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 { 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 { 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 ~ { } { mm } \times 13.8 ~ { } { 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 .