Explain how a p- n junction diode may be used to detect optical radiation. Distinguish between the photoconductive and photovoltaic modes of operation.
A photodiode is to be used, without applied reverse bias, to detect optical radiation with vacuum wavelength 0.8 μm and incident powers in the range 10 μW to 1 m W. The photodiode has a 70% quantum efficiency and a dark reverse saturation current of 1 nA.
Calculate the value of the load resistance needed to ensure shot noise limited detection at room temperature. Why cannot linear operation be achieved if the photo detector is directly connected to this load? Explain how a transimpedance amplifier may be used to overcome this problem.