AlGaN-based deep ultraviolet (UV) light emitting diodes (LEDs) and lasers are important for a broad range of applications, including water purification, disinfection, surface treatment, and chemical and biochemical sensing. To date, the performance of AlGaN quantum well UV LEDs and laser diodes has been limited by the presence of large densities of defects and dislocations, large polarization field, and extremely inefficient p-type doping. In this context, we have developed nearly defect-free AlGaN nanowire heterostructures. We have achieved highly efficient p-type conduction of Mg-doped AlN and Al-rich AlGaN nanowires. With the use of AlGaN nanowires, we have further demonstrated, for the first time, electrically pumped semiconductor lasers operating in the UV-B and UV-C bands. The AlGaN nanowires are grown on Si substrate under N-rich conditions by plasma-assisted molecular beam epitaxy. Detailed structural characterization suggests the presence of strong compositional modulation. The Ga-rich AlGaN clusters have dimensions of 0.25 to 2 nm along the growth direction and of 2 to 10 nm along the lateral direction. The localized variations in the Ga-concentration are estimated to be ~10%, thereby providing strong quantum-confinement of charge carriers. By changing alloy composition, the emission can be widely tuned in the UV-C band from 207 nm to 280 nm. We have demonstrated AlN nanowire LEDs with a small turn on voltage of ~6 V, which is significantly smaller than previously reported AlN epilayers LEDs (> 20 V). This excellent electrical performance is largely ascribed to the significantly enhanced Mg-dopant incorporation in nanowire structures and the resulting efficient impurity band conduction. For ternary AlGaN nanowire UV LEDs, the device structure consists of n⁺-GaN/Al/p⁺-AlGaN tunnel junction, which can remove the resistive p-contact layer and improve hole transport. Compared to devices without tunnel junction, the output power is improved by 2 orders of magnitude. For the device operating at 274 nm, the maximum output power is > 20 mW. It is further envisioned that, with the use of AlGaN nanowires, UV-C LEDs with efficiency >70% can be potentially achieved. By further exploiting Anderson localization of light in such nearly defect-free AlGaN nanowires, we have demonstrated, for the first time, electrically injected lasers operating in the UV-B and UV-C bands. For lasers operating at 239 nm, the threshold current is ~0.3 mA at room-temperature. These studies provide a viable path to achieve high efficiency semiconductor laser diodes operating in the UV-C band that was not previously possible.