Abstract 1/f noise calculations and experiments are presented for two types of reverse-biased silicon p-n diodes: (i) the current is determined by diffusion in the base, and (ii) the current is determined by generation-recombination in the junction. The experimental results can be interpreted in terms of 1/f mobility fluctuations. It is found that at fixed reverse bias the ratio of 1/f noise and shot noise decreases with increasing lifetime of minority carriers in the base or with increasing lifetime of carriers in the junction. The influence of 1/f noise and shot noise on the detectivity D ∗ in photodiodes is investigated. If the background temperature equals the temperature of the photodiode, the detectivity is found to be limited by 1/f noise and shot noise. From the well-known characteristics of the photon-noise limited detectivity D ∗(λ c, f) at peak wavelength λ c and frequency f for various background temperatures and from the 1/f noise and shot noise limited D ∗(λ c, f) characteristics for various detector temperatures, conclusions can be drawn about optimum performance of photovoltaic detectors.