Theoretical and experimental studies of the duo-emitter diode are described. In this diode an electron emitter and an ion emitter are facing each other. Two space-charge configurations have been found for this diode, one where the interelectrode space is occupied by a dense plasma and one where potential barriers for the emitted particles exist near the emitters. The experiment, using a low-pressure cesium-vapor diode, gives results in good agreement with theory.