A nitride semiconductor device comprises: a layer structure including an active region (102) containing AlxGayIn1-x-yN quantum dots layers (102a), and means (104a,104b) for applying an electric field across the active region to modify the spin orientation of excitons in the quantum dots. The exciton spin lifetime at 300K is, for at least a range of values of the electric field applied across the active region, at least 1 ns, more preferably at least 10 ns, and particularly preferably at least 15 ns or 20 ns. These lifetimes may be obtained by configuring the device such that the exciton binding energy is, for at least a range of values of the electric field applied across the active region, 25 meV or greater.