Photodetector Innovations for Advancing Wearable Optical Biosensors: A Review

Optical biosensors such as photoplethysmography (PPG), pulse oximetry, and near-infrared spectroscopy (NIRS) are critical health monitoring tools, providing critical insights into physiological indicators such as heart rate (HR), oxygen saturation, blood pressure (BP), and respiratory rate (RR). At the core of these sensors are photodetectors (PDs), which are available in various types, including inorganic [such as p-n, p-i-n, avalanche, CMOS, single-photon avalanche, silicon photomultiplier (SiPM), and dynamic PDs (DPDs)] and organic (such as bulk heterojunction (BHJ), photomultiplication (PM), and hybrid CMOS PDs). The choice of a PD significantly influences optical biosensor performance by affecting key factors such as photosensitivity, detectivity, signal-to-noise ratio (SNR), dynamic range (DR), low-light detection, parasitic capacitance, dark current, noise, power consumption, and design complexity. This article provides a detailed review of the latest innovations in PDs and their associated readout circuit designs, highlighting how they enhance the performance of optical biosignal sensing. It also explores future trends in PD development, including high-mobility semiconductors, ultrathin PD layers, and novel fabrication techniques that are driving the evolution of the next-generation biosignal sensing solutions.