The work presented in this thesis consists of the characterisation of monolithic CMOS sensors targeting the requirements of the outer-most layer of the ATLAS Inner Tracker after the High Luminosity upgrade of the Large Hadron Collider. Three detectors are investigated: an investigator chip and two large scale demonstrators (MALTA and mini-MALTA). The investigator chip is designed in the standard TowerJazz 180 nm technology and served as a tool to investigate the geometric parameters that affect the pixel capacitance. The MALTA chip is designed in the modified TowerJazz 180 nm technology and implements a novel asynchronous readout to minimise power consumption. The sensor is irradiated with X-rays up to 1.25 MRad to test the resistance of the front-end circuit to ionising radiation effects. The mini-MALTA chip is designed following the results obtained on MALTA and implements an improved front-end and pixel layout to enhance the radiation hardness of MALTA. A similar X-ray irradiation campaign is done for this chip showing good radiation hardness after 80 MRad of TID. Aside from the characterisation work, FPGA-based readouts for the MALTA and mini-MALTA chips were developed in collaboration with the CMOS development group at CERN
