Depleted Monolithic Active Pixel Sensors (DMAPS), also known as depleted CMOS sensors, are extremely attractive for particle physics experiments. As the sensing diode and readout electronics can be integrated on the same silicon substrate, DMAPS remove the need for hybridization. This results in thin detectors with reduced production time and costs. To achieve high speed and high radiation tolerance, DMAPS are manufactured in High Voltage (HV) processes on High Resistivity (HR) wafers. Today's most performant DMAPS are 50 µm thin and have 50 µm x 50 µm cell size with integrated mixed analog and digital readout electronics, 11 ns time resolution and 5 x 10 15 1 MeV neq/cm 2 radiation tolerance. DMAPS in HR/HV-CMOS have been adopted as the sensor technology for the pixel tracker for the Mu3e experiment and are under consideration for the ATLAS detector Phase-II Upgrade. However, in spite of the major improvements demonstrated by DMAPS, further research to achieve even more performant sensors is needed to realize the full potential of these sensors to meet the most challenging requirements for particle physics experiments planned for the future. This article describes the state-of-the-art of DMAPS in terms of time resolution and radiation tolerance, and presents specific work done by the CERN-RD50 collaboration to further develop the performance of these sensors.

Avalanche photodiodes operated in the Geiger mode present very high intrinsic gain and fast time response, which make the sensor an ideal option for those applications in which detectors with high sensitivity and velocity are required. Moreover they are compatible with conventional CMOS technologies, allowing sensor and front-end electronics integration within the pixel cell. Despite these excellent qualities, the photodiode suffers from high intrinsic noise, which degrades the performance of the detector and increases the memory area to store the total amount of information generated. In this work, a new front-end circuit that allows low reverse bias overvoltage sensor operation to reduce the noise in Geiger mode avalanche photodiode pixel detectors is presented. The proposed front-end circuit also enables to operate the sensor in the gated acquisition mode to further reduce the noise. Experimental characterization of the fabricated pixel with the conventional HV-AMS 0.35 m technology is also presented in this article.