In a time-of-flight (TOF) range sensor configured to acquire a range image, by using TOF scheme, a potential just under a gate electrode of a MOS structure is controlled in a vertical direction (depth direction) of the MOS structure. For example, a CMOS distance-measuring element and a TOF image sensor using the CMOS distance-measuring element are disclosed in patent literature (PTL) 1. The CMOS distance-measuring element has a structure such that an n-type charge-generation buried region buried in a p-type semiconductor layer, a charge-transfer buried region, a charge-read-out buried region, an insulating film covering the charge-generation buried region and the charge-transfer buried region, a transfer gate electrode provided on the insulating film to transfer signal charges to the charge-transfer buried region, a read-out gate electrode provided on the insulating film to transfer the signal charges to the charge-read-out buried region. When pulse lights are irradiated to the charge-generation buried region in the CMOS distance-measuring element recited in PTL 1, light signals are converted to signal charges in a semiconductor layer just under the charge-generation buried region, and a distance to an object is measured from a distribution ratio of charges accumulated in the charge-transfer buried region.
The CMOS distance-measuring element or the TOF image sensor using the CMOS distance-measuring element has a problem of generation of noise or dark current caused by interface defects, interface states, or the like just under the transfer gate electrode. In addition, in the case of using the transfer gate electrode disclosed in PTL 1, a potential gradient over a long distance is difficult to control, and a substantially uniform electric field over a long distance of a charge transport path is practically difficult to maintain. For this reason, in the charge-modulation elements such as distance-measuring elements having long charge transport paths, carriers are stopped in the middle of the charge transport paths, and thus, there are issues such that expected performances of the charge-modulation element are difficult to achieve.