To use a photodiode in reverse bias, you connect the cathode to a positive voltage and the anode to a negative voltage (or ground, depending on the topology). This applies an electric field across the PN junction that widens the depletion region. The direct result is a massive drop in junction capacitance, which increases the device's response speed (bandwidth) and drastically improves linearity compared to zero-bias (photovoltaic) mode. If you need to measure fast optical pulses or require strict linear correlation between light intensity and output current, photodiode reverse bias is mandatory.
The standard schematic symbol for a photodiode is a standard diode triangle with a vertical bar, accompanied by two inward-pointing arrows indicating incident light. Pin 1 is the Anode (A) and Pin 2 is the Cathode (K). In a typical through-hole epoxy package like the ubiquitous Vishay BPW34, the cathode is identified by the shorter lead and a flat spot on the side of the black epoxy casing. In surface-mount packages, a white band or dot marks the cathode.
Operation Regions and Core Specifications
Photodiodes operate in three distinct regions depending on the applied voltage. While zero-bias is excellent for low-frequency, low-noise precision, reverse bias (photoconductive mode) is the standard for general-purpose and high-speed applications. Avalanche mode is reserved for ultra-low light detection where internal gain is required.
| Mode | Bias Voltage | Typical Current | Junction Capacitance | Speed / Bandwidth |
|---|---|---|---|---|
| Photovoltaic (Zero Bias) | 0V | µA range | High (Max) | Low (kHz range) |
| Photoconductive (Reverse Bias) | 5V to 100V | nA to mA | Low (Minimized) | High (MHz range) |
| Avalanche (High Reverse) | Breakdown - 1V | mA range | Very Low | Very High (GHz range) |
When selecting a component, you must balance active area (which dictates how much light it captures) against junction capacitance (which dictates speed). Larger area means more capacitance. Below is a data-dense specification table of four industry-standard silicon photodiodes to ground your design choices in real numbers.
| Part Number | Max Reverse Voltage (Vr) | Dark Current (Id) | Capacitance (Cj @ Vr) | Peak Wavelength | Active Area |
|---|---|---|---|---|---|
| Vishay BPW34 | 60V | 2 nA (typ) | 72 pF @ 5V | 900 nm | 7.5 mm² |
| OSRAM SFH203 | 50V | 1 nA (typ) | 20 pF @ 5V | 850 nm | 1.0 mm² |
| Hamamatsu S1223 | 30V | 0.1 nA (typ) | 4 pF @ 5V | 720 nm | 0.5 mm² |
| First Sensor PIN-10D | 100V | 0.5 nA (typ) | 12 pF @ 10V | 800 nm | 10.0 mm² |
Notice the trade-off: The Vishay BPW34 offers a massive 7.5 mm² active area for capturing diffuse light, but its 72 pF capacitance will limit your bandwidth unless you apply a higher reverse voltage or use a lower feedback resistor. Conversely, the Hamamatsu S1223 is tiny, but its 4 pF capacitance allows for MHz-range response speeds, making it ideal for focused laser detection or spectroscopy.
Designing the Reverse Bias Transimpedance Amplifier (TIA) Circuit
A photodiode in reverse bias acts as a current source, not a voltage source. The current generated is strictly proportional to the incident light (irradiance). To convert this micro-current into a usable voltage without loading the diode, you must use a Transimpedance Amplifier (TIA). A simple pull-down resistor will work for basic light/dark switching, but it introduces non-linearity and thermal noise that ruins precision measurements.
Complete Application Circuit: 5V Biased Precision TIA
This circuit is designed for general-purpose visible and near-IR light metering, converting 0–5 µA of photocurrent into a 0–5V output signal.
- Photodiode: Vishay BPW34 (Cathode to +5V, Anode to Op-Amp Inverting Input).
- Op-Amp: Texas Instruments OPA340 (Low input bias current, rail-to-rail I/O, unity-gain stable).
- Feedback Resistor (Rf): 1 MΩ (Sets the transimpedance gain: Vout = I_photo × Rf).
- Feedback Capacitor (Cf): 2.2 pF (Connected in parallel with Rf to prevent oscillation).
