SILICON PHOTODIODE FOR TELECOMMUNICATIONS (FSO) WITH INCREASED SPEED AND SENSITIVITY AT 980 nm

Abstract

 One of the basic elements of Free Space Optics is a photodiode, which must have a high level of sensitivity to radiation in the range of 0.8-1 μm, and be fast-acting. In our case, the chosen wavelength is 980 nm, which corresponds to the wavelength of a vertical-cavity surface-emitting laser (VCSEL). The optimization of the photodiode design is carried out taking into account the uniform generation of the photocurrent with a uniform volume charge density inside the crystal when receiving radiation with a working wavelength. The current of the photo signal in the external circuit, in this case, depends on the modulation frequency, the width of the space charge region (SCR), the mobility of minor charge carriers and the amount of bias at the p-n junction. This allows you to significantly manipulate the rise time of the transient characteristic of the photodiode, and the use of double passage of the radiation beam throughout the SCR, you can obtain a significant increase in sensitivity.

The subject is the construction of a photodiode, which at a wavelength of 980 nm has small values of the rise time and high values of current monochromatic sensitivity.

The goal is to substantiate and create a design of a photodiode with increased current monochromatic sensitivity and reduced rise time at 980 nm wavelengths for operation in FSO.

Task: To analyze the factors affecting the sensitivity and rise time of the photodiode when working at a wavelength of 980 nm, as well as to develop the design of the photodiode and investigate it.

Methods: mathematical analysis of photoelectric parameters of a photodiode taking into account the mechanism of generation of minor charge carriers.

Modeling of the photodiode design. A planar technology for silicon was used to manufacture the photodiode. The technical solution is based on the optimization of the modulation frequency, the width of the SCR, the mobility of minor charge carriers and the amount of displacement at the p-n junction, as well as ensuring the double passage of radiation through the photodiode crystal. The result was also achieved by etching the mesastructure from the reverse side of the photodiode, the area of which is the projection of the photosensitive element of the photodiode, and the width of the silicon plate above the mesastructure should be about 50 μm.

Results: Factors affecting the current monochromatic sensitivity of the photodiode and its rise time were determined and taken into account. A new photodiode design with increased sensitivity and reduced rise time has been created. Comparative studies of the manufactured batch of created photodiodes and FD-255A photodiodes were conducted, which showed that the proposed photodiode really has a current monochromatic sensitivity at a wavelength of 980 nm of at least 0.55 A/W. At the same time, its rise time was reduced from 50 ns to 20 ns. compared to the serial photodiode FD-255A.