Photodiode

A photodiode operates by the internal photoelectric effect: absorbed photons excite electrons from the valence band to the conduction band, creating electron-hole pairs. In a p-n junction, the built-in electric field separates these carriers, producing a photocurrent. Photodiodes can operate in photovoltaic mode (zero bias) or photoconductive mode (reverse bias), the latter offering faster response and better linearity at the cost of higher dark current.¹

The spectral response depends on the semiconductor material's bandgap. Silicon photodiodes cover visible and near-infrared (190–1100 nm), while InGaAs extends to 1700 nm for telecommunications. The quantum efficiency, the ratio of collected electrons to incident photons, can exceed 90% in optimized devices.

Common photodiode types include p-n, p-i-n, avalanche, and Schottky. The p-i-n photodiode has an intrinsic layer that widens the depletion region, improving speed and quantum efficiency. Avalanche photodiodes (APDs) operate at high reverse bias to multiply carriers via impact ionization, providing internal gain for detecting weak signals.²

Photodiodes are also integrated into arrays, such as CCD and CMOS image sensors, where millions of pixels each contain a photodiode. In optical receivers, they are often paired with transimpedance amplifiers to convert photocurrent to a voltage. Specialized designs include quadrant photodiodes for position sensing and large-area photodiodes for light metering.

Photodiodes are ubiquitous in modern technology. In fiber-optic communication, InGaAs p-i-n and APD photodiodes detect modulated light at speeds up to 100 Gbit/s. They are essential in consumer electronics for ambient light sensing, proximity detection, and camera exposure control.³

Scientific applications include spectroscopy, LIDAR, and medical imaging. Photodiodes are used in smoke detectors, barcode scanners, and solar cells (though solar cells are optimized for power generation rather than signal detection). In astronomy, photodiodes in photometers measure stellar brightness with high precision.⁴

Photodiodes have a rich history: the first silicon photodiode was developed at Bell Labs in the 1950s, building on earlier selenium and germanium devices. A niche application is in radiation dosimetry, where photodiodes detect X-rays and gamma rays for medical and industrial safety.⁵

Edge cases include the 'photodiode effect' in organic semiconductors, used in flexible photodetectors. Some photodiodes are designed for ultraviolet detection using wide-bandgap materials like gallium nitride. In high-energy physics, silicon photodiodes are used in calorimeters to measure particle energies. A curious fact: photodiodes can also emit light when forward-biased, a property exploited in light-emitting diodes (LEDs).⁶

Responsivity

The ratio of photocurrent to incident optical power, typically in A/W.

Dark current

The small current that flows in a photodiode even in the absence of light, due to thermally generated carriers.

Avalanche photodiode

A photodiode that operates at high reverse bias to multiply photocurrent through impact ionization.

Quantum efficiency

The fraction of incident photons that produce a detectable electron-hole pair.