Feature: Power management
visible region of the spectrum. At this wavelength, IR light that reaches the eye can damage the retina, so safety rules force capping laser diode output power, and that cap directly limits how far a Si- based system can reach. With InGaAs APDs, however, the
window moves into the 1300-1550nm range, with 1550nm being most commonly found in practice. In this so-called “eyesafe band”, the IR lasers can be driven to powers hundreds of times higher, yet still remain compliant with safety limits. Tis allows an extended operating distance without putting users’ eyesight at risk. For years, one of the main design
headaches with InGaAs APDs has been the extra noise created by the avalanche multiplication process, which cuts sensitivity. Newer structures that incorporate antimony into the compound semiconductor stack have begun to overcome this, cutting avalanche noise yet delivering several other performance gains at the same time.
Reducing avalanche noise in InGaAs APDs In a conventional APD, the avalanche multiplication that produces the gain also generates a lot of noise, which directly undercuts sensitivity. Te usual multiplication layer materials let impact ionization create holes and electrons with similar probability, and the strong internal field then accelerates positive and negative carriers in opposite directions, so each injected carrier can trigger a very different chain reaction. Because the process is so random, one
event can run away and multiply for a long time, whilst the next may produce very little gain. Once the device is pushed to higher reverse bias, this statistical spread takes over, so beyond a gain of roughly 10-20 the noise from the avalanche process starts to dominate the signal we care about. In the newer InGaAs APD designs,
there’s a multiplication layer made from a semiconductor alloy that includes antimony. In this material, virtually every impact ionization event generates electrons, and the avalanche action is confined to a well defined region inside the
Figure 1: Reverse-biased avalanche photodiode structure showing internal gain from impact ionization
Figure 2: Typical wavelength response and gain ranges for Si, Ge and InGaAs avalanche photodiodes
www.electronicsworld.co.uk April 2026 17
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