A NEW MOS IMAGER USING PHOTODIODE AS CURRENT SOURCE

We have developed a MOS linear image sensor having a high-voltage-gain amplifier in photodiode pixels. The only difference between the amplified MOS imager (AMI) and our new device is the transfer gate added between the photodiode pixel and the source follower. In this structure, the photo-induced charge loss in video-line parasitic capacitance is compensated to improve the sensitivity. The realization of a high-voltage-gain amplifier is implemented to control the input voltage of the transfer gate so as to maintain an unchanged bias voltage of the photodiode and by operating the photodiode cell as a current source. In our device, the photoinduced charge is not divided by the photodiode capacitance C(d) or the capacitive load C(t). Therefore, it is possible to enlarge the photodiode size, and the sensor has a large saturation exposure as well as a large photocurrent/dark-current ratio of about 30:1. The device operates on a single 5-V power supply. These features make the device suitable for applications in low light levels.