Mapping of the response function of DePFET-based pixel sensors at different levels of charge injection

In DePFET (DEpleted P-channel Field Effect Transistor) Active Pixel Detectors, every pixel can be considered as a complete detector sub-unit. In fact it consists of a p-channel field effect transistor operated on a fully depleted bulk. It is a majority carrier device, as, if n-type material is used, in full depletion conditions a potential minimum for the electrons is formed. The charge collected in this potential minimum can steer the transistor current, acting as a so-called internal gate. In this way the first amplification stage is integrated on the detector chip with a detection fill factor that is around unity. Since the electrons (both coming from the signal and the leakage current) are collected in a potential minimum, the device has to be reset from time to time by emptying the corresponding internal gate. We conducted an experimental characterization of the response function of a small DePFET matrix at different levels of charge injection. In order to cover a wide dynamic range of charge injection the response function of the DePFET cell was measured both with an IR laser-based test facility and with mono-energetic protons. The aim of the measurements is two-fold. On one side we probed the detector response and the internal charge transconductance as a function of the interaction time and of the charge injection level. On the other side we qualified the experimental technique in view of the test and model of the behavior of the sensitive cell designed to match the dynamic range foreseen at the XFEL source in Hamburg (D).