Compound photodetectors, for example InGaAs, InP, or GaAs have due to their material properties higher absorption coefficients α, which lead to small penetration depths in the range up to 1 μm from the surface. Due to the wavelength dependent penetration depth of the light, the photodiodes and phototransistors will also have wavelength dependent bandwidths, since the distribution of the total photocurrent in drift and diffusion part is wavelength dependent. For example blue light with a wavelength of 430 nm generates electron–hole pairs in depths up to 0.2 μm while near infrared light with a wavelength of 850 nm has a 1/e penetration depth of about 16.6 μm. Light with short wavelength is absorbed near the semiconductor surface, while light in the near infrared region has a larger penetration depth and is therefore absorbed deep in the silicon. The generation rate G is dependent on the wavelength λ the depth x from the semiconductor surface, the photon flux Φ 0 of the incident light and the absorption coefficient α. The absorption of the photons leads to the generation of electron–hole pairs with a generation rate G : Light in the mentioned wavelength range enters the silicon and is absorbed in it.

Silicon photodetectors are able to detect wavelengths between 300 nm and 1100 nm due to the physical properties of the material. However, phototransistors as well as avalanche photodiodes have the advantage to increase the responsivity compared to simple photodiodes, due to their inherent current amplification. The most common photodetectors are photodiodes, phototransistors and avalanche photodiodes. Optical signals are converted into electrical signals by means of photodetectors. Other advantages of CMOS OEICs over III–V solutions are the possibility for cheap mass production, easy handling, packaging, etc. This for example leads to an excellent immunity against electromagnetic interference. One advantage for example is the avoidance of the bond pads and bond wires between photodetector and read out circuitry. These single-chip devices exceed assemblies of wire bonded compound photodetectors and integrated circuits in many aspects.

Compared with III–V compound semiconductors, CMOS technologies have some major advantages like the possibility to combine silicon photodetectors together with the signal processing circuitry into an optoelectronic integrated circuit (OEIC). During the last decades CMOS processes evolved to mature technologies, wherein a cheap implementation of integrated circuits is possible.

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