What Is a Photodiode and How Does It Work?

At the workbench, the answer to what is a photodiode comes down to its core function: it is a semiconductor PN-junction device that converts incident light (photons) into an electrical current. Unlike a standard rectifier diode designed to block or pass current, a photodiode is engineered with a transparent window or lens to allow light to strike the depletion region. When photons with sufficient energy hit the junction, they excite electrons, creating electron-hole pairs that result in a measurable photocurrent.

Symbol and Pinout Identification:
On a schematic, the photodiode symbol looks like a standard diode (a triangle pointing toward a vertical bar) but with two inward-pointing arrows indicating light reception. The pins are the Anode (A) and Cathode (K). Physically, through-hole photodiodes usually indicate the Cathode with a shorter lead or a flat spot on the epoxy rim. In surface-mount packages (like the popular SMA or DO-214), a printed band or notch marks the Cathode.

Bench Tip: Never confuse a photodiode with a phototransistor. A photodiode has two pins and generates current directly. A phototransistor has three pins (Base, Collector, Emitter) and provides internal gain, making it more sensitive but significantly slower.

Photovoltaic vs. Photoconductive Biasing

How you bias a photodiode dictates its speed, noise floor, and linearity. Selecting the right mode is the most critical decision in your circuit design.

1. Photovoltaic Mode (Zero Bias)

In this mode, the photodiode is left unbiased (0V across the junction). It acts like a tiny solar cell, generating a small voltage and current when illuminated. Because there is no reverse voltage, the depletion region is narrow, resulting in higher junction capacitance and slower response times. However, dark current is virtually zero, making this mode ideal for high-precision, low-frequency DC light measurements (like spectrophotometers or lux meters).

2. Photoconductive Mode (Reverse Bias)

Here, a reverse DC voltage (typically 5V to 50V) is applied across the diode (Cathode to positive, Anode to ground/virtual ground). This widens the depletion region, drastically reducing junction capacitance and allowing for nanosecond response times. The trade-off is an increase in "dark current" (leakage current that flows even in total darkness) and associated shot noise. This mode is mandatory for high-speed applications like fiber optic receivers, laser rangefinders, and barcode scanners.

Photodiode Operation Regions and Biasing Characteristics
Parameter Photovoltaic (Zero Bias) Photoconductive (Reverse Bias)
Bias Voltage 0V 5V to 60V (Reverse)
Dark Current ~0 A (pA range) nA to µA range
Junction Capacitance High (Slower) Low (Faster)
Response Speed kHz range MHz to GHz range
Linearity Excellent at low light Excellent across wide range
Primary Application Precision DC, medical sensors High-speed comms, LiDAR

Safe Default Part Numbers and Ratings

When prototyping, avoid exotic or obsolete optoelectronics. These three part numbers are industry-standard, widely available, and cover 90% of hobbyist and commercial bench needs:

  • Vishay BPW34: The undisputed king of general-purpose broadband photodiodes. It features a large active area (7.5 mm²), high sensitivity across visible to near-IR (peak at 900nm), and a maximum reverse voltage of 60V. Dark current is a mere 2nA. Cost: ~$0.60.
  • OSRAM SFH203: A fast-response PIN photodiode in a standard 5mm clear package. Peak wavelength is 850nm (ideal for IR remote and proximity sensing). Max reverse voltage is 50V, with a 10nA dark current and nanosecond rise times. Cost: ~$0.85.
  • Hamamatsu S1223: A precision UV-to-Visible silicon photodiode. If you need flat spectral response from 320nm to 1000nm for analytical instruments, this is the default. Max reverse voltage is 20V, dark current is 50pA. Cost: ~$14.00.

Complete Transimpedance Amplifier (TIA) Circuit

A photodiode outputs current, not voltage. To get a usable voltage signal, you must use a Transimpedance Amplifier (TIA). Below is a complete, single-supply (5V) photoconductive circuit using the BPW34 and an LMV358 dual op-amp (chosen for its rail-to-rail output and low cost).

Component List

  • U1: LMV358 Dual Op-Amp (using one half)
  • D1: Vishay BPW34 Photodiode
  • R1, R2: 10 kΩ (Voltage divider for virtual ground)
  • Rf: 1 MΩ (Feedback resistor / Gain setting)
  • Cf: 15 pF (Feedback capacitor / Phase compensation)
  • C1: 10 µF (Virtual ground decoupling)

