A photodetector with lateral GaAs n+-π-n+ structure is studied. This structure is fabricated by Ga ion implantation into n+-GaAs epilayer on a semi-insulating GaAs substrate using focused-ion-beam (FIB) technology. This photodetector behaves as a phototransistor in low-bias region and avalanche multiplication appears beyond a breakdown voltage. Multiplication gain of more than 50 and impulse response of less than 200 ps were obtained. These characteristics make this device suitable for application in monolithic optoelectronic circuits.