Analysis And Design Of Readout Circuit For Integrated Punchthrough Base Region Silicon Phototransistor

Photodetector is a device that converts light signal to electrical signal using photo-electrical effect. Photodetector is the most important thing of electro-optical system because its natural parameters, such as sensitivity, response time, and response wavelength, will affect the entire performance directly.With the development of digital technology, high performance photodetector is needed for light detection, optical storage and optical information detection. There are several types of photodetectors at the present time, such as p-i-n type photodiode, avalanche photodiode, and phototransistor. But p-i-n type photodiode doesn't have inner gain and the amplifier's load resistance can't be large, so its signal-to-noise ratio (SNR) is limited. Avalanche photodiode has inner gain. But it's not satisfactory, because it will introduce extra noise at the same time and need high breakdown potential even to several hundred volts. Although phototransistor has inner gain, its current gain will amplify inner shot noise. So phototransistor doesn't have good system SNR. Silicon punchthrough phototransistor(SI-PTPT) is a new type photodetector which has better SNR and could be used for low light applications, such as low light detection, and low cost fiber system.Nowadays, CMOS process based on silicon is becoming more and more important and the feature size is deceasing gradually. Although the research on compound semiconductor and other new materials has had some development, CMOS process based on silicon couldn't be replaced now. So far, silicon process has had a very powerful form of industrial capacity with the worldwide trillions of dollars in equipment and technology inputs. SI-PTPThas high gain feature and could be made by CMOS process. So devices and readout circuits could be designed on the same chip. And the speed of readout and treatment of signal could be improved. System will be simpler, smaller and lower cost.Paper structure:The first chapter introduces the background on the subject and significance.The second chapter describes the types of photodetectors and analyzes the characteristics of different photodetectors. Then it expounds the principle of SI-PTPT and simulates the dominating parameter that can obviously affect the detection performance. At last, we come to a conclusion that SI-PTPT has higher photocurrent gain.The third chapter describes a useful readout circuit. Readout circuit is the important part of photodetection system. And it can influence several aspects of detection devices, such as frequency, signal and noise, inhomogeneity, and dynamic range. So readout circuit must be designed reasonably to improve the accuracy of entire system. The readout circuit mentioned here includes four parts: bias circuit(BIAS), CTIA, Correlated Double Sampling(CDS) ,post-processing circuit. BIAS is used for providing bias voltage for entire circuit. CTIA which connects to SI-PTPT is used for integritying photocurrent. CDS is used for noise reduction especially Fixed pattern noise (FPN) and reset noise (KTC). Post-processing circuit receives the output signal of CDS and does subtraction and amplification. At last, we come to a conclusion that the readout circuit has good detecting precision from 20nA to 150nA and is suitable for SI-PTPT.