ABSTRACT
This chapter is dedicated to the noise performance of optoelectronic photodetectors. Since the basic operation principles of these devices – including the generation and propagation of electromagnetic radiation, its conversion to electronic charge by the photoelectric effect and the fundamentals of carrier transport in semiconductors – are well-known, the reader is referred to textbooks like [SN07; PN86; ST07; The95]. This chapter intends to explain the origin of noise sources and reduction methods in up-to-date CMOS active pixel sensors (APS). Additionally a novel readout structure is presented that is superior to the widely employed source follower readout implemented by enhancement MOSFETs. It yields a high output voltage swing and low noise, while requiring no additional processing steps. The readout structure consists of a low-noise JFET whose gates are formed by a floating diffusion, thus preserving inpixel accumulation capability – which additionally improves noise performance. This structure outperforms a simple in-pixel implementation of a JFET and a photodetector in terms of the necessary area consumption, thus improving fill factor. For pixels with a pitch of several microns this readout structure is a good trade-of between area, output voltage swing and, most important, noise performance. Furthermore, since only one ground connection is needed for application, fill-factor and power-grid disturbances like DC-voltage drop can be additionally improved.
