Photodiodes

  Photodetectors based on semiconductor technology. Photodiodes make use of the photovoltaic effect: the generation of a voltage across a p-n junction of a semiconductor, when the junction is exposed to light. The term is broadly used, including even solar elements; it usually refers to sensors used to measure the intensity of light. In high-energy physics they are used as readout elements associated to scintillators, e.g. for scintillating fibres and for some calorimeters ( [Fenker91]).

Their main features are:

• - excellent linearity and low noise (limited by shot noise);
• - wide spectral response and high quantum efficiency (e.g. about 80% at 800 nm);
• - easy calibration;
• - insensivity to magnetic fields;
• - compactness and mechanical ruggedness;
• - stability and long life.

Photodiodes are very similar to rectifying junction diodes, and are manufactured as p-n Si or GaAsP photodiodes, PIN Si photodiodes, GaAsP Schottky and Si avalanche photodiodes. The p layer is at the light sensitive surface, and the n-side at the substrate. An avalanche photodiode is obtained by adding to a simple PIN diode an electron multiplication region, viz. an area with large bias voltage generating secondary electrons and holes. This process multiplies the signal (and primary shot noise), and adds stochastic noise for the multiplication process; they are not suited for single-photon readout. Depending on the bias voltage, a proportional mode (allowing one to measure light intensity) or a Geiger mode (giving a larger, saturated signal) can be achieved. An application for scintillating fibre tracking has been discussed in [Nonaka96].

For an overview, see [Kazovsky96]. The radiation hardness of photodiodes has been studied in parallel with semiconductor detectors ( [Hall90]).