  We have been developing event driven X-ray Silicon-On-Insulator (SOI) pixel
sensors, called "XRPIX", for the next generation of X-ray astronomy satellites.
XRPIX is a monolithic active pixel sensor, fabricated using the SOI CMOS
technology, and is equipped with the so-called "Event-Driven readout", which
allows reading out only hit pixels by using the trigger circuit implemented in
each pixel. The current version of XRPIX has lower spectral performance in the
Event-Driven readout mode than in the Frame readout mode, which is due to the
interference between the sensor layer and the circuit layer. The interference
also lowers the gain. In order to suppress the interference, we developed a new
device, "XRPIX6E" equipped with the Pinned Depleted Diode structure. A
sufficiently highly-doped buried p-well is formed at the interface between the
buried oxide layer and the sensor layer, and acts as a shield layer. XRPIX6E
exhibits improved spectral performances both in the Event-Driven readout mode
and in the Frame readout mode in comparison to previous devices. The energy
resolutions in full width at half maximum at 6.4 keV are 236 $\pm$ 1 eV and 335
$\pm$ 4 eV in the Frame and Event-Driven readout modes, respectively. There are
differences between the readout noise and the spectral performance in the two
modes, which suggests that some mechanism still degrades the performance in the
Event-Driven readout mode.
