Lesson 4 of 12 · 6 min
Mobility
NCERT § "Mobility"
Silicon's electrons drift about thirty times more easily than copper's, yet copper conducts billions of times better. Mobility explains half of that puzzle.
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In short
Mobility μ is the magnitude of drift velocity per unit electric field: μ = |v_d|/E.
Its SI unit is m² V⁻¹ s⁻¹.
For electrons, μ = eτ/m, so a longer relaxation time gives a higher mobility.
Conductivity can be written in terms of mobility: σ = neμ for one type of carrier.
Mobility is defined as a positive quantity for every carrier; the direction of drift is decided separately by the sign of the charge.