If both electrons and holes are present the conductivities, add. This is because under the effect ofan applied electric field the holes and electrons flow in opposite directions, and a negative charge moving in the (say) +z-direction is equivalent to a positive charge moving in the –z-direction. Assume that in a certain semiconductor the ratio of electronic mobility, μe, to hole mobility, μh, is equal to 10, the density of holes is Nh =1020 m–3, and the density of electrons is Ne =1019m–3. The measured conductivity is 0.455 ohm–1m–1. Calculate the mobilities.