Mobility Variation

Strained silicon: stretch the lattice and carriers move fasterDeforming the channel raises mobility — more drive current without shrinking the transistor1 · Strain vs mobilityspacing between atoms sets carrier speedrelaxed Sislowerstrained (stretched)fasterWider atomic spacing reshapes theenergy bands so electrons scatterless and drift faster for the samefield. Higher µ → higher drive current.µ = mobility · sets Ion at fixed voltage2 · Opposite strain per typeelectrons and holes want different latticesnMOS — tensile (pull apart)gatechannel stretched → fast electronspMOS — compressive (SiGe pushes in)SiGeSiGegatechannel squeezed → fast holesBigger Ge atoms in the source/draincompress the pMOS channel; a tensilecap film stretches the nMOS one.Same die, opposite strain, both faster.3 · How strain is built inthe process levers — since ~90nmEmbedded SiGe S/D (pMOS)recessed source/drain refilled withSiGe to compress the channel.Tensile CESL nitride (nMOS)a stressed contact-etch-stop linerpulls the channel into tension.Stress-memorization annealstrain locked in during recrystallizationof the source/drain.Why it stuck aroundA "free" performance boost: more speedat the same gate length. Still used insideFinFET and nanosheet channels today, nowvia 3D-aware stressor engineering.Tensile → nMOSStretching the lattice speeds upelectrons — the majority carrier.Compressive → pMOSSqueezing the lattice speeds upholes — via embedded SiGe.Mobility = speedHigher µ lifts drive current withouta smaller, leakier transistor.

Mobility variation is the spread in carrier transport efficiency across devices caused by local differences in scattering, strain, and interface quality - it directly modulates drive current and timing at fixed geometry and bias.

What Is Mobility Variation?

  • Definition: Device-to-device and location-dependent fluctuation in effective electron or hole mobility.
  • Physical Contributors: Surface roughness scattering, phonon interactions, Coulomb scattering, and stress variation.
  • Electrical Impact: Idsat spread, gm variation, and delay distribution broadening.
  • Correlation: Often coupled with strain and process-induced local geometry effects.

Why Mobility Variation Matters

  • Timing Spread: Logic path delays shift even when Vth targets are met.
  • Analog Gain Variance: Transconductance uncertainty degrades precision circuits.
  • Power-Performance Tradeoff: Mobility tails influence both speed bins and energy targets.
  • Model Accuracy: Needs explicit treatment in compact models for robust signoff.
  • Yield Sensitivity: Combined with Vth variation, mobility spread expands failure tails.

How It Is Used in Practice

  • Extraction: Use dedicated test structures to separate mobility from threshold effects.
  • Statistical Modeling: Include mobility sigma and correlation with other parameters.
  • Mitigation: Optimize strain engineering, interface quality, and layout context uniformity.

Mobility variation is a fundamental transport-level variability source that shapes real silicon speed beyond nominal design assumptions - robust performance prediction requires mobility-aware statistical modeling.

Take mobility variation further

Ask the copilot about this term, or have our engineers assess it against your process.

Ask CFSGPT about this → Request a technical assessment → Open the CFS app