Research On Ultra Wide Dynamic Range Image Sensor

With the rapid development of the economy and the society, the technology of capturing image also has been improved obviously. Nowadays, the electronic image sensor plays an important role in the area of science research and daily life. The chargecoupled device(CCD) image sensors have been widely used during the past several decades. But now, the CMOS image sensor(CIS) has occupied the whole market except of the area of aerospace and military applications. With the popularity of smart phones and digital cameras, CISs have entered into everyone’s daily life.Dynamic range(DR) is an important performance of the image sensor, which reflects the ability of capture and processing of the maximum range of the incident light density. From the direct sunlight to the middle of the night, the dynamic range of the natural scene exceeds nearly 180 d B. The dynamic range of traditional CIS is too narrow to meet the requirement of recording natural scene. It is mainly because the limited capacity of the potential well.There are several methods to extend the dynamic range of CIS. The improvement or adjustment of the full well capacitance of the pixel is the most direct way to obtain high dynamic range. Multiple exposure or dual pixels technology is another technique. There are also in-pixel ADC and in-pixel algorithm circuit schemes that have been published. Among those known methods, logarithmic or linear-logarithmic pixel has achieved widest dynamic range. However, the above methods always require complex additional control circuit.In this paper, a charge compensation phototransistor is presented to achieve HDR for CIS. With built-in charge compensation mechanism, the proposed phototransistor works either in linear mode or in logarithmic mode, according to the incident light intensity. Furthermore, the phototransistor can switch smoothly and automatically between these two operation modes, without any auxiliary circuit participating, resulting in simplifying circuits of image sensors.Principally, the high dynamic range phototransistor is a PNP phototransistor. Unlike the convention phototransistor in which the output node is the collector, the NWELL base of the proposed phototransistor is the output anode, VP, while the collector(P+) connected to an outside voltage source VCOMP. Therefore, there are two PN junctions face to face in the device equivalently and both of them work as photodiodes under the incident light normally. The node of VP acts as a photo-generated charge well, which is reset periodically with different integration time.Charge compensation mechanism is realized by the photodiode DC with the anode and the cathode connected to VCOMP and the cathode of the main photodiode DP, respectively. Initially, DC is reverse biased as a normal photodiode as well as DP, since VP is higher than VCOMP at beginning of the integration process. When the number of photons accumulated by photodiodes is quite enough to drive the potential of VP less than VCOMP, DC is switched to forward biasing automatically. In this situation, DC works as a current source to compensate the photocurrent generated by DP, to avoid the output signal VP saturation, and to push the device into logarithmic response mode.The prototype CIS with the proposed phototransistor is fabricated with a standard CMOS process without specific imaging sensor step. The test result shows that the phototransistor can capture the signal with light density from 1.99×10-9W/cm2 to 6.99×10-3 W/cm2, i.e., 130.9d B of ultra-wide dynamic range obtained. When exposed in high power incident light, two non-ideality factors of the device should be taken into account. The one is the temperature effect and the other is the parasitic resistance effect. The measurement results can be corrected with reasonable parameters for the two nonideal effects after careful analysis. The final dynamic range of the phototransistor is 167.1d B since it can response the incident light from 1.99×10-9 W/cm2 to 0.448 W/cm2. Therefore, the proposed phototransistor architecture has a significant improvement over the traditional photodiode. Obviously, the dynamic range of CIS with proposed phototransistor can achieve dramatic improvement compared with the transitional photodiode.A prototype image sensor with a 280×210 array of 10×10μm2 pixel is fabricated using a 0.18μm 1P6 M standard CMOS process without specific imaging process. The analog output of the chip is quantized by a 12-bit off-chip ADC, to investigate the intrinsic image signal characteristics.Based on the image sensor chip, an imaging system has been designed and assembled. Due to the special opt-electronic response curve, a piecewise linear function is used for the process of the photograph generation. This reduction algorithm ensures that the detail of the objects under high light level and low light level can be preserved. Therefore, good results of HDR imaging can be obtained by using this method.Finally, for problems that appeared in the imaging test and performance measurement, we presented some preliminary solutions after careful analysis. A 12 bit Cyclic ADC is also designed for the digital output type image sensor. This ADC with more than 10 bit ENOB only occupied a very small chip area, and can be well applied in the subsequent chip design.