A semiconductor light emitting device, including: a heterojunction bipolar light-emitting transistor having a base region between emitter and collector regions; emitter, base, and collector electrodes for coupling electrical signals with the emitter, base, and collector regions, respectively; and a quantum size region in the base region; the base region including a first base sub-region on the emitter side of the quantum size region, and a second base sub-region on the collector side of the quantum size region; and the first and second base sub-regions having asymmetrical band structures.