Monte Carlo simulation of charge collection processes in Monolithic Active Pixel Sensors for the ALICE ITS upgrade

A precise simulation of the response of Monolithic Active Pixel Sensors (MAPS) is difficult to achieve because of the difficulty in describing the charge collection process both in the epitaxial layer, a typically only partially depleted active volume, and in the substrate, a low resistivity bulk, of the sensor. Transient TCAD simulations provide good description of the electric field, but are too slow to be used in the simulation and analysis chain of HEP experiments. Therefore a faster simulation of the sensor response is necessary in order to provide in-depth understanding of the detector response. Afast tooldeveloped to model the response of ALPIDE, the MAPS for the upgrade of the ALICE InnerTracking System (ITS), is presented in this paper. The basic concept is a first principles Monte Carlo (MC)simulation, using the electric fields extracted from a TCAD simulation to model the drift of the charge carriers.That is, the more complex part of the simulation is handled by TCAD while preserving the speed of the MCsimulation with only one free parameter — the external electric field strength. The tool is versatile, any MAPSarchitecture can be simulated once the proper external electric field is provided, and in this paper it has beendone for two chips, developed for the ALICE ITS upgrade, INVESTIGATOR (analogue output) and ALPIDE(digital output). It will be shown that an excellent agreement between data and simulation has been achieved,both for $^{55}$Fe X-rays and minimum ionising particles.