Silicon Photonics: Fundamentals and Devices

8.2 Optical Detection

The photodetectors that are used in imaging technologies or in optical communication systems utilize the internal photoemission effect based on Einstein's theory of photoelectric effect and Planck's quantum theory of radiation. In a semiconductor photodetector, photons from the incident optical signal are absorbed as they travel through the semiconductor material. If the photon energy is equal to or larger than the band gap energy (E_G) of the optical absorption (or active) region of the photodetector, then electron–hole (e–h) pairs are created, with the photoelectrons in the conduction band and holes in the valence band. Also, these photoexcited carriers – electrons and holes – are free to move within the semiconductor material, and under an externally applied electric field, a photocurrent in the external circuit flows.

In this book, we are primarily concerned with silicon or silicon-based photonics. Silicon is an indirect band gap semiconductor with a small absorption coefficient for light with a wavelength above 0.5 µm. However, it has a high ratio of the ionization coefficients of the holes (β_i) and electrons (α_i), so it is a good choice for avalanche photodetectors, especially for local area networks or for short-wavelength applications. However, it is not suitable for long-haul applications because the minimum attenuation in current silica-based fibers is around 1.5 µm. But if silicon is combined with germanium, then it is possible to extend the applications ...