Simulates the propagation of electrons and/or holes through the sensitive sensor volume of the detector. It allows to propagate sets of charge carriers together in order to speed up the simulation while maintaining the required accuracy. The propagation process for these sets is fully independent and no interaction is simulated. The maximum size of the set of propagated charges and thus the accuracy of the propagation can be controlled via the `charge_per_step` parameter. The maximum number of charge groups to be propagated for a single deposit position can be controlled via the `max_charge_groups` parameter.
The propagation consists of a combination of drift and diffusion simulation. The drift is calculated using the charge carrier velocity derived from the charge carrier mobility and the magnetic field via a calculation of the Lorentz drift. The correct mobility for either electrons or holes is automatically chosen, based on the type of the charge carrier under consideration. Thus, also input with both electrons and holes is treated properly. The mobility model can be chosen using the `mobility_model` parameter, and a list of available models can be found in the user manual.
This module implements charge multiplication by impact ionization. The multiplication model can be chosen using the `multiplication_model` parameter, the list of available models can be found in the user manual. By default, the model defaults to `none` and impact ionization is switched off, generating unity gain.
To simulate impact ionization, the number of newly generated electron-hole pairs is calculated for every propagation step and every charge carrier in the group, based on drawing a random number from a geometric distribution. This represents a stepwise approach to the avalanche generation process. The charge of a charge group is increased by the number of impact ionization processes per step and opposite-type charge carriers are generated at the end of the step, if the opposite-type charge carrier is selected to be propagated (see below).
The two parameters `propagate_electrons` and `propagate_holes` allow to control which type of charge carrier is propagated to their respective electrodes. Either one of the carrier types can be selected, or both can be propagated. It should be noted that this will slow down the simulation considerably since twice as many carriers have to be handled and it should only be used where sensible.
The direction of the propagation depends on the electric and magnetic fields field configured, and it should be ensured that the carrier types selected are actually transported to the implant side. For linear electric fields, a warning is issued if a possible misconfiguration is detected.
