Vis Photodiode
A visible-light photodiode generates a current when photons strike its p-n junction. In reverse-bias (photoconductive mode) it responds very fast; with no bias (photovoltaic mode) it produces a small open-circuit voltage. Either way, the current is proportional to illuminance across several decades.
Incident photons with energy above the semiconductor's bandgap create electron-hole pairs in the depletion region. Reverse bias sweeps these carriers apart before they recombine, generating a photocurrent. The response wavelength range is set by the bandgap — silicon peaks at ~800 nm and covers 350–1100 nm.
In plain terms
A tiny solar cell tuned to the visible spectrum: each photon that arrives kicks an electron across the junction, generating a current proportional to light intensity.
Why designers use it
- Detect visible light levels in ambient-light sensors and brightness controllers.
- Read bar-code or QR-code scanners, optical encoders, and line-followers.
- Build optical fibre receivers and free-space optical communication links.
- Measure solar irradiance in weather stations and MPPT controllers.
Best for
- Optical comms
- Light level sensing
- Encoders
Key specifications
- Peak wavelength: 555 nm – 580 nm
- Responsivity: 0.4 – 0.6 A/W
- Active area: 0.1 – 100 mm²
- Dark current: 0.5 nA – 100 nA
- Rise time: 5 ns – 100 ns
When not to use it
- When you need a digital output with built-in thresholding — use an ambient-light IC like the BH1750.
- When the light source is infrared — visible photodiodes have poor response above 800 nm; use a dedicated IR photodiode.
Common mistakes
- Operating in photovoltaic mode and connecting directly to a low-impedance ADC input, which loads the junction and kills linearity.
- Ignoring reverse dark current — it rises steeply with temperature and adds an offset that looks like light.
Where you will find it
- A smoke alarm's optical chamber uses a visible photodiode opposite a low-power red LED: smoke particles scatter light onto the diode, whose current rises sharply — the alarm IC compares the current to a threshold and sounds the horn when smoke scattering exceeds the normal clear-air baseline.
- A CD/DVD drive's laser pickup uses a four-quadrant photodiode to simultaneously read the data pit-and-land sequence and the tracking error: the current difference between quadrant pairs tells the servo motor whether the laser is centred on the track, closing a feedback loop that keeps the spot aligned despite disc wobble.
- A pulse-oximeter shines red (660 nm) and infrared LEDs through a fingertip onto a silicon photodiode: oxygenated haemoglobin absorbs more infrared than red, so the ratio of the two photocurrents gives SpO₂ after a firmware calibration table lookup.
A short history
A silicon photodiode relies on the internal photoelectric effect: light with energy exceeding silicon's band gap excites electrons from the valence band into the conduction band, creating electron-hole pairs. In the depletion region, the internal electric field drives electrons toward the N-region and holes toward the P-region, so connecting the diode to a circuit produces a current proportional to incident light, useful for optical sensing.
Good to know
- A silicon photodiode in reverse bias generates a current proportional to incident light — this is the heart of every digital camera pixel and every fibre-optic receiver.
- Quadrant photodiodes split the active area into four sectors, used for laser-beam alignment in DVD pickups and surgical-laser positioning.
- Avalanche photodiodes (APDs) include built-in gain (20 – 1,000×) and can detect single-photon events when cooled.