How light emitting diodes, LEDs, works! Continued Page 4

Light Emitting Diodes are much like solar cells. Both devices use n-type
and p-type crystals, but in a solar cell the process runs backwards:
instead of falling down in energy and emitting light, light hitting the
solar cell causes electrons in the p-type crystal to jump upwards in
energy. If these electrons are near the crystal junction, they can end up
in the n-type crystal, and they can flow through wires to the outside
world, falling down in energy as they do. In fact, if light shines on an
LED, the LED behaves as a tiny, inefficient solar cell. And conversely,
if a battery is used to create a current in a solar cell, the solar cell
can emit a very tiny amount of (mostly infrared) light. An LED gives
light when charge is pumped through it, and when light shines on a solar
cell, the solar cell becomes a charge pump.

Light Emitting Diodes are also like thermocouples. N-type and p-type
crystals are not the only materials whose electrons "orbit" at different
energy levels. Different metals have different levels too. If a copper
wire is twisted together with an iron wire, a junction is formed between
them which contains an energy-step like that of an LED. The energy-step
in a thermocouple is much smaller than in an LED. If electrons are forced
to flow across the thermocouple's energy step, they fall down in energy
level and emit energy. But what do they emit? Longwave Infrared light
and crystal vibrations. Together we call these by the name "heat energy".
The energy step in a thermocouple is too small, so it cannot emit photons
of visible light. Instead it creates "heat." And conversely, if heated,
a thermocouple can create an electric current. When operated one way, a
thermocouple is a bit like an LED which emits heat. When operated the
other way, it acts a bit like a "solar battery" and becomes a "heat
battery."

Summary of How LEDs work!!