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Chapter 3:
Generation-Recombination Processes: Carrier Lifetime
The competition of generation processes with recombination processes directly affects the performance of photodetectors, setting up a steady-state concentration of carriers in semiconductors subjected to the thermal and optical excitation and determining the kinetics of photogenerated signals. The decay of optically generated carriers due to recombination reduces the quantum efficiency of the device and photoelectric gain. Worse, the statistical nature of the generation-recombination processes results in fluctuation of carrier concentration, causing noise that limits the performance of photodetectors. The effects of fluctuating recombination rates can be avoided in many cases by arranging for the recombination process to take place in a region of the device where it has little effect due to low photoelectric gain − for example, at the contacts in sweep-out photoconductors or in the neutral regions of the diodes. The generation process, with its associated fluctuation, however, cannot be avoided. The narrow-gap semiconductors necessary for infrared photodetectors, especially those operated in the LWIR range at near-room temperature, exhibit very high inherent thermal generation rates. To achieve high performance, thermal generation must be suppressed to the lowest possible level. For practical purposes, the ideal situation is when the thermal generation is reduced to the level below that of the optical generation. One important example is the background-limited photodetector (BLIP), in which optical generation due to the background radiation exceeds the thermal component. The requirements for thermal generation rate can be highly reduced in heterodyne systems, when optical excita
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