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Carrier Mobility

Carrier Mobility (μn,p) is the measure of ease of charge carrier drift. That is, a measure of how quickly a charge carrier can move through a material. For example, how quickly an electron can travel through a semiconductor.


When an electric field Equation upper E is applied across a material, the electrons gain a net velocity in the direction of the field called the drift velocity, defined as

Equation v Subscript d Baseline equals plus-or-minus StartFraction q tau Subscript c Baseline Over 2 m Subscript n comma p Baseline EndFraction upper E

Where the carrier mobility Equation mu Subscript n comma p [cm2/Vs] is defined as

Equation mu Subscript n comma p Baseline equals StartFraction q tau Subscript c Baseline Over 2 m Subscript n comma p Baseline EndFraction

Note that this is for both electrons ( Equation mu Subscript n ) and holes ( Equation mu Subscript p ).


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It's worth noting that as the time between collisions ( Equation tau Subscript c ) increases, then mobility increases. Likewise, the lighter the particle ( Equation m ), then mobility also increases.

In the case of a semiconductor such as silicon, at a fixed temperature (e.g., ambient temperature), the mobility will depend on doping. For the same doping level, Equation mu Subscript n > Equation mu Subscript p , therefore holes are "heavier" than electrons. Additionally, for low doping level, Equation mu will be mostly limited by collisions with lattice (as temperature is increased, Equation mu will decrease). With medium and high doping levels collisions with ionized impurities will limit mobility.

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