Direct Energy, 2018a

150 7.3 Devices Involving Spontaneous Emission
Figure 7.3: Example current-voltage characteristics of a gas discharge
lamp. Figure usedwith permission from [89].
arc discharge is established, relatively low voltages are required to maintain
it compared to the voltages needed to maintain the glow discharge.
Fluorescent lamps are a type of gas discharge device that involves the
use of chemicals with desired optical properties, called phosphors [87, p.
542]. The gas andelectrode voltage usedin uorescent lamps is chosen
to so that the spontaneous emission produced is at ultraviolet frequencies.
These UV photons may be produced by either an arc or glow discharge.
The UV photons producedare absorbedby the phosphor molecules, andthe
phosphor molecules emit light at lower frequencies. Examples of phosphors
used include zinc silicate, calcium tungstate, and zinc sulde [87, p. 542].
7.3.3 LEDs
LEDs are devices that convert electricity to light by spontaneous emission.
They are made from pn junctions in semiconductors. Pn junctions
were discussedin Section 6.5. When a forwardbias is appliedacross a pn
junction, electrons andholes are injectedinto the junction. The energy
from the power supply excites electrons from the valence to the conduction
bands. These excited electrons can ow through the material much more