An energetic charge carrier creates an additional electron–hole pair. In a multiplication register this is stochastic, so the output charge fluctuates even when the input electron number is fixed.
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Impact ionization is a process in which an energetic charge carrier (such as an electron) collides with an atom or a lattice ion in a semiconductor or insulator material, imparting enough energy to the atom to free an additional electron. This results in the generation of electron-hole pairs, effectively increasing the number of charge carriers in the material.