Current-Mode CMOS Image Sensor

This thesis presents a current-mode CMOS image sensor using lateral bipolar phototransistors (LPTs). The objective of this design is to improve the photosensitivity of the image sensor, and to provide photocurrent amplification at the circuit level. Lateral bipolar phototransistors can be implemented using a standard CMOS technology with no process modification. Under illumination, photogenerated carriers contribute to the base current, and the output emitter current is amplified through the transistor action of the bipolar device. Our analysis and simulation results suggest that the LPT output characteristics are strongly dependent on process parameters including base and emitter doping concentrations, as well as the device geometry such as the base width. For high current gain, a minimized base width is desired. The 2D effect of current crowding has also been discussed. Photocurrent can be further increased using amplifying current mirrors in the pixel and column structures. A prototype image sensor has been designed and fabricated in a standard 0.18μm CMOS technology. This design includes a photodiode image array and a LPT image array, each 70× 48 in dimension. For both arrays, amplifying current mirrors are included in the pixel readout structure and at the column level. Test results show improvements in both photosensitivity and conversion efficiency. The LPT also exhibits a better spectral response in the red region of the spectrum, because of the nwell/p-substrate depletion region. On the other hand, dark current, fixed pattern noise (FPN), and power consumption also increase due to current amplification. This thesis has demonstrated that the use of lateral bipolar phototransistors and amplifying current mirrors can help to overcome low photosensitivity and other deterioration imposed by technology scaling. The current-mode readout scheme with LPT-based photodetectors can be used as a front end to additional image processing circuits.