Photoconductive (biased): During photoconductive (PC) operation, a reverse bias on the photodiode results in a number of response advantages, such as a faster rise time. One disadvantage is the slightly lower responsivity at higher wavelengths (see graph). This makes it ideal for low signal detection. In this case, the NEP (or Noise Equivalent Power) will be lower, thereby allowing greater sensitivity at lower wavelengths. Since Dark Current is a function of bias magnitude, PV operation eliminates Dark Current as a source of noise. Photovoltaic (unbiased): During photovoltaic (PV) operation, no external bias is applied to the photodiode. Some key issues regarding detectors are outlined in the terms and definitions box below.

While noise is certainly a key means of characterizing detectors, it is only one of the characteristics which should be considered when selecting a detector. The ratio of the overall signal output to the noise level is known as the signal to noise ratio (S/N) and can be used to determine whether noise will be a concern for a particular application. A variety of these 'non-light' contributions are present and, when summed up, make up the total noise within the detector. For example, fluctuations in thermal energy can easily be mistaken for light intensity changes. Since light is not the only source of energy that can excite an electron, detectors will produce some amount of current that is not representative of incident light. When light, with enough energy to excite an electron from the valence to the conduction band, is incident upon the detector, the resulting accumulation of charge leads to a flow of current in an attached circuit. The root of the theory behind this phenomenon is a small energy gap between the valence and conduction bands of the detector. Through the photovoltaic effect, silicon detectors provide a means of transforming light energy to an electrical current.

E current vs dark noise