Physical Analysis of the Optical Sensor Based on LED and Photodiode

Abstract

As technical and material developments in semiconductor processing are progressed, the light emitting diodes(LEDs) and laser diodes(LDs) can eject wider spectrum of wavelengths and become low-priced so that many applications built by the LEDs, LDs and Photodiodes(PDs) draw strong attentions from both the academy and industry. Our lab has reported a PBOS (Pseudo BJT Optical Sensor) system, which is the transmission based sensor built by the LED-PD, for a high sensitive measurement of the impurity concentration in the liquid. As it is implied by its name, it has similar operation characteristics with the BJT. In this paper, the mathematical modeling of the PBOS system has been proposed to relate the sensor responsivity with the concentration of the target molecules. The experimental results with bovine serum albumin(BSA) will be provided as an example to calculate the concentration and be compared with the theoretical results. The formalized equations provide in-depth understanding of the system and help to design and analyze the data. The modeling proposed in the thesis will help to design the extension of the PBOS system to other applications such as reflectance based optical sensor. Also, the modeling will help to estimate and compensate the temperature variation of the components used in the PBOS system such as resistors, quantum efficiencies of the LED and PD. In this thesis, the detail modeling of the temperature dependency of the components will be provided and a method for the temperature prediction of the PBOS system will be proposed. Also, the temperature instability has been found in the PBOS system since the LED and PD devices are highly sensitive on the ambient temperature.