Carrier Mobility — the speed at which electrons or holes move through a semiconductor when an electric field is applied, directly determining transistor switching speed.
Definition
$$\mu = \frac{v_d}{E}$$
Where $v_d$ is drift velocity and $E$ is electric field. Units: cm²/(V·s)
Typical Values (Silicon at 300K)
Electron mobility: ~1400 cm²/(V·s)
Hole mobility: ~450 cm²/(V·s)
This is why NMOS (electron carriers) is ~3x faster than PMOS (hole carriers)
Factors Affecting Mobility
Temperature: Higher T → more lattice scattering → lower mobility
Doping: Higher doping → more impurity scattering → lower mobility
Electric field: Very high fields → velocity saturation (~10⁷ cm/s)
Crystal orientation: <100> vs <110> planes have different mobilities
Mobility Enhancement Techniques
Strained Silicon: Apply tensile strain (for NMOS) or compressive strain (for PMOS) to the channel. 30-50% mobility improvement
High-mobility channels: Ge (2x electron, 4x hole), InGaAs (10x electron). Used in research
FinFET fin orientation: Choose crystal direction for optimal mobility
Why Mobility Matters
Higher mobility → higher drive current → faster switching → higher frequency
$I_{on} \propto \mu$ — drive current directly proportional to mobility
Carrier mobility is the physical foundation of transistor speed — every generation's performance gains partly come from mobility engineering.
Take electron mobility further
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