Research Of Front-end Readout ASIC For Semiconductor Radiation Detectors

The new generation semiconductor radiation detectors,typically the cadmium zinc telluride?CdZnTe?detectors,have received extensive attentions thanks to their excellent ray absorption rate and energy conversion efficiency at room temperature.They can be used to efficiently detect the ultra trace X/?rays,and have wide application prospects in many fields,such as the environmental radioactive monitoring,mineral and geological prospecting,safety inspection,biomedical imaging,space exploration,high-energy physics,etc.The functions of front-end readout ASIC are to amplify,shape and digitize the weak electrical signals generated by the radiation detector.It is required to have the characteristics of low noise,high precision,high speed,small size,low power consumption and anti-radiation.In this thesis,the front-end readout ASICs for the CdZnTe detector system have been researched,designed and implemented.The main research works are summarized as follows:1.The signal readout and processing technology for semiconductor radiation detector and the monolithic integration technology of the front-end readout circuit have been reserched.In order to achieve the low noise performance,the theoretical analysis in detail and optimization desgn were completed for the main modules of the front-end readout circuit,including the preamplifier,leakage current compensation circuit,pole-zero cancellation circuit,shaping filter,peak-detect-and-hold circuit and trigger logic.In addition,based on the application requirments of portable radiation detection instrument and radiation imaging system,the system structure of the front-end readout chip has been optimized.2.Aimed to the applications in the portable radiation detection instrument and space exploration,a low-noise front-end readout ASIC has been designed and implemented.The energy readout channel is with a simple architecture consisting of preamplifier,shaping filter,and output buffer only.The noise optimization has been performed in the circuit and layout designs of the preamplifier and shaping filter.It has the characteristics of low noise,low power consumption,high linearity and anti-radiation.The test results show that,the maximum input energy range is 200 keV,the nonlinearity is less than 2%,the equivalent noise charge?ENC?is only 52.9 e^-,and the power consumption is less than 2.4 mW/channel.By connecting this readout ASIC to a CdZnTe detector,the energy resolution of the radiation detection system is measured to be 5.9%under the irradiation of ²⁴¹Am.The anti-radiation ability of the chip can satisfy the spatial application demands.3.Aimed to the aplications in the biomedical imaging,security inspection and high energy physics experiments,a 64-channel low-noise front-end readout ASIC has been designed and implemented which can be used in the radiation detecting and imaging system.By using the preamplifier with dual source follower and the high order active filter,the output waveforms are more close to the Gauss waveform,which is beneficial to improve the reading speed.At the same time,the influences of switching noise have been reduced by using the analog timing control circuit and some crosstalk isolation techniques.The test results show that,the gain of the chip is 200 V/pC,the nonlinearity is less than 1%,the readout speed for single frame signals is about 10?s,the inconsistency among channels is less than 2.64%,crosstalk is only 0.22%,the ENC is only 66 e^-,and the power consumption is less than 8 mW/channel.The energy resolution of the radiation detection system to the ²⁴¹Am source can reach 4.4%.It can be used for the detection of X-ray and soft?-ray the energy of which are lower than 200keV.4.The radiation-hardened design technology for the front-end readout chips has been studied.The radiation-hardened design of the ASIC is considered in both the circuit and layout level.In the layout design level,the ability to resist the total ionizing dose?TID?effects is increased by using the ring-gate NMOS transistors added with P⁺type guard ring.Furthermore,in order to improve the ability to resist the single event latch-up?SEL?effects,the P⁺and N⁺type guard rings are inserted between the NMOS and the PMOS transistors.In the circuit design level,in order to eliminate the single event transient?SET?and single event upset?SEU?errors,the structures of CMOS digital circuit units have been improved,and the dual interlocked storage cell is used in the D-flip-flop.5.In order to decrease the influences on the energy resolution due to the difference between the mobilities of electron and hole in the detector,a rise time discriminator is designed which is used with the front-end readout circuit together.This circuit can filter out the input signals which are under certain time threshold,and the time threshold can be adjusted.The typical simulation results show that,this circuit can accurately remove the input signals whose width are greater than 400 ns,with a time resolution of less than 20 ns.The main innovations proposed in this thisis are as follows:1.Based on the theoretical analysis and deduction the noise performance in the front-end readout system,an efficient optimal design method for low-noise front-end readout circuits is proposed.This method is according to the specific application requirements.It would optimize the circuit structure and parameters of the front-end readout circuits from the aspects such as the structure of core amplifier,the size of input transistor,the adjustment of circuit structures,the calculation of shaping time,and the suppression of power noise.By this method,the noise performance of front-end readout circuits would be optimized.2.A circuit structure of charge sensitive preamplifier with dual output is proposed,which is suitable for the multi-channel front-end readout circuits.In this circuit,a folded cascode amplifier with a source-grounded PMOS input transistor is used as a core circuit,the influence of power supply and environment noise is effectively reduced.In addition,by connecting the grid of common-gate amplifier tube to the ground,the crosstalk between channels is effectively reduced.The dual source followers are added in the output,and the connection of the feedback network is also changed.By this way,the processing speed of readout circuit is raised.The anti-interference ability of this circuit is enhanced.And the consistency between channels is optimized.3.Since the difference between the mobilities of electron and hole in the CdZnTe detector,and the lattice defect in detector crystals,the output signal amplitude of the radiation detection system may vary with the incident position of hypervelocity particles,and that will lead to the degration of the energy resolution.In this paper,a rise time discriminator which was matched with the front-end readout circuit is proposed.By measuring the width of detector output signal,this module could judge the contribution ratio of electron and hole,and filter the output signals of energy channel.And as a result,the energy resolution of the whole system could be improved.In this thesis,based on the above innovation technologies,two types of front-end readout ASICs were developed successfully by using the commercial CMOS technology.They could be used in the portable radiation detection instruments and the radiation detecting&imaging systems.The designed chips have the characteristics of low noise,low power consumption,high linearity,high consistency and anti-radiation.The achievements obtained in this research have certain theoretical significance for the low noise front-end readout microelectronics relating to the weak signal of detectors,and also have important engineering use values for developing the X/?rays detecting&imaging system based on the semiconductor radiation detectors.