Silicon Photonics: Fundamentals and Devices

8.4 Examples of Types of Photodetectors

Semiconductor photodetectors can be broadly classified into two categories – those without internal gain and those with internal gain. Photodetectors without internal include p–n photodiodes, p–i–n photodetectors, Schottky barrier photodetectors, and metal–semiconductor–metal photodetectors (MSM-PDs). Photodetectors with internal gain include photoconductors, phototransistors, and avalanche photodetectors (APDs). The latter types of photodetectors are used in optical fiber communications systems to improve the overall sensitivity of the front-end photodetector in the optical receiver. In this section, we discuss several of these types of photodetectors.

A p–n photodiode is a p–n junction diode operating under reverse-bias conditions. The incident photons are absorbed in both the depletion and the neutral regions, where the number of the generated electron–hole (e–h) pairs is proportional to the optical power. The e–h pairs get separated in the depletion region and drift under the effect of the applied electric field. In general, the depletion region should be wide enough to achieve high conversion of light to e–h pairs – or to have a high quantum efficiency. However, the thickness of the absorption layer should not be so large because the drift time through the depletion region is long, resulting in a low bandwidth of the photodiode. This illustrates one of the compromises between quantum efficiency and speed for many photodetector structures. ...