In order to exploit the potential of GaAs/Ge heterojunctions for high‐speed heterojunction bipolar transistors and hot‐electron transistors, aPnpGaAs/Ge floating base phototransistor, sensitive to 1.1–1.55‐&mgr;m wavelength range, was made. The gallium‐doped Ge substrate was first exposed to the As2flux in the growth chamber of a molecular beam epitaxy system to form apnjunction in Ge. This was followed by the growth of ap‐type Be‐doped GaAs layer which served as the emitter. The fabricated mesa devices showed an optical gain of 85 at 1.15 &mgr;m incident wavelength. The gain was found to be nearly independent of the incident light power indicating good quality of the GaAs/Ge heterointerface.