mportance of the absorption coefficient is evident and it is this characteristic that selects the photodiode material once the wavelength region is determined. The photocurrent created by a PIN photodiode as a consequence of the detection of a light beam is composed by four components: The first term is constituted by the photon-generated carrier flux that is given by Cpc=nq, where q is the electron charge and Q is the photon flux, that is, the number of photons per unit time arriving on the photodiode front area. It is important to underline that is a ran- dom variable, corresponding to the measure of the quantum operator number (î) relative to the incoming field quantum state. In almost all the telecommunication applications, the incoming field can be considered a time sequence of quasi-stationary coherent states; thus Q is a Poisson variable whose parameters slowly change as a function, for example, of the field modulation. Assuming a constant incoming field and separating the average and the fluctuations, the photo-gener- ated carrier flux can be written as (5.25) where P is the power of the incoming optical beam, Rp = qn/hw is the so-called photodiode responsivity, and the random term Nsh(t), which is in all respects a noise term, is the so- called shot noise. The shot noise has a distribution determined by the fact that the photon flux is a Poisson variable and that an incoming photon has a probability n of generating a photocarrier. Thus, using the probability composition law, the probability distribution of the photo- current flux can be written as (5.26) However, if the number of photons arriving on the photodiode in the relevant time unit is much higher than 1, the distribution of the number of photocarriers can be assumed Gaussian, with a standard deviation equal to the incoming optical power. The second current term is the dark current, which is the current that always pass through an inversely polarized p-i-n junction. The third term is the current generated by thermally excited carriers and, finally