We aim to achieve a breakthrough in the performance of "dilute nitride" semiconductor materials to enable the development of novel light sources and photodetectors which can operate in the mid-infrared spectral range. The 3-5 um wavelength range is technologically important because it is used for applications including; remote gas sensing, range-finding and night vision, bio-medical imaging for diagnosis in healthcare and sensitive detection in optical spectroscopy. However, the development of instrumentation is limited by the availability of efficient, affordable light sources and photodetectors, which is directly determined by the semiconductor materials which are currently available. By introducing small amounts (~ 1%) of N into InAs(Sb) we have shown that it is possible to access the mid-infrared using a new (dilute nitride) semiconductor and we are now seeking to engineer its band structure in order to significantly enhance the material's optical properties and increase quantum efficiency for light detection and emission. To enable the development of new photodetectors we will exploit the sensitivity of the conduction band to the resonant interaction of the N-level with the extended states of the host InAsSb crystal lattice to tailor the photoresponse and create a near ideal situation for electron acceleration and avalanche multiplication, resulting in a much larger detectable signal. To minimise the unwanted processes causing excessive noise and dark current, which compete with the avalanche multiplication and light detection in the detector, we shall arrange for the avalanche multiplication to be initiated by only one carrier type (electrons in our case). Many applications rely on the detection of very weak signals consisting of only a few photons. Conventional photodiodes have a limited sensitivity, especially if high speed detection is needed. In applications which are "photon starved", avalanche photodiodes (APDs) can provide an effective solution. However, at present effective avalanche multiplication in the mid-infrared spectral range can only be obtained by using exotic CdHgTe (CMT) semiconductor alloys. The resulting detectors require cooling, thus making CMT-based APDs prohibitively expensive for all except military applications. Simpler fabrication, low noise, low operating voltage, inexpensive manufacturing and room temperature operation, together with monopolar electron ionisation are all significant advantages of APDs based on the dilute nitride materials compared to existing technologies. Similarly, we shall enable the development of more efficient mid-infrared light sources. By adjusting the N content within InAsN(Sb) quantum wells and carefully tailoring the residual strain and carrier confinement, we shall be able to defeat competing non-radiative recombination processes whilst simultaneously enhancing the light generation efficiency. These novel quantum wells would then form the basis of the active region from where the light is generated, either within an LED or a diode laser. Currently mid-infrared LED efficiency is low at room temperature, and with the improvements which we shall deliver; we envisage that devices with significantly higher dc output power will be developed following our lead. Mid-infrared diode lasers incorporating our strained dilute nitride quantum wells are also expected to exhibit a reduced threshold current and could offer an affordable alternative to existing technology, especially in the 3-4 um spectral range. We will produce prototype photodetectors and LEDs and use these to demonstrate the above-mentioned avalanche behaviour and quantum efficiency improvements respectively. We shall validate our dilute nitride materials and structures in close collaboration with our collaborators at NPL, SELEX, CST and INSTRO to evaluate performance for use in practical applications and help ensure uptake of our technology.