Analysis of the Spectral Characteristics of the Responsivity of Diffusion <i>p-i-n</i> Photodiode Based on High Resistivity <p>p</p>-Si
Authors: Kukurudziak Mykola Stepanovych 
Published: August 2023

This Taking into account the market need for highly responive silicon p-i-n photodiodes (PD) for detecting YAG-laser radiation (wavelength 1.064 μm), it was decided to investigate methods of increasing their responsivity, in particular, shifting the maximum of the spectral responsivity characteristic of the photodetector towards longer wavelengths, as well as to analyze the influence of various technological factors in its appearance.
Research was conducted on silicon four-element p-i-n photodiodes with guard ring, manufactured by diffusion-planar and mesa technology. Single crystal p-type silicon with [111] orientation, resistivity of 13-21 kΩ and life time of minor charge carriers of 1.2-2 ms was used.
It was found that when the reverse bias voltage increases, the maximum of the spectral characteristic shifts to longer wavelengths. This is caused by the expansion of the space charge region (SCR) with an increase in the bias voltage and, accordingly, an increase in the collection coefficient of charge carriers. When studying the spectral characteristic of samples with different resistivity, it was noticed that when the resistivity increases, the maximum of the spectral characteristic can be reached at a lower bias voltage. Because with a higher resistivity, the area of the SCR expands over the entire thickness of the substrate at a lower voltage. When expanding the space charge region to the maximum value, the responsivity level of the PD also reaches saturation. Accordingly, it makes no sense to use a bias voltage of the photodiode higher than the one at which the width of the SCR reaches the reverse side of the crystal.
 It is also seen that when the life time of minor charge carriers and silicon resistivity increases at the same bias voltage, the maximum of the spectral characteristic shifts to longer wavelengths.
The maximum value of the spectral characteristic of 1.01-1.02 μm was reached. It was not possible to achieve a further shift, since the shape of the spectral characteristic of the sensitivity of photodetectors is primarily determined by the dependence of the collection coefficient of charge carrier on the wavelength, the maximum of which under normal conditions is at a wavelength of 0.8-0.9 μm. It is possible to achieve the maximum collection coefficient of charge carriers by optimizing the concentration of impurity in the p+- and n+- regions of the crystal, which allows minimizing the absorption of radiation in these regions.
When using mesa technology, which minimizes the degradation of the resistivity of the material, no further shift of the maximum occurs, but the absolute value of responsivity increases compared to planar technology. For a wavelength of 1064 μm, a value of pulse responsivity of 0.5-0.53 A/W and 0.46-0.48 A/W was achieved for mesa- and planar PD, respectively (with a specific resistance of 20-21 kΩ), which is about 60% of the maximum value.
It was also seen that a change in the depth of the PD diffusion layers can negatively affect the noise level. It was found that at a depth of n+-p-junction of 4.25 μm, the level of responsivity, at wavelengths below the maximum of the spectral characteristic, is the lowest, accordingly, the influence of background radiation noise on the level of the useful signal at this depth of p-n-junction is minimal.