Design And Implementation Of High Responsivity Photodetector And Pixel Unit Circuit

With the rapid development of CMOS image sensor technology,the applications of CMOS image sensor have been extended from the strong light detection to the weak light detection,such as biological fluorescence detection,disease diagnosis,observation of weak astronomical signal.However,it is not suitable for the applications of weak light detection due to the lack of charge amplification in Pinned Photodiode(PPD).What's more,Avalanche Photodiode(APD)with high responsivity can realize the detection of weak light signal.But the APD is not compatible to the standard readout circuits due to the high bias voltage for avalanche multiplication.Therefore,research on high responsivity photodetector compatible with standard CMOS process and its pixel unit circuit,have become an urgent problem in the weak light detection.To achieve high responsivity photodetectors and pixel unit circuit based on UMC0.18?m CMOS technology,the research works carried out in this thesis are as following:1.Two kinds of gate-body tied MOSFET type photodetector with different depth of grid interconnection have been designed.The one of photodetectors is composed of the gate-body tied PMOS transistor and photodiode consisting of N-well and P-type substrate.The other of photodetectors is composed of gate-body tied NMOS transistor and photodiode consisting of N-well and T-well.Simulation results have shown that the detector of gate-body tied PMOS has extra high responsivity for the wavelength window of 500~700 nm.Under the condition of weak light(<10?W/cm~2),the detector has a responsivity more than 10~4 A/W.With the increase of light intensity,the responsivity decreases but it still better than 10~2A/W in the interesting range.Then,the detector of gate-body tied NMOS has extra high responsivity for the wavelength window of 450~650 nm.Furthermore,it has a low dark current and a higher signal-to-noise ratio at a drain bias of 0.2V.By comparison,NMOS detector has better performance than PMOS detector in the same condition.2.A novel composite Ultraviolet/blue photodetector was proposed,which is constructed by a lateral/vertical PN diode and an NMOS transistor.The shallow PN diode which formed by the Twell layer and Nwell layer is used to enhanced the absorption efficiency of Ultraviolet/blue light.The simulation results have shown that the detector has extra high responsivity and wide dynamic range for the wavelength window of 300~550 nm.Under the condition of weak light,the detector has a responsivity more than 10~5 A/W.With the increase of light intensity,the responsivity decreases but it still better than 10~3A/W in the interesting range.3.The layout design and chip-testing of the gate-body tied PMOS type and NMOS type detectors were carried out.The testing results have shown that the chip of detectors can work normally in hundreds of KHz bandwidth with good current responsivity.And it can meet the basic requirements of CMOS image sensor.4.Based on the structure of the gate-body tied MOSFET detector,two kinds of pixel unit circuits were designed.The one of pixel unit circuits is traditional 4-T active pixel unit.The other one is composed of inverter and current comparator.The simulation results have shown that the 4-T active pixel unit with narrow dynamic range was suitable for weak light detection.And the other pixel unit circuit with wider dynamic range was suitable for weak light detection and met the requirements of strong light application.