  We have been developing monolithic active pixel sensors for X-rays based on
the silicon-on-insulator technology. Our device consists of a low-resistivity
Si layer for readout CMOS electronics, a high-resistivity Si sensor layer, and
a SiO$_2$ layer between them. This configuration allows us both high-speed
readout circuits and a thick (on the order of $100~\mu{\rm m}$) depletion layer
in a monolithic device. Each pixel circuit contains a trigger output function,
with which we can achieve a time resolution of $\lesssim 10~\mu{\rm s}$. One of
our key development items is improvement of the energy resolution. We recently
fabricated a device named XRPIX6E, to which we introduced a pinned depleted
diode (PDD) structure. The structure reduces the capacitance coupling between
the sensing area in the sensor layer and the pixel circuit, which degrades the
spectral performance. With XRPIX6E, we achieve an energy resolution of $\sim
150$~eV in full width at half maximum for 6.4-keV X-rays. In addition to the
good energy resolution, a large imaging area is required for practical use. We
developed and tested XRPIX5b, which has an imaging area size of $21.9~{\rm mm}
\times 13.8~{\rm mm}$ and is the largest device that we ever fabricated. We
successfully obtain X-ray data from almost all the $608 \times 384$ pixels with
high uniformity.
