The diffusion length is the average distance, that the excess carriers can cover before they recombine. It depends on the lifetime and mobility of the carriers.

It is important to keep in mind, that the diffusion constant D is not a constant, but is dependent of the carrier mobility.

For the resolution of every electrical measurement the diffusion length is the limitation. This means for very high quality silicon samples that only a maximal resolution of about 1 mm can be achieved.