photodiodesas

Photodiodesas are semiconductor devices that detect light and convert it into an electrical signal. They rely on the generation of charge carriers in a p-n junction or similar structure when illuminated, producing a current proportional to light intensity when a bias is applied.

Most photodiodes are operated with reverse bias (photoconductive mode) to increase speed, linearity, and dynamic range; zero-bias operation (photovoltaic mode) is used in low-noise or self-powered applications. Common types include PIN photodiodes, which incorporate an intrinsic layer to expand the depletion region; avalanche photodiodes (APDs), which provide internal gain at the cost of higher bias and noise; and Schottky or metal-semiconductor photodiodes for specific spectral or speed requirements.

Materials vary by spectral range. Silicon covers visible to near-infrared wavelengths; germanium and indium gallium arsenide (InGaAs) extend sensitivity to longer infrared wavelengths; other compound semiconductors are used for ultraviolet or specialized bands. Performance depends on responsivity (current per optical watt), spectral response, dark current, bandwidth, and noise. Key figures of merit include noise-equivalent power (NEP) and specific detectivity (D*).

Applications span fiber-optic communications, LIDAR, spectroscopy, imaging, and medical instrumentation. In many systems, photodiodes are paired with transimpedance amplifiers and integrated optical components to form fast, sensitive detectors. Design considerations include reverse-bias requirements, junction capacitance, temperature sensitivity, and packaging to minimize parasitics and thermal drift.

Photodiodesas thus represent a foundational technology in modern photonics, enabling precise, high-speed light detection across a broad range of wavelengths and applications.