Research On Low Voltage And High Dynamic Range Image Sensor Based On PEPT

Image Sensor is the key element to obtain visual information, which plays an important rolein information system. Recently, the applications of image sensor have penetrated into all aspectsof people’s life and work with the prevalence of mobile phone camera, and portable notebookcomputer, digital camera and various monitoring equipment popularization. However, there arestill two unsolved problems for the applications of image sensor.First of all, portable multimedia electronic development requires power consumption ofimage sensor shouldn’t be too large. Reduce the operating voltage is an important way to achievelow power consumption. However, the low operating voltage always results in low output voltageswing, leading to a bad response to photo signal and SNR. On the other hand, dynamic range isanother way to measure the imaging quality because the application of traffic monitoring andinstallation of monitoring require to capture the strong light and weak light information at thesame time. The traditional CMOS image sensor normally has a low dynamic range about70dBdue to the limitation of the full well capacity. Thus, it cannot record high dynamic scene.With the low consumption requirement for the portable multimedia electronic products, andhigh dynamic range requirement for the high-quality CMOS image sensor, we design a lowvoltage and high dynamic range CMOS image sensor based on PEPT. The designed image sensorhas a low operating voltage of1.8V, and demonstrated a good response for the incident light,leading to a high dynamic range.In this thesis, firstly we propose a punchthrough enhanced phototransistor （PEPT）, which canbe applied in CMOS image sensor. The phototransistor adopts a unique combined structure for thelateral transistor. The photogenerated current amplification mechanism lies in the diffusion of thephoto carriers, resulting in a high internal optical gain. To research PEPT, we set up atwo-dimensional model for simulation. Based on the diffusion theory of photo carriers and simulating results, an in-depth analysis for the working mechanism is proposed. We optimize thedesign of the device parameters with the results of the analysis. Then we use0.5m standardCMOS process to tape-out the device. With the measurement results of the tape-out chip, weanalyze the optoelectronic properties of electrical properties of device, including photo responseand signal-to-noise characteristics. The PEPT is demonstrated to improve the dark currentcharacteristics of traditional punchthrough phototransistor, also have high photo responsivity inweak light conditions. For instance, with an incident light of4.45×10^-3 W/cm², the photo currentobtained is above100nA at a bias voltage of1V, and the corresponding responsivity is1.87×10⁷A/W. Therefore, the phototransistor has a wide application prospect in the field of weaklight detection. Meanwhile, the high response under weak light condition and the unsaturatedcharacteristics in strong light condition, making the device possible to apply in high dynamicrange image sensor.The thesis proposes a new current source bias architecture for PEPT, and then put forward aconstant current source mode, which is suitable for the application of CMOS image sensor. Thisconstant current source pixel structure is mainly composed of a constant current source Ibiasandthe developed punchthrough enhanced phototransistor. The two devices are series connected. Theconnecting point is made an output voltage terminal. When working, the constant current sourcecontrol current flowing through the device constantly. Under different light intensity, the outputterminal will produce a continuous voltage signal Voutchanging with the light intensity. The pixelstructure is tested can respond a range of light intensity from¹0-2μW/cm²to105W/cm².Theresponding light intensity dynamic range is more than7orders of magnitude, that is,140dB. Thischaracteristic makes PEPT have a great application prospect in the wide dynamic range imagesensor applications. At the same time, using this operating mode, the output voltage swing is ashigh as1.3V, which is obtained without using any auxiliary amplifier for the pixel. Afterdiscussion and analysis, we determine a reasonable range of bias current in the constant currentsource operating mode for PEPT, making the photoelectric response and signal-to-noise of thedevice ratio compromise to achieve optimal. In this design, the bias current value of1μA to10μAcan make the device show a good response rate and signal to noise ratio.When PEPT working, there occurs punchthrough effect. The punchthrough base region isvery narrow, causing an inevitable introduction of large dark current. Compared with the traditional phototransistor, the power consumption of the system is much larger. In order toovercome the problem of large dark current of transistor operating in continuous mode, in thispaper, we proposed a new discrete operating mode for PEPT. The pixel circuit consists of a pulsevoltage source and a PTPT transistor connected in series. Because there is no dark current withinintegration time, this operating mode is suitable for low power applications. Meanwhile, it ispossible for the punchthrough phototransistor to design large pixel array CMOS image sensor.Research results show that, in the incident light power density of10-3μW/cm2, maximumphotocurrent of1nA can be get, far greater than the current value of the continuous mode ofoperation. At the same time, high responsivity of2×107A/W can be obtained in the same incidentlight power density, with an effective device size as small as1m2.This paper also invented a CMOS image sensor, in which the photodiode is replaced byPEPT, leading to a more sensitive photo respond. This design proposes system level design for theimage sensor and solves the key problem, such as the different type of PEPT used in image sensorpixel, special restrictions of constant current source, load and source follower. Then it givesdifferent elements corresponding to different types of PEPT. Due to the image sensor using acontinuous working mode of constant current source architecture as the pixel circuit, a high outputvoltage of1.3V is obtained with this pixel architecture, which does not need any auxiliaryamplifier. Therefore, there is no need to use column amplifier in the new system architecture.Moreover, due to the continuous working mode based on the pixel circuit, there is no resettransistor with large KTC noise. Therefore, the system architecture can also omit the correlateddouble sampling circuit. Analog voltage generated by the pixel circuit directly put into12bit ADC.It greatly simplifies the complex signal processing process of traditional image sensor, and alsosimplifies the design of sensor system, saving chip area. In addition, giving special considerationfor the selection of the image sensor pixel circuit or the transistor current source, it can realize lowoperating voltage of1.8V, suitable for low power applications.Finally, according to the system of CMOS image sensor SoC integrated chip, this paperdesigned a cap-less low dropout linear regulator. The cap-less LDO using a two-stage amplifier,uses Miller compensation to ensure system stability. In view of cap-less LDO system low PSRRcharacteristics in the middle frequency, we propose a new PSRR boosting circuit. Under thepremise of no effect on system stability, the zeros of PSRR are rearranged. Firstly, the AC small signal circuit the PSRR booster is analyzed. After a careful design of the circuit parameters, weuse GSMC0.18m process to tape-out. Measurement results demonstrate that the cap-less LDOhas an excellent DC characteristic and good power supply ratio in mid-frequency, which is63dBPSRR at100kHz and about40dB at1MHz. Therefore, the high PSRR of the cap-less LDOdemonstrate a good inhibitory ability of power supply ripple interference in mid-frequency.