  The Soft X-ray Imager (SXI) is an imaging spectrometer using charge-coupled
devices (CCDs) aboard the Hitomi X-ray observatory. The SXI sensor has four
CCDs with an imaging area size of $31~{\rm mm} \times 31~{\rm mm}$ arranged in
a $2 \times 2$ array. Combined with the X-ray mirror, the Soft X-ray Telescope,
the SXI detects X-rays between $0.4~{\rm keV}$ and $12~{\rm keV}$ and covers a
$38^{\prime} \times 38^{\prime}$ field-of-view. The CCDs are P-channel
fully-depleted, back-illumination type with a depletion layer thickness of
$200~\mu{\rm m}$. Low operation temperature down to $-120~^\circ{\rm C}$ as
well as charge injection is employed to reduce the charge transfer inefficiency
of the CCDs. The functionality and performance of the SXI are verified in
on-ground tests. The energy resolution measured is $161$-$170~{\rm eV}$ in full
width at half maximum for $5.9~{\rm keV}$ X-rays. In the tests, we found that
the CTI of some regions are significantly higher. A method is developed to
properly treat the position-dependent CTI. Another problem we found is pinholes
in the Al coating on the incident surface of the CCDs for optical light
blocking. The Al thickness of the contamination blocking filter is increased in
order to sufficiently block optical light.
