  We have been developing a new type of X-ray pixel sensors, "XRPIX", allowing
us to perform imaging spectroscopy in the wide energy band of 1-20 keV for the
future Japanese X-ray satellite "FORCE". The XRPIX devices are fabricated with
complementary metal-oxide-semiconductor silicon-on-insulator technology, and
have the "Event-Driven readout mode", in which only a hit event is read out by
using hit information from a trigger output function equipped with each pixel.
This paper reports on the low-energy X-ray performance of the "XRPIX6E" device
with a Pinned Depleted Diode (PDD) structure. The PDD structure especially
reduces the readout noise, and hence is expected to largely improve the quantum
efficiencies for low-energy X-rays. While F-K X-rays at 0.68 keV and Al-K
X-rays at 1.5 keV are successfully detected in the "Frame readout mode", in
which all pixels are read out serially without using the trigger output
function, the device is able to detect Al-K X-rays, but not F-K X-rays in the
Event-Driven readout mode. Non-uniformity is observed in the counts maps of
Al-K X-rays in the Event-Driven readout mode, which is due to region-to-region
variation of the pedestal voltages at the input to the comparator circuit. The
lowest available threshold energy is 1.1 keV for a small region in the device
where the non-uniformity is minimized. The noise of the charge sensitive
amplifier at the sense node and the noise related to the trigger output
function are ~$18~e^-$ (rms) and ~$13~e^-$ (rms), respectively.
