could someone explain to me how does a photodiode work? I read article but i still don ge tthe n junction and stuff... what does it mean it works reversed biased?
A photodiode can work in two ways:
1) When it is reverse-biased then light causes it to leak current. The reverse bias voltage causes its capacitance to be less so it switches fast. It has a temperature-dependent leakage current that is called "dark current".
2) Without bias voltage then it is a tiny solar cell which produces a tiny current when it receives light. It is slower but has no dark current.