How It Works: The Photoelectric Effect

The core of a PD is the photoelectric effect: when photons strike the semiconductor material (typically InGaAs or silicon), their energy is absorbed and electron-hole pairs are generated. Under the electric field created by reverse bias, electrons and holes drift toward their respective electrodes, producing a photocurrent proportional to the incident optical power.

How precisely the photocurrent maps to optical power determines whether the PD can faithfully "translate" the optical signal. This precision is defined by the key metrics below.

The Two Mainstream Types: PIN and APD

The two most common PDs in optical communications are the PIN photodiode and the APD (avalanche photodiode). They differ fundamentally in principle and positioning.

PIN photodiode: the simple, reliable workhorse

A PIN diode consists of three layers: P-layer / Intrinsic layer / N-layer. The widened intrinsic (depletion) layer absorbs more photons while lowering junction capacitance and improving response speed.

Fast response: suitable for high data rates;

Low dark current, low noise, excellent linearity;

No internal gain: responsivity is capped at ~0.9~1.0 A/W (limited by the physics of quantum efficiency < 100%);

Low cost, high reliability: the standard receive-side choice for the vast majority of optical transceivers; typical reverse bias 5~12 V.

APD: the amplifier for weak light

An APD adds a high-field multiplication region to the structure: photogenerated carriers are accelerated in the strong electric field and collide to ionize more electron-hole pairs, producing an avalanche multiplication effect - a single photon "fissions" into hundreds or thousands of electrons, achieving internal gain.

Internal gain of tens to hundreds of times - sensitivity an order of magnitude higher than PIN;

Higher SNR, ideal for detecting weak optical signals;

Requires >100 V reverse bias, and gain is sensitive to temperature and bias - needs bias control circuitry;

Higher cost, typically reserved for sensitivity-critical applications.

In one sentence: the PIN is good enough and cheap; the APD is sensitive but demanding - choose PIN when received optical power is ample, choose APD when the power budget is tight.

Five Parameters You Must Know

Responsivity: Measured in A/W - how many amperes of photocurrent are produced per watt of incident optical power. PIN diodes typically reach 0.9~1.0 A/W at 1550 nm. Responsivity directly determines the opto-electrical conversion efficiency of the receive chain.

Quantum Efficiency: The probability (as a percentage) that each incident photon successfully generates an electron-hole pair. It is governed by material absorption coefficient, depletion-layer thickness, and surface reflection - the physical root of responsivity.

Dark Current: The tiny leakage current that flows even with no light. Dark current is a major noise source in receivers and grows exponentially with temperature - one reason industrial-grade optical modules require temperature compensation.

Bandwidth: The PD's ability to track rapid changes in optical power, determined jointly by carrier transit time and junction capacitance. Bandwidth directly constrains the maximum supported line rate: 10G, 25G, 50G and 100G modules need PDs of different bandwidth grades.

Sensitivity & NEP (Noise Equivalent Power): Sensitivity is the minimum received optical power required for a target bit-error rate - the core metric of an optical receiver. NEP is the minimum optical power needed to achieve an SNR of 1; the lower, the better. With internal gain that overcomes the thermal noise of the load resistor, the APD holds a generational advantage over the PIN on both counts.

Common Packages and Component Forms

In optical modules, the PD rarely appears alone - it is usually integrated into component-level assemblies:

TO package: metal stem + lens (e.g., TO-46) - the basic package for PD chips;

ROSA (Receiver Optical Sub-Assembly): PD + TIA transimpedance amplifier + fiber interface in one package, outputting electrical signals directly - the standard receive-side form in optical modules;

BOSA (Bidirectional Optical Sub-Assembly): transmitter laser and receive PD co-packaged for single-fiber bidirectional operation - the core component of ONUs;

Butterfly package: high-speed applications with cooling requirements, e.g., 100G coherent receivers.

Typical Application Scenarios

Optical transceivers and ONUs: Every SFP/QSFP module and every ONU contains a PD. From 1G to 100G, the PD's bandwidth and sensitivity grade determine how fast and how far the module can go.

OLT and PON systems: OLT-side modules also rely on high-sensitivity PDs (often APDs) to meet the ~20 km optical power budget of GPON/EPON.

OTDR and fiber testing: OTDR needs to detect weak backscattered light - high-sensitivity APDs with low-noise transimpedance amplifiers are standard.

Laser ranging and LiDAR: Pulsed time-of-flight ranging depends on the APD's high sensitivity and fast response - from industrial ranging to automotive LiDAR.

Optical power meters and instruments: Precision optical power measurement requires PIN diodes with excellent linearity and minimal dark current, calibrated to 0.01 dB-level accuracy.

Conclusion

From the PIN's "simple and reliable" to the APD's "sensitive multiplication," photodetectors draw a clear dividing line between sensitivity and cost. Understand the underlying parameters - responsivity, dark current, bandwidth - and you can precisely control receive-side performance in module selection and link design.

GOODMIND ELECTRONICS (SHENZHEN) LTD has been deeply engaged in optical communications. GME-branded optical modules, ONUs, and OLTs all employ rigorously screened PIN/APD photodetectors, validated layer by layer from component to system, committed to delivering stable and reliable communication products to global customers. Visit www.rfgme.com to learn more.