We have fabricated a silicon phototransistor using low-temperature processing. The emitter of the bipolar transistor is an Al-SiO2-p-Si tunnel junction. The quantum efficiency gain is 200 at λ = 0.6328 µm. The common-emitter current gain of the analogous three-terminal metal-insulator-semiconductor (MIS) transistor also is 200. This demonstrates the high minority-carrier injection efficiency of the MIS emitter.