The presence, in a volume region, of a differing density of particles across that volume. In semiconductor devices there are two kinds of concentration gradient. The first is the fixed variation in the concentration of n-type and p-type impurities as an intended consequence of their manufacture. The second arises as the impurities lead to the generation of a corresponding concentration gradient associated with the mobile carrier density. This leads to diffusion currents that work to flatten this gradient, and it is the interaction of these with voltage-driven drift currents that determines how the device operates. See n-type semiconductivity; p-type semiconductivity.