A photodiode works by absorbing incident photons to generate electron-hole pairs within a semiconductor's depletion region, producing a measurable current strictly proportional to light intensity. Unlike solar cells that generate a forward voltage, signal photodiodes are typically operated as precision current sources. If you shine 1 milliwatt of 850nm light onto a typical silicon photodiode with a responsivity of 0.5 A/W, it will output exactly 0.5 milliamps of current, regardless of the voltage across it (within its breakdown limits).

This linear relationship between optical power and electrical current makes photodiodes the undisputed standard for precision light measurement, fiber optic receivers, and medical pulse oximeters. Below, we break down the physical mechanism, how to select and bias the right component, and how to build a stable readout circuit.

The Core Mechanism: How a Photodiode Converts Light to Current

At the bench level, a photodiode is simply a P-N junction optimized for light exposure rather than current rectification. When a photon with sufficient energy (greater than the semiconductor's bandgap) strikes the depletion region, it knocks an electron loose, creating an electron-hole pair.

If the diode is reverse-biased, the strong internal electric field immediately sweeps the electron toward the cathode and the hole toward the anode. This movement of charge carriers constitutes the photocurrent ($I_p$). The total current flowing through the device is the sum of this photocurrent and the dark current ($I_d$)—the tiny leakage current that flows even in total darkness.

Bench Insight: Dark current doubles approximately every 10°C rise in junction temperature. If your low-light sensor is drifting, check your board's thermal profile before blaming the photodiode. Cooling the sensor (or selecting one with a smaller active area) is the standard fix for high-precision applications.

Symbol, Pinout, and Safe Default Part Numbers

The schematic symbol for a photodiode is the standard diode triangle-and-bar, but with two arrows pointing inward toward the junction, indicating light absorption. Physically, the Cathode (negative) is usually marked by a flat edge on the plastic package or the longer lead on a bare TO-18 can. The Anode (positive) is the shorter lead or the unmarked side.

When selecting a component, avoid random eBay assortments. Here are the industry-standard, safe default part numbers with their exact ratings for 90% of hobbyist and prototyping jobs:

Safe Default Photodiode Part Numbers (Silicon)
Part Number Manufacturer Peak Wavelength Active Area Responsivity (A/W) Max Reverse Voltage Typical Price
BPW34 Vishay / OSRAM 850 nm (NIR) 7.5 mm² 0.55 @ 850nm 60 V $1.20
BPW21R Vishay 565 nm (Green) 4.4 mm² 0.35 @ 565nm 50 V $3.50
SFH2030 OSRAM 850 nm (NIR) 1.0 mm² 0.62 @ 850nm 50 V $1.80
TEMD6200 Vishay 550 nm (Vis) 0.5 mm² 0.22 @ 550nm 32 V $0.90

Note: The BPW34 is the undisputed workhorse for general-purpose light detection and laser tripwires. The BPW21R includes an integrated optical filter that mimics the human eye's spectral response, making it mandatory for ambient light sensing (lux meters).

Biasing Modes: Photovoltaic vs. Photoconductive

How you bias the photodiode dictates its speed, linearity, and noise floor. You must choose between zero-bias (photovoltaic) and reverse-bias (photoconductive) operation based on your application's bandwidth requirements.

Back to Learn

Related guides

Photodiode Operation Regions
Parameter Photovoltaic Mode (Zero Bias) Photoconductive Mode (Reverse Bias)
Applied Voltage 0 V Typically 5 V to 50 V (Cathode to VCC)
Junction Capacitance High (Limits bandwidth) Low (Widens depletion region, increases speed)
Dark Current Effectively Zero Non-zero (Increases with reverse voltage and temp)
Linearity