Wavelength sensitivity is determined by the material composition of the photodiode. In general, silicon photodiodes are used for detection in the visible portion of the spectrum, InGaAs crystals are used in the near-infrared portion of the spectrum between 1000 nm and 1700 nm, and germanium detectors are used between 800 nm and 1500 nm. Figure 8.2 depicts the various types of detectors and their wavelength dependence.