The 3-5 μm mid-infrared spectral region is of great interest as it contains the fundamental molecular fingerprints of a number of pollutants and toxic gases, which require remote real-time monitoring in a variety of applications. Consequently, the development of efficient optoelectronic devices operating in this wavelength range is a very fascinating and pertinent research. In recent years, there has been a rapid development of optical technologies for the detection of carbon dioxide (CO2), where the detected optical intensity at the specific gas absorption wavelength of 4.26 μm is a direct indication of the gas concentration, the main applications being in indoor air quality control and ventilation systems. The replacement of conventional infrared thermal components with high performance semiconductor light-emitting diodes (LEDs) and photodiodes in the 3-5 μm range allows to obtain sensors with similar sensitivity, but with an intrinsic wavelength selectivity, reduced power consumption and faster response. Gas Sensing Solutions Ltd. has developed a commercial CO2 optical gas sensor equipped with an AlInSb-based LED and photodiode pair, which has demonstrated a significant reduction in the energy consumption per measurement. The aim of this Ph.D. project, supported by an EPSRC Industrial CASE Studentship, was to improve the performance of mid-infrared AlInSb LEDs. This was achieved through the optimisation of the layer structure and the device design, and the application of different techniques to overcome the poor extraction efficiency (~ 1 %) which limits the LED performance, as a consequence of total-internal reflection and Fresnel reflection. A key understanding was gained on the electrical and optical properties of AlInSb LEDs through the characterisation of the epi-grown material and the fabrication of prototype devices. Improved LED performance, with a lower series resistance and stronger light emission, was achieved thanks to the analysis of a number of LED design parameters, including the doping concentration of the contact layers, the LED lateral dimensions and the electrode contact geometry. A Resonant-Cavity LED structure was designed, with the integration of an epitaxially-grown distributed Bragg reflector between the substrate and the LED active region. The advantage of this design is twofold, as it both redirects the light emitted towards the substrate in the direction of the top LED surface and adds a resonant effect to the structure, resulting in a three-times higher extraction efficiency at the target wavelength of 4.26 μm, spectral narrowing and improved temperature stability. Finally, 2D-periodic metallic hole array patterns were integrated on AlInSb LEDs, showing potential advantages for spectral filtering and enhanced extraction of light emitted above the critical angle.