A novel JFET readout structure applicable for pinned and lateral drift-field photodiodes

Enhancement of the dynamic range of photodetectors used in advanced image sensors such as time-of-flight sensors or image sensors for automotive applications is a major research topic. In this paper an improved unipolar 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 is consisting of a low-noise JFET whose gates are formed by a floating diffusion, thus preserving in-pixel accumulation capability - which additionally improves noise performance. This structure outperforms a simple in-pixel implementation of a JFET and a photodectector 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-off between area, output voltage swing and, most important, noise performance. Furthermore, since only a ground connection is needed for application, fill-factor and powder-grid disturbances like DC-voltage drop can be additionally improved.