Basic Principles of Silicon Detectors

Through the photovoltaic effect, silicon detectors provide a means of transforming light energy to an electrical current. The root of the theory behind this phenomenon is a small energy gap between the valence and conduction bands of the detector. 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. 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. For example, fluctuations in thermal energy can easily be mistaken for light intensity changes. A variety of these "non-light" contributions are present and, when summed up, make up the total noise within the 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. 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. Some key issues regarding detectors are outlined in the terms and definitions box below.