The transition metal dichalcogenides (TMDs) have great optical absorption and mobility in a subnanometer thickness. The MoSe2 among TMDs is significantly interested as an active layer in optical devices because it is superior to other TMDs in light absorption. Herein, we investigated the mechanism and characteristics of phototransistors using bi-layer MoSe2 film grown by molecular beam epitaxy. The interaction between the Al2O3 and MoSe2 was explored by X-ray and ultraviolet photoelectron spectroscopy, which showed that the chemical bonding and energy-band bending in the MoSe2 due to the electron doping. The optical devices exhibited well-controlled performance of the phototransistors with gate bias and illumination power-density, resulting in superior photoresponsivity (R) of 242 A/W. The mechanism of the R was fully interpreted by the change in quasi Fermi levels, defect states, mobility, and metal contact. Especially, capturing the electrons (holes) generated in the defect states increases the probability that the untrapped holes (electrons) reach the source and drain electrode, resulting in the high R. In addition, the decay mechanism was described by lowering the temperature, which persistent photocurrent was not observed at the low temperature. Finally, the performance of logic gates composed of the MoSe(2)d demonstrates its applicability to future photo-sensing systems.