Semiconductors, modulated by periodicnandpdoping, possibly with intrinsic regions in‐between (‘‘n‐i‐p‐icrystals’’) represent a new class of semiconductors, because of their tunable electronic properties. A large number of intriguing phenomena, predicted by the theory, have been observed recently on GaAs doping superlattices grown by molecular beam epitaxy. In this paper we summarize the theory and experimental results with special emphasis on the aspects of the metastability of large deviations of the carrier concentrations from thermal equilibrium. Approximate analytical expressions for the lifetime enhancement, its dependence of the ‘‘design parameters’’ and on the state of excitation are derived. A brief outlook on possible extensions of the original concept is given.