RESUMEN
Miniaturized and wireless near-infrared (NIR) based neural recorders with optical powering and data telemetry have been introduced as a promising approach for safe long-term monitoring with the smallest physical dimension among state-of-the-art standalone recorders. However, a main challenge for the NIR based neural recording ICs is to maintain robust operation in the presence of light-induced parasitic short circuit current from junction diodes. This is especially true when the signal currents are kept small to reduce power consumption. In this work, we present a light-tolerant and low-power neural recording IC for motor prediction that can fully function in up to 300 µW/mm2 of light exposure. It achieves best-in-class power consumption of 0.57 µW at 38° C with a 4.1 NEF pseudo-resistorless amplifier, an on-chip neural feature extractor, and individual mote level gain control. Applying the 20-channel pre-recorded neural signals of a monkey, the IC predicts finger position and velocity with correlation coefficient up to 0.870 and 0.569, respectively, with individual mote level gain control enabled. In addition, wireless measurement is demonstrated through optical power and data telemetry using a custom PV/LED GaAs chip wire bonded to the proposed IC.
RESUMEN
Photovoltaic modules at the mm-scale are demonstrated in this work to power wirelessly interconnected mm-scale sensor systems operating under low flux conditions, enabling applications in the Internet of Things and biological sensors. Module efficiency is found to be limited by perimeter recombination for individual cells, and shunt leakage for the series-connected module configuration. We utilize GaAs and AlGaAs junction barrier isolation between interconnected cells to dramatically reduce shunt leakage current. A photovoltaic module with eight series-connected cells and total area of 1.27-mm2 demonstrates a power conversion efficiency of greater than 26 % under low-flux near infrared illumination (850 nm at 1 µW/mm2). The output voltage of the module is greater than 5 V, providing a voltage up-conversion efficiency of more than 90 %. We demonstrate direct photovoltaic charging of a 16 µAh pair of thin-film lithium-ion batteries under dim light conditions, enabling the perpetual operation of practical mm-scale wirelessly interconnected systems.
RESUMEN
Wireless biomedical implantable devices on the mm-scale enable a wide range of applications for human health, safety, and identification, though energy harvesting and power generation are still looming challenges that impede their widespread application. Energy scavenging approaches to power biomedical implants have included thermal [1-3], kinetic [4-6], radio-frequency [7-11] and radiative sources [12-14]. However, the achievement of efficient energy scavenging for biomedical implants at the mm-scale has been elusive. Here we show that photovoltaic cells at the mm-scale can achieve a power conversion efficiency of more than 17 % for silicon and 31 % for GaAs under 1.06 µW/mm2 infrared irradiation at 850 nm. Finally, these photovoltaic cells demonstrate highly efficient energy harvesting through biological tissue from ambient sunlight, or irradiation from infrared sources such as used in present-day surveillance systems, by utilizing the near infrared (NIR) transparency window between the 650 nm and 950 nm wavelength range [15-17].
RESUMEN
The design and characterization of mm-scale GaAs photovoltaic cells are presented and demonstrate highly efficient energy harvesting in the near infrared. Device performance is improved dramatically by optimization of the device structure for the near-infrared spectral region and improving surface and sidewall passivation with ammonium sulfide treatment and subsequent silicon nitride deposition. The power conversion efficiency of a 6.4 mm2 cell under 660 nW/mm2 NIR illumination at 850 nm is greater than 30 %, which is higher than commercial crystalline silicon solar cells under similar illumination conditions. Critical performance limiting factors of sub-mm scale GaAs photovoltaic cells are addressed and compared to theoretical calculations.
RESUMEN
Silicon photovoltaics are prospective candidates to power mm-scale implantable devices. These applications require excellent performance for small-area cells under low-flux illumination condition, which is not commonly achieved for silicon cells due to shunt leakage and recombination losses. Small area (1-10 mm2) silicon photovoltaic cells are studied in this work, where performance improvements are demonstrated using a surface n-type doped emitter and Si3N4 passivation. A power conversion efficiency of more than 17% is achieved for 660 nW/mm2 illumination at 850 nm. The silicon cells demonstrate improved power conversion efficiency and reduced degradation under low illumination conditions in comparison to conventional crystalline silicon photovoltaic cells available commercially.
RESUMEN
GaAs photovoltaics are promising candidates for indoor energy harvesting to power small-scale (≈1 mm2) electronics. This application has stringent requirements on dark current, recombination, and shunt leakage paths due to low-light conditions and small device dimensions. The power conversion efficiency and the limiting mechanisms in GaAs photovoltaic cells under indoor lighting conditions are studied experimentally. Voltage is limited by generation-recombination dark current attributed to perimeter sidewall surface recombination based on the measurements of variable cell area. Bulk and perimeter recombination coefficients of 1.464 pA/mm2 and 0.2816 pA/mm, respectively, were extracted from dark current measurements. Resulting power conversion efficiency is strongly dependent on cell area, where current GaAs of 1-mm2 indoor photovoltaic cells demonstrates power conversion efficiency of approximately 19% at 580 lx of white LED illumination. Reductions in both bulk and perimeter sidewall recombination are required to increase maximum efficiency (while maintaining small cell area near 1 mm2) to approach the theoretical power conversion efficiency of 40% for GaAs cells under typical indoor lighting conditions.
RESUMEN
Arrays of floating neural sensors with high channel count that cover an area of square centimeters and larger would be transformative for neural engineering and brain-machine interfaces. Meeting the power and wireless data communications requirements within the size constraints for each neural sensor has been elusive due to the need to incorporate sensing, computing, communications, and power functionality in a package of approximately 100 micrometers on a side. In this work, we demonstrate a near infrared optical power and data communication link for a neural recording system that satisfies size requirements to achieve dense arrays and power requirements to prevent tissue heating. The optical link is demonstrated using an integrated optoelectronic device consisting of a tandem photovoltaic cell and microscale light emitting diode. End-to-end functionality of a wireless neural link within system constraints is demonstrated using a pre-recorded neural signal between a self-powered CMOS integrated circuit and single photon avalanche photodiode.
RESUMEN
A key challenge for near-infrared (NIR) powered neural recording ICs is to maintain robust operation in the presence of parasitic short circuit current from junction diodes when exposed to light. This is especially so when intentional currents are kept small to reduce power consumption. We present a neural recording IC that is tolerant up to 300 µW/mm2 light exposure (above tissue limit) and consumes 0.57 µW at 38°C, making it lowest power among standalone motes while incorporating on-chip feature extraction and individual gain control.
RESUMEN
Dual-junction GaAs photovoltaic (PV) cells and modules at sub millimeter scale are demonstrated for efficient wireless power transfer for Internet of Things (IoT) and bio-implantable applications under low-flux illumination. The dual-junction approach meets demanding requirements for these applications by increasing the output voltage per cell with reduced area losses from isolation and interconnects. A single PV cell (150 µm × 150 µm) based on the dual-junction design demonstrates power conversion efficiency above 22% with greater than 1.2 V output voltage under low-flux 850 nm near-infrared LED illumination at 6.62 µW/mm2, which is sufficient for batteryless operation of miniaturized CMOS IC chips. The output voltage of dual-junction PV modules with 4 series-connected cells demonstrates greater than 5 V for direct battery charging while maintaining a module power conversion efficiency of more than 23%.