Avalanche photodiode innovation addresses cost challenges of LiDAR

According to MEMS Consulting, Phlux Technology’s InGaAs technology enables avalanche photodiodes (APDs) to operate at a wavelength of 1550 nm, with a sensitivity 12 times that of traditional APDs and overcoming many challenges associated with 905 nm lasers.

Without sensors, self-driving cars would not be able to perceive their surroundings and potential hazards. The same is true for a large number of advanced driver assistance systems (ADAS). Some ADAS functions (such as blind spot detection and adaptive cruise control) need to detect different distances, which prompts automotive engineers to choose the right sensor technology, such as ultrasonic sensors, radar (RADAR) and laser radar (LiDAR). Increasingly, sensor fusion technology allows data obtained from multiple detection methods to be "fused" together to create a more accurate and multi-dimensional image of the vehicle's surroundings.

LiDAR is a commonly used method for detecting objects - lasers emit infrared light and detect the reflected light, using time of flight (ToF) to calculate the distance between the LiDAR and the object, and can usually detect objects up to 200 meters away. Initially, scanning subsystems for LiDAR were very expensive, especially those that used rotating mechanical motors to control the field of view (FOV). However, with the increasing popularity of LiDAR and the development of field of view control based on MEMS micromirrors, the cost has been greatly reduced.

The APD is operated at a reverse voltage, which makes its operating voltage slightly lower than the breakdown voltage to achieve maximum gain.

As with many sensing applications, the specifications of the sensor determine the performance characteristics of the entire subsystem. The receiving sensor in a lidar is typically a photodiode, which converts reflected light into an electrical signal. APDs are highly sensitive because they involve an avalanche process that produces a multiplication factor, producing more electronic output from a given number of reflected photons.

Cost remains a major consideration

Until now, the manufacture of APDs has often used indium gallium arsenide (InGaAs) processes. Phlux Technology announced that it has developed an InGaAs technology that can operate at a wavelength of 1550 nm, which is 12 times more sensitive than traditional APDs and overcomes many challenges associated with 905 nm lasers. According to MEMS Consulting, EE Times Europe recently interviewed Ben White, CEO and co-founder of Phlux Technology, to understand the secret of the 1550nm wavelength providing better performance for automotive sensing applications.

Ben White

“Phlux Technology started as a research project at The University of Sheffield, where my co-founder and professor and I made an early breakthrough by discovering that a particular antimony alloy composition exhibited some unique electronic properties that were very low noise and could be used in detectors,” Ben White told EE Times Europe.

He continued: “LiDAR is increasingly being used in the automotive sector, however, the cost of lasers remains a limiting challenge to the mass rollout of LiDAR. Utilizing our highly sensitive APDs, subsystem designers can use lower-cost lasers to achieve the same sensing performance as the more expensive lasers matched to traditional InGaAs APDs. This reduces the overall material cost to a level that can be mass-produced.”

EE Times Europe asked Ben White if there were other limiting factors besides cost, and he responded: “Reliability is a huge metric for the automotive industry, and even the failure of just a few components can have a substantial negative impact, given the cost of recalls. Therefore, it is critical to have a very low failure rate over the entire life cycle of the product, regardless of the circumstances, and as far as we know, this is one of the challenges of today’s lidar systems.”

905 nm vs. 1550 nm

Ben White told EE Times Europe that at the moment, the lidar industry seems to be divided into two camps regarding the best wavelength to use. “At 905 nm, one can use an inexpensive gallium arsenide laser and a low-cost silicon detector.”

However, he warned: "At 905nm, which is very close to what the human eye can absorb, the cornea can still focus light onto the retina, so one has to use a lower-power laser. But to do that, there is not the required photon budget to see the required 250 meters with a single channel within a reasonable field of view. We noticed that most companies use 256-line laser detectors to solve this problem. Although one can use low-cost components, you end up using a large number of components to make up for the lack of performance. So the solution of using cheap components makes the system more complicated."

Ben White said Phlux Technology's highly sensitive Aura Noiseless InGaAs APD series focuses on the 1550 nm wavelength. "Working at this wavelength, one can use more laser power without damaging the eye. The cornea does not focus the light, and the mucus in the eye also absorbs this wavelength."

Aura Noiseless InGaAs APD developed by Phlux Technology

Improved 1550 nm APD performance

While the 1550 nm wavelength is safer for the human eye, many APDs are not particularly sensitive to short-wave infrared wavelengths. "With the Aura Noiseless APD, we will be more competitive, balancing the system trade-offs of low-cost lasers compared to cheaper silicon-based avalanche photodiodes," said Ben Ben White. "Looking back over the past 25 years, the detectors currently in use have not changed much. New applications such as lidar bring higher performance requirements. What differentiates us from other companies making 1550 nm detectors is that we add antimony alloys to compound semiconductors during the manufacturing process. For a given laser power, the lidar image resolution is improved by 12 times, and with an APD gain of up to 120, we can identify the smallest signal above the noise floor of the connected transimpedance amplifier (TIA) and reduce size, weight, and bill of materials (BOM) cost."