How does an LED emit light?

How does an LED emit light?
(c)1997 William J. Beaty

Light from a Light Emitting Diode (LED) is created in much the same way
that light is created in a flourescent tube or neon sign. In an LED
crystal the electrons of its atoms ar 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 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.

Do not you love the way that different parts of physics can hang together
and seem the same?

.....................uuuu / oo \ uuuu........,.............................
William Beaty voice:206-781-3320 bbs:206-789-0775 cserv:71241,3623
EE/Programmer/Science exhibit designer http://www.eskimo.com/~billb/
Seattle, WA 98117 billb@eskimo.com SCIENCE HOBBYIST web page
e pumped up to higher energy states,
and when they fall back down again, each atom gives off a particle/wave of
light. However, the electrons in an LED are not exactly the same as the
gas molecules in a neon sign. They are not in orbitals stuck to
individual atoms. Instead the electrons occupy a contiguous "sea of
charge," and they continually wander among all the atoms in the material.
But while they do this, they maintain a particular energy level just like
they do when stuck to individual atoms. It is as if each electron in an
LED crystal was "orbiting" among all the atoms of the substance as a
whole, and the electron always "orbits" at a particular "height" above
each of the atoms it passes.

Be aware that *all* substances contain electrons. The electrons I am
discussing here are not supplied by the battery, they instead occur
naturally in the wires, crystals, etc. They are in the LED all the time,
even when the battery is not connected. Do not make my original mistake by
imagining that electrons are injected into the LED by the power supply.
In fact, they are already in the material, and the power supply simply
forces them to flow.

To create LED light, first we connect two conductive crystals of different
characteristics together. Both types of crystal contain movable
electrons. In one type of crystal the electrons "orbit" naturally at a
high energy level, and in the other, they always "orbit" low. When a
voltage is applied across the joined crystals, the electrons inside are
forced to flow across the boundary between the pair of crystals. If the
flow direction is correct, electrons in the "high" crystal flow into the
"low" crystal and must begin orbiting at the lower energy level. As they
fall to the lower energy level, they give off light. The frequency of the
light (which we see as the color) is determined by the difference in
energy levels between the two crystals. By manufacturing different types
of crystals having different natural energy levels, various colors of
light can be created. Crystals with similar levels create low-energy
photons of red light or even infrared light. With a larger difference in
energy levels, green light can be created. An even larger energy-step can
create blue light.

The "high" and "low" crystals are usually called "n-type" and "p-type."
In n-type crystals the movable electrons wander around while staying at
the upper energy level of an unfilled outer atomic orbital. During an
electric current they travel at this level. In "p-type" crystals the
mobile electrons naturally exist at a deeper orbital level. When the two
crystals are connected to each other and then connected properly in a
circuit with a battery, the battery creates a current in the entire
circuit. It sucks electrons out of the end of the p-type crystal and into
the wire. At the same time it pushes electrons into the far end of the
n-type crystal. The electrons already in the n-type crystal then are
forced to flow across the crystal junction, fall down in energy, emit
light, and end up back in the p-type crystal.

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 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 is another way to visualize LEDs. In a neon sign, the electrons
around each neon atom get pumped up in energy as they are 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 is so large that we can build in different characteristics, and
change the color of its flourescence.

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,