Ultra-sensitive PIN and avalanche photodiode receivers

Optical sensors and receivers with so-called single-photon avalanche diodes (SPADs), which can detect the smallest units of light being called photons, may show false detections from dark counts, afterpulsing and optical crosstalk (in case of arrays), which lead to bit errors. Therefore, complex, area and power hungry signal processing circuits are needed to obtain bit error ratios low enough for error correction. Furthermore, after about a decade of research hype on SPADs and on their applications, the potential of SPADs approaches to its limits. In USPAR it will be investigated how few photons are sufficient in one bit to still obtain low bit error ratios, i. e. what sensitivities, data rates, light-sensitive area, chip area and power consumption of optical receivers are possible using the principle of integration of photogenerated charges on low-capacitance photodiodes or on small integration capacitors like in widely used image sensors. It will be determined how these properties scale with reduction of structure sizes in modern CMOS technologies. Integration of photodiodes and amplifiers on the same chip are essential to realize low capacitances of the input node. The low-capacitance photodiodes and low-noise amplifiers will be designed in the most difficult, so-called full custom, style to reduce all parasitics in integrated circuits as far as possible. Test chips will be fabricated as Application Specific Integrated Circuits (ASICs) to be able to verify the innovative approaches with measurements. PIN photodiodes and linear-mode avalanche photodiodes in PIN photodiode CMOS technology are known to possess small capacitances. They will be reduced further in their capacitance by using thicker low-doped epitaxial layers. High-gain amplifiers will enable a large integration efficiency also for larger photodiode capacitances and make much larger light-sensitive areas of photodiodes possible than in active pixel image sensors. The USPAR receivers will be much easier to be applied in practice due to the following advantages. The quantum efficiency of PIN photodiodes is larger than the photon detection probability of SPADs and a lower photodiode reverse bias voltage is sufficient. The USPAR receivers will not have a dead time, afterpulsing and optical crosstalk. A much lower bit error ratio than with SPADs is possible, skipping the need for error correction.

Optical sensors and receivers with single-photon avalanche diodes (SPADs), which can detect the smallest units of light being called photons, show false detections from dark counts, afterpulsing and optical crosstalk (in case of arrays), which lead to bit errors. Therefore, complex, area and power hungry signal processing circuits are needed to obtain bit error ratios low enough for error correction. Furthermore, after about a decade of research hype on SPADs and on their applications, the potential of SPADs approached to its limits. In USPAR it was investigated how few photons are sufficient in one bit to still obtain low bit error ratios, i. e. what sensitivities, data rates, light-sensitive area, chip area and power consumption of optical receivers are possible using the principle of integration of photogenerated charges on low-capacitance photodiodes or on small integration capacitors like in widely used image sensors. It was determined how these properties scale with reduction of structure sizes in modern CMOS technologies. Integration of photodiodes and amplifiers on the same chip are essential to realize low capacitances of the input node. The low-capacitance photodiodes and low-noise amplifiers were designed in the full custom style to reduce all parasitics in the integrated circuits as far as possible. Test chips were fabricated as Application Specific Integrated Circuits (ASICs) in 0.35 m, 0.18 m and 55 nm CMOS technologies to verify the innovative approaches with measurements. Dot PIN photodiodes and linear-mode avalanche photodiodes with very low capacitance by using thicker low-doped epitaxial layers were realized. Multi-dot photodiodes made much larger light-sensitive areas of photodiodes possible. With dot PIN photodiodes, the same sensitivity as with 4-SPAD receivers in the same 0.35 m CMOS technology was achieved. Data rates were increased by a factor of four to 200 Mbit/s. With avalanche photodiodes, a better sensitivity as with SPAD receivers was shown. The gap to the quantum limit was reduced to 10 dB. The USPAR receivers are much easier to apply in practice due to the following advantages. The quantum efficiency of PIN photodiodes is larger than the photon detection probability of SPADs and a lower photodiode reverse bias voltage is sufficient. The USPAR receivers do not have a dead time, afterpulsing and optical crosstalk. A much lower bit error ratio than with SPADs is possible, skipping the need for error correction.