A photodetector works by converting light (photons) into an electrical signal.
The Basic Principle
At its core, a photodetector leverages the interaction of light with semiconductor materials. When light energy, carried by particles called photons, strikes the semiconductor, it can excite electrons within the material.
Photon Absorption and Electron Release
As described in the reference, detection occurs when a photon of sufficient energy kicks an electron from the valence band to the conduction band. Think of the valence band as where electrons are normally bound within the atom's structure, and the conduction band as an energy level where electrons are free to move.
- Valence Band: Electrons are tightly held.
- Conduction Band: Electrons are free to flow, creating an electric current.
Only photons with energy greater than or equal to the band gap energy of the semiconductor material can successfully "kick" an electron into the conduction band, leaving behind a "hole" in the valence band. This electron-hole pair is the fundamental basis for detection. The reference specifically mentions this process for infrared photons, implying the semiconductor material is chosen to be sensitive to that part of the spectrum.
Signal Generation
Once an electron is moved to the conduction band (and a hole is created), a voltage or electric field applied across the semiconductor material causes these charge carriers (electrons and holes) to move. This movement constitutes an electrical current.
The reference notes that such an electron is collected by a suitable external readout integrated circuits (ROIC) and transformed into an electric signal. The ROIC is crucial; it gathers the tiny electrical currents generated by many individual detection sites (pixels) and converts them into a usable voltage or digital signal that can be processed, displayed, or stored.
Types of Photodetectors
While there are many types, a common example is the photodiode. The reference highlights that LEDs which are reverse-biased can act as photodiodes. Normally, LEDs emit light when forward-biased. However, when reverse-biased, they can detect light, operating on the same principle of photon absorption creating charge carriers.
- Photodiodes: Semiconductor devices designed to convert light into current.
- Reverse Bias: Applying a voltage across the diode in a direction that normally prevents current flow; this creates an electric field that helps separate the electron-hole pairs generated by light.
In summary, a photodetector works by absorbing photons, using their energy to free electrons within a semiconductor material, and then collecting these free electrons (or the resulting current) using circuitry like an ROIC to create a detectable electrical signal.
