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Avalanche photodiodes (APDs) are widely used to detect and amplify weak optical signals by utilizing the impact ionization process. The choice of material is critical for the detection of a particular wavelength, and it is often expedient to use a combination of different materials to optimize the overall device performance. The APDs are now capable of covering a wide spectrum from the infrared down to the ultraviolet wavelengths. This paper will review the material requirements to achieve high gain with low excess noise at the different wavelength regions.