Research And Study Of Ga N Based 640×8 Ultraviolet Focal Plane Array For Aerospace Application

Ultraviolet（UV）detection technology is widely used in military and civil applications.With the rapid development of semiconductor technology and the third generation semiconductor technology became mature,GaN based UV detector attracted more and more attention recently.As a direct band gap semiconductor material,GaN has many advantages such as wide band gap,high thermal conductivity,good chemical stability,strong irradiation hardening,etc.The band gap of GaN can adjust continuously by adjusting the doping elements and components of its ternary alloy compounds.So,GaN is a ideal material for UV detector,GaN based UV detector also become a hot research topic.GaN based ultraviolet detector has many advantages,such as high quantum efficiency,low cost,small volume,good anti-seismic ability and strong irradiation hardening.Electro-optical systems that used GaN based detectors don’t need filters because of the ultra low out-of-band spectral response,which makes GaN based UV detectors surpass the traditional UV detection devices,such as photomultiplier tubes,micro-channel plates and ultraviolet enhanced CMOS sensors.UV focal plane array consist of detector array and readout circuit.The photocurrent is amplified by readout circuit,and then the signal transmits to the load through the multiplexer.Meanwhile,the readout circuit also provides a stable bias for the detector,and can eliminate noise on the chip.Therefore,readout circuit will affect the performance of the UV focal plane arrays.Readout circuit is of great importance in ultraviolet detection system.Recently,the result on readout circuit is abundant.But most of the results are focused on new readout mode and readout circuit structure.Few studies had been done on how to design high-performance readout circuit and analysis performance after interconnection of readout circuit and detector array.In this paper,we start with introducing operation principle and characteristics of UV detector,and then studied the design method of the UV detector readout circuit.Finally,a 640×8 UV detector readout circuit has been completed.Then,a comprehensive test of the UV focal plane array which fabricated by this readout circuit has been carried out.According to the test result,several key performance indicators are analyzed and discussed.In order to make the circuit can be used in space application;we studied the radiation hardening of the readout circuit.Firstly,the operation principle and preparation method of GaN based UV focal plane array are introduced.The detector model and parameters of UV detector are determined through theoretical analysis and test results.After that,the typical input structure of the readout circuit is introduced and analyzed,then compared the performance of different input structures.Considered the performance of UV detector and input structure of readout circuit,capacitor feedback transimpendance amplifier（CTIA）has been selected as the input structure of the readout circuit.Based on the equivalent model of the UV detector,analyzed the injection efficiency,then the expression of injection efficiency is obtained.The correctness of the theory is verified by the experiment.Based on the analysis of injection efficiency,designed a CTIA circuit with high injection efficiency,large output swing and small area for ultraviolet detector.Through theoretical calculation,the injection efficiency is 98.2%,and the output swing is 3.5V.Secondly,a correlation double sample（CDS）circuit has been designed which can achieve the whole frame transfer and compatible with the integration when read mode and integration then read mode.Next,a column stage buffer with low power consumption has been implemented by timing controlled.Then,a Class AB output stage buffer with low noise,high speed and low power consumption has been designed.In addition,cascaded D flip-flops and 2 to 4 line decoders are used in the circuit to realize the row and column selection.Finally,we completed the schematic and layout design of the 640×8 readout circuit.Furthermore,the test method and test system of the UVFPA are introduced,and the method how to measurement modulation transfer function（MTF）is designed.After the interconnection of the readout circuit and the ultraviolet detector array,the UVFPA has been comprehensively tested.The test results show that the external quantum efficiency of the UVFPA is 51.2%（the quantum efficiency is 55.2%before flip-chip）,the noise electron number is 252e-(C_int=62.5fF),the linear dynamic range is 75dB,the power consumption is 150mW,and the out-of-band spectral response is 1.14%.In order to meet the needs of space missions,the performance of radiation hardening is also studied in this paper.In this readout circuit,the digital part of the circuit was hardened by using ring gate MOSFET and double ring protection.Furthermore,the Si O₂-Si₃N₄ composite passivation layer is alternately grown on the chip surface readout circuit.Then the hardened readout circuit is tested by gamma irradiation,the result shows that,the total dose of anti-ionizing radiation is increased from 35krad（Si）to 50krad（Si）.The anti single event upset ability of the readout circuit is evaluated by single event irradiation test.The test results show that the readout circuit doesn’t have a single event upset under the irradiation condition of LET=57.36MeV/mg/cm²,which is greater than the threshold of 37MeV/mg/cm²requested in the space integrated circuit.The 640×8 UVFPA readout circuit has good performance in injection efficiency,radiation hardening and so on.Each indicators of the UVFPA fabricated by the readout circuit meet the needs of space applications.And the UVFPA had been used in space engineering projects.