Carrier Transport

Syllabus: Carrier Transport: Diffusion current, drift current, mobility and resistivity, generation and recombination of carriers, Poisson and continuity equations.

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Drift and diffusion

Drift is driven by a field, diffusion by a concentration gradient. The two coefficients are linked by the Einstein relation, so knowing mobility gives diffusivity for free.

$J_{drift}=q(n\mu_n+p\mu_p)E$
conductivity $\sigma=q(n\mu_n+p\mu_p)$
$J_{diff}=qD_n\frac{dn}{dx}-qD_p\frac{dp}{dx}$
$\frac{D}{\mu}=\frac{kT}{q}=V_T\approx26\ \text{mV at }300\ \text{K}$

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Diffusion length and continuity

Excess minority carriers injected at one point decay exponentially with distance. The diffusion length is how far they get before recombining.

$L_n=\sqrt{D_n\tau_n}$

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Carrier Transport

Syllabus: Carrier Transport: Diffusion current, drift current, mobility and resistivity, generation and recombination of carriers, Poisson and continuity equations.

3 hand-picked visual resources on this topic — under each card, look for See it. Every link is checked against the live site before it ships.