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Excellent choice for detecting weak light: How do SPADs operate in Geiger mode?
With the advancement of science and technology, our demand for light detection technology continues to grow. Especially in high-tech fields such as quantum computing and biomedicine, finding equipment that can sensitively capture weak light has become an important issue. Among them, single-photon avalanche diodes (SPAD) have shown strong potential in the field of light detection due to their unique working principles. This article will delve into the operating principles of SPAD, especially in Geiger mode, and reveal its value in various applications.
The basic working principle of SPAD
A standard SPAD is a photodiode that operates at a high reverse bias and is designed to allow photons to enter, creating electrons and positive holes. Collisions of these carriers can cause additional electrons to be released, triggering an "avalanche" effect that further enhances the signal.
In Geiger mode, the SPAD's gain can reach 10^5 to 10^6, making it an extremely sensitive single-photon detector.
Geiger pattern and its implementation
Geiger mode is a special operating state. In this mode, the reverse voltage of the SPAD is set above its breakdown voltage. This means that even the incidence of a single photon can trigger a huge increase in current. However, due to the high-gain nature, when a signal is generated, it must be quickly limited to prevent continued conduction. This is achieved through active and passive kill techniques.
The effective operation of SPAD in Geiger mode depends on the control of its dark count rate and post-pulse probability, which are important indicators to ensure detection accuracy.
SPAD material selection
SPAD performance is also affected by the materials used. Silicon can effectively detect visible light and near-infrared light, while Germanium can detect infrared light to a wavelength of 1.7 microns. However, the gain noise of germanium is relatively high, while diodes using amorphous InGaAs can be optimized for detection up to 1.6 microns, and their gain noise is relatively low, suitable for high-speed communication needs.
SPAD application scope
SPAD’s high sensitivity allows it to play a key role in multiple fields, including quantum communications, medical imaging (such as positron emission tomography), and particle physics research. Because SPAD can accurately count the arrival of individual photons, this is critical for studying the behavior of light at extremely tiny scales.
Performance limits and challenges
Although SPAD has many advantages, its performance is also limited by multiple factors, including quantum efficiency, high leakage current, and additional noise sources such as gain noise and Fano noise. These factors may affect the final sensitivity and accuracy of the device, so not only the reverse bias voltage needs to be strictly controlled, but the materials and design also need to be continuously optimized to reduce the impact of noise.
When asked how all these technological advantages will affect our future, can SPAD become the mainstream of optical detection technology in the future?
