Research Of Mechanism,model And Device Optimization Of CMOS Single-photon Avalanche Diodes

Single photon avalanche diode is Geiger-mode avalanche photodiode.It is a kind of detector which can detect very weak optical signal.Because of its advantages of single photon detection sensitivity,picosecond response speed and high gain coefficient,it has been widely used in photonics,laser ranging and other fields.The SPAD based on CMOS technology can be integrated with readout and signal processing circuits,therefore it has gained great interest in many single photon detection applications,especially high performance SPAD array detectors in high energy physical detection,medical imaging,quantum communication and other fields of weak light detection.Based on the technology of 0.18-?m CMOS image sensor,the principle of single photon avalanche diode is studied based on the physical mechanism and theory of the device.The physical model and circuit model of SPAD are established.The design has high detection efficiency and fast working speed.The SPAD with low noise and other characteristics has carried out.The works in this paper are as follows.(1)The establishment of numerical model for SPAD.Considering not only the absorption of photons in the depletion layer and quasi-neutral layer,but also the avalanche breakdown probability of the injected-carriers and distributed-carriers in the depletion region,a new single photon detection probability model is proposed and simulated with Matlab.In this model,the factors that affect the performance of single photon detection probability are theoretically analyzed,as well as the relationship between the single photon detection probability and the dead layer thickness D and the excess bias voltage Vex is obtained,which will provide a theoretical basis for the optimal design of SPADs.Based on the single photon detection probability model and considering the influence of dark count rate,a new single photon quantum efficiency model is proposed.The generation mechanisms of dark count rate,including heat generation,band-to-band tunneling and SHR generation centers,are studied deeply.By analyzing the relationship between the dark count rate and excess bias voltage and temperature,the relationship between single photon quantum efficiency and excess bias voltage and temperature is calculated.(2)The establishment of equivalent circuit model for SPAD.Based on the tested I-V characteristic of SPAD,considering the physical mechanism,the EDA circuit model is established by using series resistance,exponential current source and voltage control switches.This model can not only simulate the static characteristics of SPAD,but also simulate the dynamic behaviors of SPAD,such as trigger,self-sustaining,self-quenching and recovery.At the same time,it can simulate the dead-time of SPAD,which is helpful to predict the dead-time of devices.The consistency between the experimental results and the simulation results proves the validity and accuracy of the model.(3)Design and optimization of SPAD.In order to shorten the dead-time of SPAD,the P+/DNW SPAD is proposed based on the traditional P+/N-well SPAD,by analyzing the dead-time of passively quenched SPAD.The test results show that the dead time and the dark count rate of P+/DNW SPAD are less than P+/N-well SPAD.When the diameter of the active region is 20 ?m,the minimum dead time is 0.76 ?s when the quenching resistance is 10 k?,and the dark count rate is 1.63 kHz at room temperature.In order to further reduce the size of SPAD,the guard ring size of P-well/DNW SPAD is designed and analyzed by Silvaco TCAD simulation.Four SPADs with effective diameter of 20 ?m and guard ring sizes of 1.2 ?m,0.8 ?m,0.4 ?m and 0.2 ?m were designed and tested.The test results show that when the guard ring size is 0.2 ?m,the premature edge breakdown will occur.It is found that the guard ring with a decreasing size to 0.4 ?m is still effective in preventing premature edge breakdown,and exhibit superior avalanche breakdown characteristics with a breakdown voltage of 15.8 V.Moreover,the SPAD of 20 ?m diameter has dark count rate of 812 Hz at 1V excess bias voltage.(4)Application study of SPAD.Based on the principle of TOF and TAC pixel,using SPAD as a photodetector,the TAC pixel circuit is designed and simulated.Finally,the array design and layout of the pixel array is done after the analysis of the system structure and sequence.The SPAD is designed with an octagonal layout,the array size is 32×32,and the array area is 1.975×2.0 mm2.