AbstractIn order to obtain an image sensor for X‐rays with long absorption length and energies greater than 1 ± 2 keV, a CCD with a thick depletion region has been investigated. A thick depletion region can be obtained by using a semiconductor substrate having high purity (high resistivity) and by applying a high gate voltage. However, it is not easy to apply a high voltage since the gate voltage in a CCD is a high‐speed clock pulse for charge transfer. This paper proposes a mode (dc bias‐voltage mode) in which a DC bias voltage is superimposed, keeping the charge‐transfer clock pulse voltage low. the conditions of SCCD and BCCD device parameters are investigated in order to determine useful ranges of these parameters for normal operation and for preventing dielectric breakdown. It is shown that when a substrate with usual purity (doping density 1020 m3) has an applied dc bias voltage of about 300 V, a depletion region with thickness of about 60m can be obtained. the effectiveness of the dc bias voltage mode is confirmed. When CCD characteristics other than depletion region thickness and simplicity of device structure are considered, a BCCD with the frame transfer system is suitable for X‐ray image sensors.