What are UV LEDs?

Background
UV LED sources utilize the unique properties of semiconductors which were first reported by Michael Faraday in 1883.  A high level overview of how semiconductors can be used to produce light follows: 

Materials can be grouped by their ability to support a flow of electric current as follows: 

1) Insulators with no current flow
2) Conductors where current flows easily
3) Semiconductors with limited current flow that is a function of temperature

In the semiconductor manufacturing process, impurities (or dopants) are added to the base semiconductor material to add either free electrons (in negative or n-type regions) or create holes to attract electrons enabling current flow (in positive or p-type regions).  Semiconductor light-emitting diode (LED) technology consists of a p-n junction formed by two dissimilarly doped semiconductors.  When no external voltage is applied, the electrons near the junction zone balance and there is no current flow.  When an external electric voltage bias is applied across the junction, current can be made to flow, and when the holes from the p-type and electrons from the n-type meet at the junction and combine, a photon of light is released (see Figure 1). 

The wavelength of the light emitted is a function of the bandgap energy of the materials used in the p-n junction (bandgap energy refers to the energy threshold that electrons must pass in order to flow across the junction).  The more aluminum added to the semiconductor material, the shorter the output wavelength. However, with increased aluminum content in the semiconductor material, it becomes harder for electrical current to flow and the semiconductor material becomes much harder to grow in the manufacturing process. By adjusting the materials used and the doping, a wide range of wavelengths are possible, including wavelengths in the ultraviolet (UV) portion of the electromagnetic spectrum.