Wiring Steps

  1. Create a Virtual Ground: Connect R1 from 5V to the non-inverting input (+) of the op-amp. Connect R2 from the non-inverting input to GND. This creates a 2.5V reference. Place C1 (10µF) in parallel with R2 to stabilize the 2.5V rail.
  2. Wire the Photodiode: Connect the BPW34 Cathode to the 5V rail. Connect the Anode to the inverting input (-) of the op-amp. This applies a 2.5V reverse bias across the diode.
  3. Set the Gain and Compensation: Connect the 1 MΩ feedback resistor (Rf) between the inverting input (-) and the op-amp output. Crucial step: Solder the 15 pF capacitor (Cf) directly in parallel with Rf. This compensates for the BPW34's junction capacitance and prevents the op-amp from oscillating at high frequencies.
  4. Calculate Output: The output voltage will sit at 2.5V in total darkness. As light hits the sensor, current flows through Rf, pulling the output voltage down. Vout = 2.5V - (I_light × 1,000,000).
Safety & ESD Warning: Photodiodes are highly susceptible to Electrostatic Discharge (ESD). The BPW34 has an ESD rating of roughly 2kV (HBM). Always use a grounded wrist strap when handling bare photodiodes, and never touch the Anode/Cathode pins directly with ungrounded tweezers.

How Photodiodes Fail and How to Test Them

Photodiodes rarely fail from normal use, but they do die from electrical abuse and environmental factors.

Common Failure Modes:

  • ESD Punch-Through: A static shock permanently shorts the PN junction. The device will read as a dead short in both directions.
  • Thermal Runaway: In photoconductive mode, if the reverse voltage is too high or ambient temperature spikes, dark current increases. This current generates heat, which generates more dark current, eventually melting the junction.
  • Optical Degradation: Prolonged exposure to high-intensity UV light can degrade the epoxy encapsulant, causing it to yellow and permanently drop the sensor's responsivity.

How to Test a Photodiode with a Multimeter

You can verify a photodiode's health in about 30 seconds using a standard digital multimeter (DMM).

  1. Diode Test (Forward Bias): Set your DMM to Diode Test mode. Place the red probe on the Anode and the black probe on the Cathode. A healthy silicon photodiode will read between 0.400V and 0.550V. (Standard rectifier diodes read ~0.600V).
  2. Reverse Bias Check: Swap the probes (red to Cathode, black to Anode). The meter must read OL (Over Limit). If it reads a low resistance or voltage, the junction is shorted (ESD damage).
  3. Photovoltaic Light Test: Set the DMM to DC millivolts (mV). Connect red to Anode, black to Cathode. Shield the sensor with your hand, then expose it to a bright desk lamp or flashlight. You should see the voltage jump from near 0mV to 100mV - 350mV depending on light intensity. If it stays at 0mV under bright light, the device is dead.

Frequently Asked Questions

What is the difference between a photodiode and a photoresistor (LDR)?

A photoresistor (Light Dependent Resistor, or CdS cell) is a passive component whose resistance drops when exposed to light. They are slow (response times in tens of milliseconds) and contain toxic cadmium, making them obsolete in modern commercial designs. A photodiode is an active semiconductor that generates current. Photodiodes are millions of times faster, highly linear, and temperature-stable, making them the required choice for any precision or high-speed application.

Can I use a standard LED as a photodiode?

Yes, but with severe limitations. Every LED is technically a photodiode sensitive to its own emission wavelength. If you reverse-bias a green LED and shine a green laser on it, it will generate a tiny photocurrent. However, standard LEDs lack the optimized depletion region, anti-reflective coatings, and low junction capacitance of a dedicated photodiode. The generated current will be in the picoamp range, requiring highly specialized electrometer op-amps to read. For anything beyond a science fair demo, buy a dedicated BPW34.

Why does my photodiode circuit oscillate or output high-frequency noise?

This is almost always caused by missing or incorrectly sized phase compensation in your Transimpedance Amplifier. A photodiode has inherent junction capacitance (often 50pF to 100pF at zero bias). When connected to the high-impedance inverting input of an op-amp, this capacitance introduces a pole in the feedback loop, destroying phase margin and causing the op-amp to oscillate at MHz frequencies. You must place a small feedback capacitor (Cf) in parallel with your feedback resistor. Calculate it using the formula: Cf = √(Cj / (2 × π × Rf × GBW)), where Cj is junction capacitance and GBW is the op-amp's gain-bandwidth product. Usually, a 10pF to 22pF capacitor solves the issue.

How do I calculate the exact output voltage of my photodiode circuit?

You need the photodiode's Responsivity (R), measured in Amps per Watt (A/W) at your specific light wavelength. For example, the BPW34 has a responsivity of roughly 0.62 A/W at 900nm. If your IR LED source delivers 5 µW of optical power to the sensor surface, the generated current is: I = 5 µW × 0.62 A/W = 3.1 µA. If your TIA feedback resistor (Rf) is 100 kΩ, the output voltage swing is: Vout = 3.1 µA × 100,000 Ω = 0.31V. Always verify that the optical power reaching the sensor is calculated using the inverse-square law if the light source is more than a few millimeters away.