This paper describes development of a novel mid-infrared light emitting diode (LED) and photodiode (PD) light source/detector combination and use within a non-dispersive infrared (NDIR) carbon dioxide gas sensor. The LED/PD based NDIR sensor provides fast stabilisation time (time required to turn on the sensor from cold, warm up, take and report a measurement, and power down again ≈1 second), longevity (>15 years), low power consumption and low cost. Described performance is compatible with "fit and forget" wireless deployed sensors in applications such as indoor air quality monitoring/control & energy conservation in buildings, transport systems, horticultural greenhouses and portable deployment for safety, industrial and medical applications. Fast stabilisation time, low intrinsic power consumption and cycled operation offer typical energy consumption per measurement of mJ's, providing extended operation using battery and/or energy harvesting strategies (measurement interval of ≈ 2 minutes provides >10 years operation from one AA battery). Specific performance data is provided in relation to measurement accuracy and noise, temperature performance, cross sensitivity, measurement range (two pathlength variants are described covering ambient through to 100% gas concentration), comparison with NDIR utilizing thermal source/pyroelectric light source/detector combination and compatibility with energy harvesting. Semiconductor based LED/PD processing together with injection moulded reflective optics and simple assembly provide a route to low cost high volume manufacturing.
In recent years there has been a rapidly increasing demand for CO2 sensors for applications in health monitoring, control of air quality and horticulture. Amongst the various approaches reported so far, the AlGaInSb quaternary alloy shows great promise for the development of compact Light Emitting Diodes (LEDs) as it offers bandgap Type-I alignments, which enable the design of effective multi-quantum well (MQW) active regions. In this paper we show a more than fourfold improvement in wall-plug efficiency by optimising both the strain in AlGaInSb MQW active regions and the fabrication process flow of LEDs emitting at 4.26 um.
In recent years there has been a rapidly increasing demand for energy-efficient and cost effective gas sensors. Of particular interest are CO2 sensors that can find numerous applications in health monitoring, control of air quality and horticulture. A major hurdle comes from the fact that the main CO2 absorption band lies above 4um, where very few cheap and compact sources are commercially available. Amongst the various approaches explored, the indium antimonide material system stands out as a very effective solution for the development of compact Light Emitting Diodes (LEDs). In particular, the quaternary compound AlGaInSb shows great promise as it offers a bandgap type-I alignment, which enables the design of effective multi-quantum well (MQW) active regions. In this paper we show the great potential of LED structures with strained GaInSb MQWs and AlGaInSb barriers for the next generation of mid-IR emitters at 4.3 um. Different quantum well and barrier compositions were examined through k.p simulations to extract momentum matrix elements and energy levels. The simulations were also used to assess the impact of strain and quantum well width on the efficiency of the radiative transition and to optimise the profile of the carrier injection. Based on the theoretical analysis, a number of different epilayer structures were grown by molecular beam epitaxy and the performance of LEDs with varying geometries were compared. Results confirm that strained MQW structures suppress unwanted transitions by at least one order of magnitude and provide a substantial enhancement in the internal quantum efficiency of the LEDs.
We report on the impact of lateral current spreading on light emission from aluminium indium antimonide (AlInSb) mid-infrared p-i-n light-emitting diodes (LEDs) grown by molecular beam epitaxy on a GaAs substrate. Due to the high effective mass of holes in AlxIn1−xSb, the resistivity of p-type material determines the 3-D distribution of current flow in the devices. This work shows that maximum light emission, as measured by electroluminescence, and 3-times wall-plug efficiency improvement were obtained at room temperature from devices with a p-type contact grid geometry with a spacing of twice the current spreading length in the p-type material, which was measured by spatially resolved photocurrent. The LED with the optimal contact geometry exhibits improved performance at high injection current levels thanks to the more uniform carrier distribution across the device area.
A 50 mm × 20 mm × 15 mm indoor photovoltaic (PV) energy harvesting power module (IPEHPM) has been developed for powering an Internet of Things (IoT) sensor node containing a low-power CO 2 sensor for automatic building ventilation. It is composed of a high efficiency PV energy harvesting module and a supercapacitor to produce 3.6-4.2 V output voltage with 100 mA pulse current for up to 600 ms. Storage efficiency analysis and storage efficiency tests of the IPEHPM have demonstrated that with the adopted simple power management scheme, which exempts the commonly used power management blocks of the voltage regulator and the maximum power point tracking to save power, 88.7% average storage efficiency has been achieved at 200 lux. With the newly established PV powering model, the power consumption requirements of an IoT node can be directly converted into the illumination requirements of the PV energy harvester, making the IPEHPM easy to use. IPEHPM powered IoT experiments with a low-power CO 2 gas sensor have demonstrated that the IPEHPM is suitable for IoT-based building ventilation applications, where the CO 2 concentration level is measured every 150 s at the indoor lighting condition down to 200 lux.
This work presents the experimental investigation of different techniques to improve the electrical and optical performance of mid-infrared antimonide-based semiconductor light-emitting diodes. A study of the current crowding effect, supported by spatially-resolved photocurrent measurements, allows the design of an optimal contact geometry. Additionally, a higher fraction of the generated light is redirected towards the top surface of the device thanks to the integration of a back reflector and a resonant-cavity design. Enhanced performance for mid-IR LEDs represents a significant step forward towards power efficient optical sensors for environmental, safety and health applications.
To develop a connected, portable, point-of-care medical device (capnometer) for the diagnosis, monitoring and management of acute and chronic cardiorespiratory diseases at home and in primary and secondary care environments, facilitating timely, evidence-based treatment interventions and
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