Lecture 131 © Jeffrey Bokor, 2000, all rights reservedLecture 13Semiconductor Photodetectors• Seimiconductor band structure• Optical absorption across the bandgap• p-n JunctionEGelectronsholesECEVdopingEFEFn-type p-typeEChEVIf , an electron and hole (pair) is createdafter photon absorption.In a suitable structure, the electron and the holecan contribute to an electric current through thedevice.h EGNNDNA–=zNDNA–equilibriumEFEVbizLecture 132 © Jeffrey Bokor, 2000, all rights reserved• Depletion approximation: Assumes carriers diffuse across junction and create regions that are totallydevoid of free carriers• Reverse biasPhotodiodeReverse bias condition: electron and hole created in the depletion region follow the electric field and sep-arate.0lp–lnzzEUnder reverse bias, no current flowsbecause the barrier to diffusion increases.Under forward bias, barrier to diffusion isreduced.depletion widths widerhVThese carriers are pulled apartby the field.Lecture 133 © Jeffrey Bokor, 2000, all rights reservedThe electric field exists only inside the depletion region. So the light absorption must also occur there tocreate current.Construction• Photodiodes can be used at longer wavelength than photomultiplier – • Typically fast response time < 10 nsec• Compact, inexpensivelightly doped p-regionbackside electrodethin electrode passes lightthin, heavily doped n+ layerdepletion regionEGnpip-i-n photodiodeNPconstant -field in the i-layerLecture 134 © Jeffrey Bokor, 2000, all rights reservedSolar cells (Reference: Solar electricity, 2nd edition, Tomas Markvart, ed., John Wiley, 2000)silicon p-n juntion diode:Solar radiation -II0qVkT-------1–exp=IVI0Lecture 135 © Jeffrey Bokor, 2000, all rights reservedAM 1.5 - on a clear day, the typical maximum solar irradiance is ~1kW/m2 or 100 mW/cm2, whichtranslates to ~4.4X1017 photons/cm2-sec.In principle, when absorbed, this photon flux could produce a ‘generation current’ ofwhere N is the number of photons absorbed per second, and A is the area that is exposed to light. Forthe entire solar spectrum, this corresponds to about 70mA/cm2. The band gap for crystalline silicon is1.1 eV, so only the part of the spectrum shown above that is shaded in black can be absorbed. Thus,for silicon, the maximum generation current is about 44