Avalanche photodiodes are highly sensitive, fast photodiodes and count among the avalanche diodes, i.e. they are optical sensors based on a silicon substrate. They form the basis for modern silicon photomultipliers (SiPM) and use the internal photoelectric effect to generate charge carriers and the avalanche effect for internal amplification. Similar to conventional photomultiplier tubes (PMT), they use the effects of impact ionization to generate secondary charge carriers. However, they get by with a significantly lower supply voltage and reach limit frequencies of up to the Gigahertz range. Their extremely high sensitivity enables the detection of weak light right down to individual photons. They are therefore also used for the detection of individual particles in high-energy physics. With regard to TCSPC, modern analog SiPM offer the possibility of inferring the photon rate directly from the voltage output. Read also: SPADs and HPDs.