In the Gunn effect, which occurs in certain semiconductors in strong electric fields, electrons are driven out of a low-mass central valley into a heavy-mass side valley in k (momentum) space, ultimately resulting in negative differential resistance (NDR). Recently, there has been interest in the possibility of exploiting this phenomenon in heterostructured semiconductors, as a means to realize novel terahertz sources. Here, we demonstrate that a GaAs/AlGaAs heterostructure planar Gunn diode exhibits both NDR and electroluminescence (EL), the characteristics of which are consistent with the combined action of simultaneous k-space and real-space electron transfer. By making simultaneous electrical-transport and EL measurements, we reveal the following scenario as a function of the applied voltage. At sufficiently large bias, the onset of k-space transfer gives rise to NDR and results in the formation of traveling high-field domains. As each domain travels toward the anode, impact ionization taking place wi...