Charge transfer image sensor with antiblooming and exposure control

Abstract

A charge transfer image sensor comprises a signal-readout section, overflow drain section, overflow gate electrodes, photosensing regions and transfer gate electrodes. A first barrier gate electrode is formed between each photosensing region and the overflow drain section and a second barrier gate electrode is provided between each photosensing region and the signal-readout section. During the operation of the image sensor, a prescribed amount of invariable charge is always stored in the photosensing region. One charge-integration period is divided into first and stored integration subperiods. A first variable charge stored in the photosensing region and the semiconductor regions under the first and second barrier gate electrodes during the first integration subperiod is drawn off to the overflow drain section through the semiconductor region under the overflow gate electrode. A second variable charge stored in the photosensing region and the semiconductor region under the first and second barrier gate electrode is transferred to the signal-readout section through the semiconductor region under the transfer gate electrode.