Minority carrier lifetime (${\partial }_t\Delta n=-\Delta n/\tau$)

In a weak-injection regime where the majority population is nearly fixed, a small excess minority charge carrier population often relaxes exponentially with time constant $\tau$. This lifetime describes carrier removal by recombination. It differs from the momentum relaxation time governing charge carrier mobility: many momentum-scattering events can occur before a carrier recombines. Traps and spatial transport can make a single exponential inadequate.