The present invention relates to an ultra-high speed semiconductor phototransistor which comprises a substrate. A conductive collector layer, on which a collector electrode is formed, is formed on the substrate. A collector barrier layer for collector electric potential is formed on the conductive collector layer. A conductive base layer, on which a base electrode is formed, is formed on the collector electric potential barrier layer. An emitter barrier layer for emitter electric potential is formed on the conductive base layer for injecting hot-electrons onto the conductive base layer. The emitter barrier layer for emitter electric potential further comprises various sizes of quantum-dot array combination structures for absorbing an infrared ray. A blocking barrier layer positioned beneath the quantum-dot array combination structures reduces a dark current passed through the quantum-dot array combination structure. A second buffer layer positioned beneath the blocking barrier layer absorbs an electric potential change in the quantum-dot array combination structure due to the applied voltage. A conductive emitter layer, on which an emitter electrode is formed, is formed on the emitter barrier layer for emitter electric potential.