Materials
In principle any semiconductor material can be used as a multiplication region:
- Silicon will detect in the visible and near infrared, with low multiplication noise (excess noise).
- Germanium (Ge) will detect infrared out to a wavelength of 1.7 µm, but has high multiplication noise.
- InGaAs will detect out to longer than 1.6 µm, and has less multiplication noise than Ge. It is normally used as the absorption region of a heterostructure diode, most typically involving InP as a substrate and as a multiplication layer. This material system is compatible with an absorption window of roughly 0.9-1.7 µm. InGaAs exhibits a high absorption coefficient at the wavelengths appropriate to high-speed telecommunications using optical fibers, so only a few micrometres of InGaAs are required for nearly 100% light absorption. The excess noise factor is low enough to permit a gain-bandwidth product in excess of 100 GHz for a simple InP/InGaAs system, and up to 400 GHz for InGaAs on silicon. Therefore high speed operation is possible: commercial devices are available to speeds of at least 10 Gbit/s.
- Gallium nitride based diodes have been used for operation with ultraviolet light.
- HgCdTe based diodes operate in the infrared, typically out to a maximum wavelength of about 14 µm, but require cooling to reduce dark currents. Very low excess noise can be achieved in this material system.
Read more about this topic: Avalanche Photodiode
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