Modeling Of Single Photon Avalanche Diode And Design Of Pixel Circuit For Fluorescence Lifetime Imaging Application

Geiger mode single photon avalanche diodes SPAD(Single Photon Avalanche Diode) is a detector that can detect extremely weak optical signal. It has been widely used in quantum physics, biology, astronomy, and so on. However, the theoretical research on SPAD devices is just derived from the empirical formula and the theoretical formula of semiconductor physics. All the theoretical results need to be verified experimentally, which increases the research cost of the device. In the application of EDA(Electronic Design Automation) simulation model, it lacks an exact circuit simulation model provided by Complementary Metal Oxide Semiconductor technology foundry. In addition, in the application for fluorescence lifetime, the current relatively mature technology is TCSPC(Time-Correlated Single Photon Counting).However,it covers a large silicon area and the vast majority of fluorescence lifetime imaging are single exponential or double exponential decay, thus the application of TCSPC is a waste of the circuit performance.Aiming at the above mentioned problems, first of all, this paper studies three physical mechanisms, including photon detection efficiency, dark count rate and afterpulsing probability. For the photon detection efficiency calculation, three physical mechanisms of surface absorption, bottom absorption, depletion layer absorption are considered. By means of SILVACO simulation, the distribution of electric field and avalanche breakdown probability can be extracted. As a result, the relationship between photon detection and the wavelength could be precisely calculated. Besides, the model calculated results and the reported experimental results are compared to verify the theoretical correctness. Additionally, this article discusses the dark count rate of three physical mechanisms comprising thermal generation, trap assisted tunneling and band-to-band tunneling, and analyses the dominant mechanism in the low and high electric field. For the afterpulsing probability prediction, two trap energy levels are assumed, and the trapping and releasing process of trapped electrons and holes is studied.Secondly, based on the above theory model, the single photon detector EDA simulation model is established. The presented simulation model not only simulates the current voltage characteristic of the devices, but also transforms the calculated photon detection efficiency, dark count rate and afterpulsing probability into the device performance parameter. The simulation model is written in the hardware description language Verilog-A. Using some random distribution function of Verilog-A, the characteristics of the behavior modeling can be realized.Finally, a fluorescence lifetime detection of pixel circuit is design. The circuit uses time gated detection method, utilizing two gating window. Because most of the fluorescence lifetime are in nanoseconds, the width of the gating window width is set about 1ns. Therefore, the photon pulse should be cut to an accuracy pulse of about 300 ps by monostable circuit. Through a large number of experiments, the two gating window gives the photon number statistics, and eventually the fluorescence lifetime can be calculated. Compared with the time-correlated single photon counting, the circuit does not need to record the arrival time of each photon, thus the area of a single pixel can be reduced, which makes it possible to integrate a large array of pixels. At the same time, the circuit can work in two states, the gated mode can be used as the measurement of fluorescence lifetime, and the continuous mode can be used as a single photon analog counting circuit.