Enhanced Near-Infrared Sensitivity and Extended Detection Wavelength in Silicon Phototransistors Using a Backside Aluminum Reflector

Silicon is the quiet workhorse of the modern world, forming the backbone of the computers and smartphones in our pockets. It is a material that loves to conduct electricity when we want it to, but it has a specific limitation when it comes to light. Silicon is excellent at catching visible light and the near-infrared light that sits just beyond the red edge of what our eyes can see. However, as light waves get longer and move deeper into the infrared spectrum, silicon becomes increasingly transparent. Instead of absorbing these longer waves to create an electrical signal, the light simply passes right through the material, like sunlight through a clear window. This physical barrier has long prevented silicon-based sensors from seeing the full infrared spectrum, forcing engineers to use more expensive and complex materials for applications that require detecting these faint, long-wavelength signals.

Researchers at Chungbuk National University in South Korea have found a way to overcome this transparency without changing the silicon itself or using exotic new materials. They focused on a simple, clever modification to the back of a standard silicon phototransistor, a device that converts light into an electrical current. By adding a thin layer of aluminum to the rear of the silicon chip, they created a mirror that bounces light back into the material. This single addition allowed the device to detect light at wavelengths up to 1300 nanometers, a range where silicon usually lets light escape. The result is a sensor that is not only capable of seeing deeper into the infrared but is also significantly more sensitive and quieter than its predecessors, all while remaining compatible with the standard manufacturing processes used to make the chips inside our everyday electronics.

The team began with a standard silicon wafer, a thin slice of the material used to build electronic circuits. They constructed a phototransistor on this wafer, a device designed to generate a current when light hits it. In a typical setup, light enters from the top, passes through a gate, and strikes the silicon. If the light is too long-waved, it travels all the way through the silicon and disappears out the bottom, never triggering a response. To fix this, the researchers deposited a 100-nanometer-thick layer of aluminum on the very back of the silicon wafer. This aluminum layer acts as a reflector. When a photon of near-infrared light enters the silicon and fails to be absorbed on its first pass, it hits the aluminum mirror at the bottom and is reflected back up into the silicon. This gives the light a second chance to be absorbed, effectively trapping it inside the material and increasing the likelihood that it will generate an electrical signal.

The impact of this simple mirror was immediate and dramatic. When the researchers tested the devices, they found that the version with the aluminum reflector could detect light at wavelengths up to 1300 nanometers, whereas the device without the mirror stopped responding effectively around 1200 nanometers. In the range between 900 and 1100 nanometers, the sensitivity of the new device increased by more than twelve times compared to the old one. At the extreme edge of 1300 nanometers, where the silicon is almost completely transparent, the improvement was even more striking, with the device becoming roughly twenty-six times more responsive. This means that light which would have previously been lost is now being captured and converted into a usable signal.

Beyond just seeing further, the new design also became much quieter. In electronic sensors, "noise" often comes from a small, unwanted current that flows even when no light is present, known as dark current. This background noise can drown out faint signals. The researchers observed that adding the aluminum reflector reduced this dark current by a factor of forty-six, dropping it from a level of 2.5 × 10–10 amperes down to 5.4 × 10–12 amperes. This reduction is crucial because it allows the sensor to distinguish a real signal from the background static with much greater clarity. Remarkably, this improvement in sensitivity and noise reduction did not come at the cost of the device's normal performance; the current flowing when the device is turned on remained strong and consistent, proving that the mirror improved the light capture without disrupting the electrical flow of the transistor.

To understand exactly how this worked, the researchers used computer simulations to visualize the path of light inside the silicon. These models showed that without the mirror, a significant amount of light simply passed through the silicon and was lost. With the aluminum reflector in place, the simulations revealed that the light was bouncing back and forth, staying trapped within the silicon layer for longer. This multi-pass effect meant that even at wavelengths where silicon is naturally poor at absorbing light, the photons had multiple opportunities to interact with the material and generate electricity. The simulations confirmed that the aluminum layer was successfully concentrating the optical power within the silicon, particularly at the longer wavelengths where the material is usually transparent.

The study demonstrates that a straightforward, cost-effective modification can significantly extend the capabilities of silicon-based sensors. By simply adding a reflective layer to the back of the chip, the researchers were able to push the detection limit of silicon phototransistors into a range that was previously difficult to access. This approach does not require complex nanostructures or expensive new materials, making it a practical solution for improving near-infrared sensing. The findings suggest that with this simple optical trick, silicon can be made to see further and more clearly, offering a path toward more sensitive and versatile sensors that can be built using the same manufacturing lines that produce the chips in our current technology.