • Oxide thickness variation

    • Impurities from poly gate (particularly B)

    • Reliability and lifetime problems

    • High gate current

  • Gate leakage current (VG = 1V):

    • 1pA/cm2 at 3.5 nm

    • 10A/cm2 at 1.5 nm

Gate dielectrics4
Gate Dielectrics

  • Solutions to gate problems:

    • Add nitrogen to SiO2

    • Use high-k dielectrics

      • High-k dielectrics must meet a number of criteria

      • Must be thermally stable

      • Good electronic properties

      • Microstructural stability

      • Deposition tools and chemistry

      • Process compatibility


  • Poly-silicon is used for gates because:

    • Adjustable work function through doping

    • Process compatibility

  • Drawbacks include:

    • It’s a semiconductor, so it forms a depletion layer which adds to the EOT (effective oxide thickness)

    • High resistivity

  • Metal is considered as the successor to poly-silicon gates

Junctions and contacts
Junctions and Contacts

  • Other resistances must be less than 10% of the channel resistance (Rchan)

  • Rchan = [(W/L)  (ox/tox (VG – VT)]-1

    • L  Rchan (scaling)

    •   Rchan (new substrates)

    • ox  Rchan (high-k dielectrics)

    • tox Rchan (high-k dielectrics and scaling)

    • VT  (VG – VT)   Rchan (doping)

Junctions and contacts1
Junctions and Contacts

  • Contacts connect the metal lines to the source/drain/gate of a MOSFET

  • Contact resistance becomes a problem as geometries shrink

  • This can be partially solved by using silicides:

    • Silicides are metal/silicon alloys with a low resistance

Junctions and contacts2
Junctions and Contacts

  • Formation of self-aligned silicides (salicides)

  • Metal is deposited over entire wafer

  • Reacts with exposed silicon

  • Unreacted metal is selectively etched off

Alternate mosfet structures
Alternate MOSFET Structures

  • Silicon On Insulator (SOI) wafers eliminate capacitive coupling to the substrate

  • An oxide layer is buried below the transistors, eliminating coupling to the substrate

  • SOI:

    • reduces leakage

    • reduces capacitance

    • higher speed

    • less susceptible to soft errors

Alternate mosfet structures1
Alternate MOSFET Structures

  • New technologies for coming years:

    • High-k gate dielectrics

    • Low-k Dielectrics

    • Metal gate electrodes

    • SOI

    • Strained silicon

    • Vertical multi-gate structures

Thank you
Thank You!

  • This presentation will be available on the web.