  Two different depleted monolithic CMOS active pixel sensor (DMAPS) prototypes
with a fully synchronous column-drain read-out architecture were designed and
tested: LF-Monopix and TJ-Monopix. These chips are part of a R&D effort towards
a suitable implementation of a CMOS DMAPS for the HL-LHC ATLAS Inner Tracker.
LF-Monopix was developed using a 150nm CMOS process on a highly resistive
substrate (>2 k$\Omega\,$cm), while TJ-Monopix was fabricated using a modified
180 nm CMOS process with a 1 k$\Omega\,$cm epi-layer for depletion. The chips
differ in their front-end design, biasing scheme, pixel pitch, dimensions of
the collecting electrode relative to the pixel size (large and small electrode
design, respectively) and the placement of read-out electronics within such
electrode. Both chips were operational after thinning down to 100
$\mathrm{\mu}$m and additional back-side processing in LF-Monopix for total
bulk depletion. The results in this work include measurements of their leakage
current, noise, threshold dispersion, response to minimum ionizing particles
and efficiency in test beam campaigns. In addition, the outcome from
measurements after irradiation with neutrons up to a dose of
$1\times10^{15}\,\mathrm{n_{eq} / cm}^{2}$ and its implications for future
designs are discussed.
