May 16, 2024

White Paper: High-performance photodiodes for wearable health monitoring

Wearable healthcare technology has gained widespread popularity, both in clinical settings and daily health monitoring. ElFys introduces an advancement with high-performance photodiodes (PDs) utilizing Black Silicon Induced Junction technology, aiming to enhance the accuracy and efficiency of wearable health monitors, particularly in photoplethysmography (PPG).

Black Silicon Induced Junction Technology

ElFys Black Silicon Induced Junction technology represents a radical departure from conventional PDs. This innovation is designed to eliminate optical losses and establish a recombination-free p-n junction, achieving near-ideal external quantum efficiency (EQE) across the UV-VIS-NIR spectrum. The nanostructured black silicon on the PD's front surface plays a pivotal role, offering virtually zero reflection for a wide range of wavelengths.

Figure 1. ElFys PDs are based on the patented Black Silicon Induced Junction technology. They have a nanostructured black Si front surface to eliminate optical losses. Charge carrier collection is realized via induced junction technology: a highly-charged thin film coating on the black Si surface inverts the polarity of the PD front surface and enables a “recombination-free” p-n junction. Note that the figure presents a generalized PD structure and not any specific ElFys product.

At the core of wearable health monitoring is Photoplethysmography (PPG), a technique dependent on LEDs illuminating the skin while PDs measure transmitted or reflected light. ElFys' approach focuses on induced junction technology, creating a p-n junction without external dopants, effectively minimizing electrical losses and enhancing measurement accuracy.

Figure 2. (a) PPG is based on illuminating the skin with LEDs and measuring the reflected or transmitted light with a PD. (b) Portion of the incident light is absorbed by tissues, venous blood, and arterial blood, and the rest is transmitted through or reflected from the body. The amount of absorbed light varies periodically, since the arterial blood volume changes depending on the phase of the cardiac cycle. HR can be determined by measuring this variation.

ElFys PDs achieve remarkable EQE, approaching 100% across the UV-VIS-NIR spectrum. This exceptional efficiency ensures the capture of every incident photon, surpassing the performance of traditional PDs optimized for specific wavelength ranges. The SM series, tailored for wearable health monitoring, exhibits noteworthy sensitivity improvements, particularly for green and red light.

Figure 3. The patented Black Silicon Induced Junction technology enables near 100 % external quantum efficiency (EQE) over a wide range of wavelengths in UV-VIS-NIR, meaning that ElFys PDs can “Capture every ray of light”. The improvement compared to other technologies is clear. Notably, the conventional PDs can be optimized only for a narrow wavelength region.

High-performance photodiodes for wearable health monitoring

Selection of the PD has a major impact on the accuracy of the PPG measurement. Since the device is responsible for catching the transmitted or reflected light signal, its performance determines the quality of the raw measurement data regardless of the used software algorithm

As presented in the previous section, ElFys provides high-performance PDs based on the patented Black Silicon Induced Junction technology. The ElFys SM product series is specifically designed for wearable health monitoring applications, where high sensitivity is required. These PDs are packaged in a surface-mount type of package with the PD attached on a printed circuit board (PCB) and molded in optical epoxy for surface protection. Standard sizes of 3.22 mm2 and 4.46 mm2 are available off the shelf for easy drop-in replacement, but the products can also be flexibly customized to fulfill customer-specific requirements.

One of the most important parameters on PD performance for PPG applications is photoresponse, which tells how large output current the PD produces per Watts of incident light. Higher value means that smaller amount of light is enough to produce equally strong signal, or analogously, the electrical signal is stronger for the same amount of light. Figure 4 presents a typical response spectrum of packaged ElFys SM series products compared with another state-of-the-art PD often used for PPG. ElFys patented Black Silicon Induced Junction technology provides ~50 % improvement for the sensitivity at green, and the photoresponse is as high as 0.40 A/W at 540 nm wavelength. The response is nearly ideal also at red, where ElFys technology improves the performance by >15 % to 0.47 A/W at 630 nm.

