Two single-photonic devices are presented. A single-photon turnstile device is based on the simultaneous Coulomb-blockade effect for electrons and holes in a mesoscopic, double-barrier, pn-tunnel junction. By periodically modulating the bias voltage between electron and hole resonant tunneling, a single electron and a single hole can periodically be injected into the central island, followed by single-photon emission. A solid-state photomultiplier (SSPM) utilizes the impact ionization of shallow As donor levels in Si as the multiplication mechanism. A SSPM, combined with a cold GaAs cascode amplifier that operates at 4 K, achieves single-photon detection with high quantum efficiency, high gain, and short response time.