The design, fabrication, and evaluation of a silicon junction field-effect photodetector

A new device for photodetection is introduced in this study that is capable of responsivities comparable to devices operating in the charge-storage mode. Since the physical process involved corresponds to a photodiode in series with a high value of resistance, the device operates in real time and hence avoids the problems of switching encountered in the charge-storage mode at low light levels. The device utilizes this physical process in a new technique that significantly reduces the required surface area. Although the junction field-effect transistor (FET) serves as the prototype for the device, considerations for improving the magnitude of the output response and its linearity along with the responsivity-bandwidth product lead to a device having a very low transconductance and a large cutoff voltage VGC. Field-effect photodetectors fabricated in accordance with the principles developed for optimum photodetection are evaluated. Epitaxial silicon of conventional doping was used in the fabrication. The devices are found to exhibit the photodetecting characteristics expected from theoretical considerations. A simple two-lump approximation of the distributed gate-channel interface is found to adequately describe the frequency response of the devices. In contrast to the area-independent responsivity of the charge-storage mode, it is shown that the responsivity of the field-effect photodetector is proportional to the square of the device surface area. Depending upon the illumination level and the deviation from linearity that is tolerable, this area dependence presents one of the fundamental drawbacks of the field-effect photodetector for applition in dense arrays.