PMTs are based on vacuum technology, which almost disappeared after it peaked in the 60s of the last century. However, PMTs have advantages over traditional semiconductor devices. A PMT uses two effects (which are described in detail in textbooks of physics). The first effect is the conversion of photon energy into the kinetic energy of released electrons . If a photon interacts with material, the photon energy can be absorbed by the electronic system in the atoms or molecules. If the energy is high enough (shorter wavelength), the excitation can cause an electron to leave the structure. This dissociation requires a material-specific minimum energy . The photocathode of a photomultiplier is made from materials that have sufficiently low dissociation energies to allow red photons down to 800 nm to cause electron dissociation. Unfortunately, red-sensitive materials also show higher thermal noise. Electrons from the photocathode are accelerated by an electric potential to a second electrode (dynode). Here the accumulated energy is used to release more electrons from the material. This is the second effect used in photomultiplier tubes. The result is a gain from the first photoelectron in up to 3 or 4 secondary electrons. These electrons are then amplified by cascaded additional dynodes (typically 6 ... 12). The entire PMT voltage is typically in the range of about 300 V to 1,200 V (tunable), thus the potential difference between dynodes is maximally ca 100 V. If the average gain for a given voltage is 3 per dynode, then the total gain in a 10-dynode PMT is 50,000-fold. After the last dynode the electrons are discharged into an anode, where the electrical current is measured. In analog mode, the current over a time interval is integrated, and the brightness of the sample is coded by that charge. If the circuitry is fast enough, single photons cause current peaks that can be resolved. The intensity then corresponds to the number of photons in a given time interval.
Multi-alkali photocathodes are standard in visible light applications . These tubes exhibit 25 % quantum efficiency for blue-green light. Obviously, there is much room for improvement as regards the sensitivity of such devices . The number of secondary electrons is randomly distributed. When at a certain voltage the gain is 3 electrons on average, then the signal-to-noise ratio (S/N) is 1.7 (Poisson statistics). This noise is transferred to the output signal. Since only whole electrons are released, a variation of only one electron more or less changes the signal by about 30%. A second objective for improving light sensors is therefore to increase the yield in the first amplification step.
A third important parameter is the width of the generated electrical pulses of a single photon. The pulsewidth is broadened by the temporal variation of electrons arriving at the anode, and by the circuitry. At high photon rates, the pulses must be very short in order to separate them for counting. Another discussion point is the dynamic range. Small dynamic ranges allow only limited variation in light intensity. High dynamic ranges indicate acceptance of very different illumination intensities with comparable results.
Avalanche photo-diodes  are PIN diodes with an additional p-layer inserted between the i and n layers. The pn-region is called "multiplication zone" and has very high field strength. The photons are absorbed in the insertion layer, where it can generate an electron-hole pair (intrinsic photoelectric effect). Electrons are accelerated to the multiplication zone, where additional pairs are created by impact ionization. This process amplifies the original charge by a factor of 100 to 1,000, depending on the applied voltage. The charge multiplication occurs over a very short interval and is referred to as avalanche effect. Obviously, the sudden increase and the self-amplifying current can easily damage the device, so special precautions must be taken to avoid high currents. Much higher gain can be achieved with avalanche photodiodes, when the applied voltage exceeds the breakdown voltage, allowing amplifications of up to 108 ("Geiger mode"). The high gain is sufficient for direct detection of individual photons. Since the current is not stoppable, the diode circuitry must include specific measures to avoid demolition. Geiger-mode avalanche photodiodes are therefore very sensitive to damage from too much light and improper operation.
Avalanche photodiodes amplify by 100 ... 1,000 fold in non-Geiger mode and up to 100,000,000 fold in Geiger mode. In non-Geiger mode the