A photodiode, comprising: a first photodiode, comprising a p type substrate and a signal output region; a positive electric field providing structure, provided in the P type substrate, configured to receive a positive voltage to provide a positive electric field according to the positive voltage to attract electrons in the P type substrate; wherein an avalanche region is formed when the positive voltage is larger than a threshold voltage, thereby a current path is formed between the signal output region and the positive electric field providing structure. The present invention also discloses an optical sensor comprising pixels with different light sensitivities.