How light emitting diodes, LEDs, works! Continued Page 3

Where did the electrons get the energy to emit light? How do they get to
a higher energy level so they can enter the n-type crystal? Well, in
order for the battery to push electrons through the LED, it had to apply
electrical attraction and repulsion forces to the electrons in the
crystal. To apply force to the electrons in the crystal, it had to apply
a force to the electrons in the negative wire. This squeezes all the
electrons on the surface of the negative wire together, which raises the
voltage of the entire wire. (If electrons were like water, then the wire
is like a long trough. The battery pumps water into one end of the
trough, and this makes the water level 'voltage' rise everywhere in the
trough.) When the negative wire's electrons get to the energy level equal
to the n-type crystal, they start flowing into the crystal and falling
"down" the junction, emitting light as they go. (This analogy is
incomplete: at the same time that the battery was pumping up the "water
level" of the negative wire to match the n-type crystal level, it also was
REDUCING the "water level" of the positive wire so that the low-energy
electrons of the p-type crystal could be sucked into the wire.)

Here's another way to visualize LEDs. In a neon sign, the electrons
around each neon atom get pumped up in energy as they're whacked by
incoming high-speed electrons. In an LED the battery pumps up the
electrons directly. In a neon sign, each atom emits light when an
electron falls back to its original energy level. In an LED, the whole
crystal junction emits light as electrons drop back to a lower level.
Therefor an LED resembles a gigantic single neon atom! An LED/atom is so
large that we can connect its electron cloud directly to a battery with
wires. It's so large that we can build in different characteristics, and
change the color of its flourescence.

Next Lesson on How LEDs work....