In the search for new physics, the Large Hadron Collider at CERN will be upgraded around year 2025, increasing its integrated luminosity by an order of magnitude. For the Inner Tracker (ITk) of the ATLAS experiment, the elevated levels of radiation, pile-up and data rate will require a complete replacement. Active pixel sensors built in sub-micron High Voltage CMOS (HV-CMOS) processes are candidates for the ITk outer pixel detector layers. Being high voltage tolerant, thus supporting drift-based charge collection, they allow to build radiation hard detectors in industry standard technology. On-sensor signal processing permits replacing complex and cost-intensive hybridisation techniques by simpler ones, ultimately leading to fully monolithic devices. In this thesis, sensors built in the ams H18 180nm process were investigated as hybrid detectors. Several design versions were characterised in laboratory and in-beam measurements. The FE-I4 Telescope, a particle tracker for beam based detector characterisation was built to facilitate measurements on HV-CMOS sensors. Its construction and performance as well as the findings on detection performance of the HV-CMOS prototypes, especially after irradiation, are presented.