  In view of the tracking detector application to the ATLAS High Luminosity LHC
(HL-LHC) upgrade, we have developed a new generation of High Voltage CMOS
(HV-CMOS) monolithic pixel-sensor prototypes featuring the AMS aH18 (180 nm)
commercial CMOS technology. By fully integrating both analog and digital
readout-circuitry on the same particle-detecting substrate, current challenges
of hybrid sensor technologies, i.e., larger readout input-capacitance, lower
production-yield, and higher production and integration cost, can be
downscaled. The large electrode design using high-resistivity substrates
actively helps to mitigate the charge-trapping effects, making these chips
radiation hard. The surface and bulk damage induced in high irradiation
environment change the effective doping concentration of the device, which
modulates high electric fields as the reverse-bias voltage increases. This
effect can cause high leakage current and premature electrical breakdown,
driven by impact ionization. In order to assess the characteristics of heavily
irradiated samples, we have carried out dedicated campaigns on ATLASPix1 chips
that included irradiations of neutrons and protons, made at different
facilities. Here, we report on the electrical characterization of the
irradiated samples at different ambient conditions, also in comparison to their
pre-irradiation properties. Results demonstrate that hadron irradiated devices
can be safely operated at a voltage high enough to allow for high efficiency,
up to the fluence of 2E15 neq/cm2, beyond the radiation levels (TID and NIEL)
expected in the outermost pixel layers of the new ATLAS tracker for HL-LHC.
