  Following the Phase-II upgrade during Long Shutdown (LS3), the LHC aims to
reach a peak instantaneous luminosity of $7.5\times 10^{34}$cm$^{-2}$s$^{-1}$,
which corresponds to an average of around 200 inelastic proton-proton
collisions per beam-crossing (every 25 ns). To cope with these conditions, the
ATLAS Inner Detector will be replaced by a new all-silicon system -- the Inner
Tracker (ITk). The ITk will be operational for more than ten years, during
which time ATLAS is expected to record approximately 4000 fb$^{-1}$ of data.
The ITk's pixel sub-system is based on hybrid pixel modules with new silicon
sensors and readout chips. These studies focus on testbeam campaigns undertaken
to study the spatial resolution and efficiencies of hybrid pixel detector
modules based on the first large-structure prototype front-end readout chip --
the RD53A -- using planar silicon sensors. These devices have been irradiated
to replicate the effect of the high radiation environment present during
operation in the ATLAS detector. Results for devices using sensors with
different punch-through bias structures and using different readout chips are
summarised. Those with sensors incorporating a punch-through bias structure are
found to exhibit systematically lower efficiency than those without, as a
result of local areas of relative inefficiency around the punch-through dots.
Despite this, all devices measured are found to satisfy the requirement of 97%
efficiency at $V_\mathrm{bias}=400$ V after being irradiated to end-of-life
fluence.
