A method for photo-detecting and an apparatus for the same are provided. The apparatus for photo-detecting includes a first P-N diode and a second P-N diode. The first P-N diode, has a first P-N junction which has a first thickness, by which a first electrical signal is generated when irradiated by light, and the second P-N diode has a second P-N junction which has a second thickness, by which a second electrical signal is generated when irradiated by light. The second thickness is larger than the first thickness and an operation of the first electrical signal and the second electrical signal is proceeded for obtaining a third electrical signal.