e p-n junction [27]. If the junction is inversely polarized, all the carriers created by absorbed photons are immediately removed from the junction area due to the inverse polarization field and constitute the photocurrent. Since the number of generated carriers is proportional to the incoming photon number, the intensity of the photocurrent is proportional to the optical power, SO that a photodiode performs quadratic detection of the incoming field. Since for telecom applications the interesting wavelength region is the near infrared, telecom photodiodes are built using materials with a high-absorption coefficient in this region of the spectrum. Almost all the photodiodes used in telecom equipments are built using III-V alloy like InGaAsP or InP in order to optimize the absorption in the desired wavelength region [105], but the research on new materials has recently pointed out new alternatives that could give good results in the future [28,29]. Essentially two types of photodiodes are used: PIN photodiodes, where the junction is a p-i-n junction SO as to optimize the absorption efficiency, and Avalanche photodi- odes (APD), where internal gain is achieved by the avalanche multiplication of the photo- generated carriers. Considering PIN photodiodes, the first important characteristic is the quantum effi- ciency n of the photodiode, defined as the probability that an incoming photon will gener- ate a carrier couple that will contribute to the photocurrent. The expression of the quantum efficiency is given by =(1-f)r(1-e-ad) (5.24) Technology for Telecommunications: Integrated Optics and Microelectronics where f is the facet reflectivity, SO that (1 - f) is the probability that the photon is not reflected a is the absorption coefficient of the junction zone d is its thickness, SO that (1 - e-ad) is the probability that the photon is absorbed in the junction zone creating free carriers is the probability that the created carrier exits from the junction depletion zone with- out recombination From Equation 5.24 the i