- Bias Supply: Clean 5.0V DC source (Cathode connected here).
- Non-Inverting Input (+): Tied to Analog Ground (0V).
The feedback capacitor (Cf) is not optional. The BPW34's 72 pF junction capacitance interacts with the op-amp's input capacitance to create a pole in the feedback loop, causing severe ringing or outright oscillation. Cf introduces a zero that cancels this pole. You can calculate the required Cf using the formula: Cf = √( (Cj + Cin) / (2 × π × Rf × GBW) ), where GBW is the op-amp's gain-bandwidth product. For the OPA340 (5.5 MHz GBW) and BPW34, 2.2 pF provides a safe, slightly under-damped response.
Testing, Failure Modes, and Multimeter Diagnostics
Photodiodes are robust but can fail silently, especially when subjected to electrostatic discharge (ESD) or thermal runaway. The most common failure mode is a degraded PN junction that exhibits excessive 'dark current' (current that flows in reverse bias when no light is present), which drowns out low-light signals in noise.
How to Test a Photodiode with a Multimeter
- Forward Bias Check (Diode Mode): Set your DMM to diode test. Place the red probe on the Anode and black on the Cathode. A healthy silicon photodiode will read between 0.35V and 0.55V. If it reads 0.00V, the junction is shorted.
- Reverse Bias Check (Resistance/Diode Mode): Swap the probes (red on Cathode, black on Anode). The meter should read 'OL' (Open Loop) or infinite resistance. If it reads any finite resistance or a low voltage drop, the junction has suffered punch-through or ESD damage. Discard the part.
- Photo-Response Verification (Current Mode): Set the DMM to the µA range. Connect the red probe to the Cathode and black to the Anode (the meter's internal battery will provide a small reverse bias). Cover the sensor with your hand and note the baseline (dark current). Shine a bright white LED flashlight directly onto the epoxy. The current should immediately spike into the 10–100 µA range. If it does not react to light, the wire bond inside the package is likely broken.
Thermal Runaway Warning: Dark current doubles for approximately every 10°C rise in junction temperature. If you are operating a photodiode in a high-ambient-temperature environment (like inside an enclosed outdoor luminaire), a 50°C ambient temp will increase your dark current by roughly 16x compared to room temperature. You must either select a part with a sub-nA dark current spec (like the S1223) or implement software baseline subtraction.
Safe Default Part Numbers and Selection Matrix
When prototyping or moving to production, picking the right default part prevents costly redesigns. Here is a decision matrix for the three most reliable, widely available silicon photodiodes on the market.
| Application Scenario | Recommended Default Part | Why It Wins | Design Caveat |
|---|---|---|---|
| General Light Metering, Lux Sensors, Hobbyist Builds | Vishay BPW34 | Massive 7.5mm² active area captures diffuse room light easily; costs under $0.50 in volume. | High 72pF capacitance limits bandwidth to ~100kHz in standard TIA circuits. Do not use for high-speed data. |
| IR Remote Receivers, Optical Encoders, Pulse Oximetry | OSRAM SFH203 | Optimized for 850-900nm IR; fast 50ns rise time; excellent rejection of visible light if paired with an IR-pass filter. | Smaller 1.0mm² area requires precise optical alignment or a focusing lens to capture enough photons. |
| UV Detection, Fluorescence, Precision Spectroscopy | Hamamatsu S1223 | Ultra-low 0.1nA dark current and 4pF capacitance; excellent UV-to-visible response curve. | Higher cost (~$3.00+); fragile epoxy window requires careful handling during wave/ream soldering. |
For 90% of bench projects involving Arduino or ESP32 microcontrollers reading ambient light or basic laser tripwires, the BPW34 paired with an LM358 (for low speed) or OPA340 (for precision) is the undisputed starting point. Keep your reverse bias voltage between 3.3V and 5V to stay well below the 60V breakdown limit while still depleting enough of the junction to maintain linearity. If your application demands reading nanosecond laser pulses, step down to the SFH203, increase your reverse bias to 15V-30V to minimize capacitance, and use a high-GBW amplifier like the OPA657.
