Electron multiplication is a process occurring directly on the EMCCD chip behind the serial register, in which the signal is amplified to such an extent that it is possible to exceed the read noise and detect even extremely weak signals.
The principle of EM is based on charge travelling through the Multiplication Register region (a series of deeper potential pits) - when an electron "falls" into the pit, the Impact Ionization process occurs and another electron is created. At the same time, a specific noise induced by charge transfer occurs, the so-called Clock Induced Charge. This type of noise is also found in conventional CCDs, where it is negligible (less than read noise), but in EMCCDs it is multiplied by amplification - a defined amount of it can be added to the charge to create a gain(Gain) of up to 1000x. With this signal amplification strategy, it is important to keep the thermal noise to a minimum by cooling the sensor very intensively.
