What is a Diode?
A diode is the simplest sort
of semiconductor
device. Broadly speaking, a semiconductor is a material with a
varying ability to conduct electrical current. Most
semiconductors are made of a poor conductor that has had
impurities (atoms
of another material) added to it. The process of adding
impurities is called doping.
In the case of LEDs, the conductor material is typically
aluminum-gallium-arsenide (AlGaAs). In pure
aluminum-gallium-arsenide, all of the atoms bond perfectly to
their neighbors, leaving no free electrons
(negatively-charged particles) to conduct electric current. In
doped material, additional atoms change the balance, either
adding free electrons or creating holes where electrons
can go. Either of these additions make the material more
conductive.
A semiconductor with extra electrons is called N-type
material, since it has extra negatively-charged
particles. In N-type material, free electrons move from a
negatively-charged area to a positively charged area.
A semiconductor with extra holes is called P-type
material, since it effectively has extra
positively-charged particles. Electrons can jump from
hole to hole, moving from a negatively-charged area to a
positively-charged area. As a result, the holes themselves
appear to move from a positively-charged area to a
negatively-charged area.
A diode comprises a section of N-type material bonded to a
section of P-type material, with electrodes on each end. This
arrangement conducts electricity in only one direction. When
no voltage is applied to the diode, electrons from the N-type
material fill holes from the P-type material along the
junction between the layers, forming a depletion
zone. In a depletion zone, the semiconductor material is
returned to its original insulating state -- all of the
holes are filled, so there are no free electrons or empty
spaces for electrons, and charge can't flow.
At the junction, free electrons from the
N-type material fill holes from the P-type material.
This creates an insulating layer in the middle of the
diode called the depletion
zone.
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To get rid of the depletion zone, you have to get electrons
moving from the N-type area to the P-type area and holes
moving in the reverse direction. To do this, you connect the
N-type side of the diode to the negative end of a circuit and
the P-type side to the positive end. The free electrons in the
N-type material are repelled by the negative electrode and
drawn to the positive electrode. The holes in the P-type
material move the other way. When the voltage difference
between the electrodes is high enough, the electrons in the
depletion zone are boosted out of their holes and begin moving
freely again. The depletion zone disappears, and charge moves
across the diode.
When the negative end of the circuit is
hooked up to the N-type layer and the positive end is
hooked up to P-type layer, electrons and holes start
moving and the depletion zone
disappears.
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If you try to run current the other way, with the P-type
side connected to the negative end of the circuit and the
N-type side connected to the positive end, current will not
flow. The negative electrons in the N-type material are
attracted to the positive electrode. The positive holes in the
P-type material are attracted to the negative electrode. No
current flows across the junction because the holes and the
electrons are each moving in the wrong direction. The
depletion zone increases. (See How
Semiconductors Work for more information on the entire
process.)
When the positive end of the circuit is
hooked up to the N-type layer and the negative end is
hooked up to the P-type layer, free electrons collect on
one end of the diode and holes collect on the other. The
depletion zone gets
bigger.
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The interaction between electrons and holes in this setup
has an interesting side effect -- it generates light!
In the next section, we'll find out exactly why this is.
