Particle Detectors: (4.24) Resonant-Cavity-Enhanced Photo Detector: (2) Theory of RCE Photo Detectors: This shows for an optimum value of quantum efficiency the bandwidth has to sacrifice. A p-i-n RCE photodetector can reduce the absorption region to a much smaller scale. In this case the carriers need to traverse a smaller distance as well, L1 (< L) and L2 (< L) for electrons and holes respectively. The length of L1 and L2 can also be optimized to match the delay between the hole and electron drift. And the transition bandwidth becomes: As in most of semiconductors   is more than   the bandwidth increases drastically. It’s been reported that for a large device of L=0. 5μm 64 GHz of bandwidth can be achieved and a small device of L=0. 25μm can give 120 GHz bandwidth, where conventional photodetectors have bandwidth of 10–30 GHz. (2. 3) Wavelength selectivity of RCE photo detectors: A RCE structure can make the detector wavelength selective to an extent due to the resonance properties of the cavity. The resonance condition of the cavity is given as 2βL+ ф1 + ф2 = 2mп. For any other value the efficiency η reduces from its maximum value, and vanishes when 2βL+ ф1 + ф2 = (2m+1) п. The wavelength spacing of the maxima of η are separated by the Free Spectral Range of the cavity, given as: Where neff is the effective refractive index and Leff, i[] are the effective optical path lengths of the mirrors. Finesse, the ratio of the FSR to the FWHM at the resonant wavelength, gives the wavelength selectivity of the cavity. This shows that the wavelength selectivity increases with higher reflectivity and smaller values of L