  Depleted Monolithic Active Pixel Sensors (DMAPS) constitute a promising low
cost alternative for the outer layers of the ATLAS experiment Inner Tracker
(ITk). Realizations in modern, high resistivity CMOS technologies enhance their
radiation tolerance by achieving substantial depletion of the sensing volume.
Two DMAPS prototypes that use the same "column-drain" readout architecture and
are based on different sensor implementation concepts named LF-Monopix and
TJ-Monopix have been developed for the High Luminosity upgrade of the Large
Hardon Collider (HL-LHC). LF-Monopix was fabricated in the LFoundry 150 nm
technology and features pixel size of $50x250~\mu m^{2}$ and large collection
electrode opted for high radiation tolerance. Detection efficiency up to 99\%
has been measured after irradiation to $1\cdot10^{15}~n_{eq}/cm^{2}$.
TJ-Monopix is a large scale $(1x2~cm^{2})$ prototype featuring pixels of
$36x40~\mu m^{2}$ size. It was fabricated in a novel TowerJazz 180 nm modified
process that enables full depletion of the sensitive layer, while employing a
small collection electrode that is less sensitive to crosstalk. The resulting
small sensor capacitance ($<=3~fF$) is exploited by a compact, low power front
end optimized to meet the 25ns timing requirement. Measurement results
demonstrate the sensor performance in terms of Equivalent Noise Charge (ENC)
$\approx11e^{-}$, threshold $\approx300~e^-$, threshold dispersion
$\approx30~e^-$ and total power consumption lower than $120~mW/cm^2$.
