Chapter 7 Optical detectors Incident light with an energy of hv>Eg excites an electron and causes it to jump from the valence band to the conduction band, thereby creating an electron-hole (e-h) pair. An incident photon can cause the production of e-h pairs in three different regions (A, B and C) in the p-n diode. However, rapid detection is only achievable in B, the depletion region. Schematic description of field strength variations in a p-n diode. Reverse voltage and external load applied. By applying an intrinsic layer between the p- and n-layers, the absorption region obtained is much broader than that of a p-n diode. The three layers of a front-illuminated PIN diode. PIN diode for longer wavelengths. Semiconductor materials: InP and InGaAsP. A high reverse voltage of 100-300 V gives the electrons such high kinetic energy that multiplication of e-h pairs occurs through collision ionization. Example of the distribution of field strength in an APD among the different layers. Construction of an RAPD. The responsivity of a photodiode is dependent both on the wavelength of the incident light and the materials used. Model of a PIN diode with internal resistance Ri, internal capacitance Ci , and external load RL. Relationship between BER and optical SNR.