Floating-base bipolar transistors are widely used semiconductor devices because they could both amplify signal current and suppress noise. Employing two-dimensional (2D) materials of ultrahigh photoelectric properties could further improve the device performance. Due to the difficulty in doping, homojunctions are usually not realizable for many 2D materials. Instead, a heterojunction of various 2D materials of different Fermi levels is usually needed. However, the material interface of a heterojunction deteriorates device performance and makes the fabrication process difficult. Here, the doping difficulties have been solved by utilizing a solid ionic dielectric material (LiTaO3) and a floating-base bipolar transistor based on a 2D material (monolayer MoS2 here) homojunction is realized. The transistor shows tunable ambipolar transport characteristics. Particularly, under illumination, the amplification coefficient of a phototransistor can be optimized by changing the gate voltage. The optimized photoresponsivity of the device could reach up to 7.9 A W-1 with an ultrahigh detectivity of 3.39 × 1011 Jones. The overall fabrication processing is compatible to conventional processing. This design can effectively extend the application of 2D materials.