Figure 4. Photoresponse of ElFys Black Silicon Induced Junction PDs compared to other state-of-the-art PD products often used in PPG. ElFys technology provides ~50 % sensitivity improvement for green light and produces >15 % stronger signal for red light.

As the noise levels of the products compared in the figure 4 are similar, difference in response directly tells the difference in signal to noise ratio. ElFys SM components hence produce up to ~50 % higher signal with a given light intensity compared to other state-of-the-art products, meaning a great improvement in the PPG measurement accuracy. The higher sensitivity also means that smaller light intensity can be measured more accurately, and a weaker light signal produces equally strong electrical output from the PD. The LEDs of the PPG module can hence be driven with a smaller current, which reduces the power consumption of the measurement and provides a possibility to improve the device battery life. A third benefit of the higher photoresponse is that the same signal level can be obtained from a PD with a smaller footprint. This gives wearable device manufacturers more freedom in optimizing their design and enables making the devices ever smaller.

Applications and Implications

Pulse oximeter .Non-copyright photo from

As the use of wearable healthcare technology proliferates in clinical and everyday contexts, the accuracy of vital sign monitoring, such as heart rate and blood oxygen levels, becomes crucial. ElFys Black Silicon Induced Junction PDs bring forth a host of benefits, from heightened sensitivity to reduced power consumption, setting a new standard for wearable health monitoring devices.

Improved photodiode performance translates into tangible benefits for users of wearable health monitoring devices. Higher sensitivity enables the detection of new body parameters, potentially opening avenues for new applications. In medical use, heightened accuracy leads to more precise monitoring of vital signs, improving diagnosis and potentially saving lives.

In sports, optical measurements have enabled heart rate-based training without the need for a separate chest strap. ElFys improved sensitivity addresses limitations in certain sports, ensuring high accuracy where needed. Moreover, in endurance sports, higher PD performance contributes to reduced energy consumption, crucial for activities lasting several days.

ElFys Black Silicon Induced Junction technology is poised to usher in a new era of wearable health monitoring. As the demand for precision, portability, and energy efficiency continues to surge, ElFys PDs stand out as a pioneering solution. The implications stretch from medical applications to sports and fitness tracking, promising a future where wearables not only monitor health but do so with unprecedented accuracy and efficiency

Read more:

J. Heinonen et al.  “Modeling field-effect in black silicon and its impact on device performance”, IEEE Transactions on Electron Devices, 67(4):1645–1652 (2020).

M. A. Juntunen et al. “Near-unity quantum efficiency of broadband black silicon photodiodes with an induced junction”, Nature Photonics, 10(12): 777-781 (2016).

Juha Heinonen, “High-sensitivity photodiodes using black silicon and induced junction”, Aalto University publication series DOCTORAL THESES, 36/2022.

M. A. Juntunen et al., ”N-type Induced junction Black Silicon photodiode for UV detection”, Proc. SPIE 10249, Integrated Photonics: Materials, Devices, and Applications IV, 102490I (2017).

M. Garin et al. ”Black silicon UV photodiodes achieve >130% external quantum efficiency”, Physical Review Letters, 125:117702 (2020).

Author biography:

Dr.Toni Pasanen

Dr.Toni Pasanen works at ElFys, Inc. as a Senior Project Engineer and is one of the co-founders of the company. He holds a D.Sc. (Tech.) degree in Micro- and Nanosciences from Aalto University, Finland, and has a background in applied research on semiconductor-based optoelectronic devices. He has several years of experience in the design and fabrication of various types of light and radiation detectors and solar cells, as well as strong expertise on nanostructured black silicon surfaces and thin films. Dr. Pasanen has authored tens of peer-reviewed scientific articles, written a book chapter about the properties and applications of black silicon, and presented his research in several international conferences.

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