The invention discloses a light sensing semiconductor unit, which comprises a substrate; a first doped region of a first conductivity type over the substrate; a second doped region of a second conductivity type over the first doped region; a third doped region of the first conductivity type over the second doped region; and an isolation structure for isolating the first doped region from the thirddoped region. Two depletion regions are formed, and photon-generated carriers which are driven into the two depletion regions can be swept out by the built-in electric field to form a photoconductivecurrent effect. Compared with the traditional diode with one depletion region, the design of the two depletion regions greatly improves the efficiency of absorbing photon-generated carriers; secondly, specific bias voltage is applied outside the two depletion regions, so that the two depletion extension regions can be expanded specifically and maximally, but no punchthrough effect is formed, different voltages and data information of photon-generated carrier absorption efficiency can be formed, and the light resolution sensitivity can be improved.