This paper presents an in-pixel dual exposure synthesis technique to extend dynamic range of CMOS image sensors (CISs). The transfer transistor of the 4-T pixel acts as an overflow transistor in this proposed method. The full well capacity of photodiode (PD) can be very well controlled by biasing the transfer gate at different states. The long and short time exposed signals can be both stored in PD by adjusting the transfer gate voltage, and such the wide dynamic range signal can be obtained without extra readout operation. A proto chip with 64 x 48 pixel array is fabricated in 0.18-mu m CIS process. The sensor structure is based on a typical 4-T pixel image sensor with modified transfer gate driver and timing. This structure not only maintains the good features of 4-T pixels, such as good fill factor and low noise, but also achieves a wide dynamic range. The dynamic range of the proto chip is extended from 51.59 to 90.53 dB.
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