PIN/APD vs. SPAD/SiPM LiDAR Detectors: Key Differences

Introduction : In the evolution of LiDAR technology, the shift from traditional semiconductor detectors to modern “photon counting” sensors has been the primary driver behind the move toward autonomous vehicles and high resolution 3D mapping. Here is an original breakdown of these technologies, comparing the early Legacy Detectors (e.g. PIN and APD) with the Modern detectors such as SPAD and SiPM.

Legacy Detectors : PIN diodes and Avalanche Photodiodes (APDs)

Before the rise of photon-counting technology, LiDAR relied on linear detectors that produced a current proportional to the amount of light received.

1. PIN Photodiodes : These are simple detectors which convert light into electrical current at 1:1 ratio.

• Benefits : Inexpensive, easy to manufacture
• Drawbacks : No internal gain i.e. amplification. Because LiDAR return signals are extremely weak, the electronic noise often drowns out the signal, making them useless for long-range detection.

2. Avalanche Photodiodes : They use a high-voltage “bias” to create an internal gain effect. One photon triggers a small “avalanche” of electrons.

• Benefits : Much more sensitive than PIN diodes; can detect weaker signals.
• Drawbacks : They require very high voltages (often 100V to 500V) and are extremely sensitive to temperature changes. If the temperature fluctuates, the gain changes, making the distance measurement inaccurate.

Modern Detectors : SPADs and SiPMs

The industry has largely moved toward Single-Photon Avalanche Diodes (SPADs) and Silicon Photomultipliers (SiPMs).

1. SPAD (Single Photon Avalanche Diode): A SPAD is essentially a digital switch. When a single photon hits it, it triggers a massive electrical discharge.

• Benefit: It offers incredible timing precision. Because the output is essentially digital “ON” signal the moment a photon hits, it provides the most accurate Time of Flight (ToF) measurements.
• Drawback: Once a SPAD “fires,” it needs a few nanoseconds to reset (known as dead time). During this time, it is blind to any other incoming photons.

2. SiPM (Silicon Photomultiplier) : A SiPM is an array consisting of hundreds or thousands of tiny SPAD cells all connected in parallel.

• Benefit : It combines the sensitivity of a SPAD with the ability to measure intensity. If 10 photons hit the sensor, 10 SPAD cells fire, telling the system exactly how “bright” the reflection was. They operate at much lower voltages (~ 30V) compared to APDs.
• Drawback : Like all high sensitivity sensors, they can be overwhelmed by ambient sunlight (i.e. solar noise), if not properly filtered.

Comparison between legacy vs Modern LiDAR detectors

Summary

Modern LiDAR detectors offer lower power and cost, longer range and higher reliability.