Page 53 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

34 Chapter FQUY

A Note on Carborundum
To the editors of Electrical World:
Sirs: During an investigation of the unsymmetrical passage of current through
a contact of curborundum and other substances a curious phenomenon was
noted. On applying u potential of 10 volts between two points on a crystal of
carborundum, the crystal gave out a yellowish light.

Mr. Round went on to note that some crystals gave out green, orange, or
blue light. This is quite possibly the first documented reference to the effect
upon which special components called light-emitting diodes (LEDs) are based.9
Sad to relate, no one seemed particularly interested in Mr. Round’s
discovery, and nothing really happened until 1922, when the same phenomenon
was observed by 0.V Losov in Leningrad. Losov took out four patents between
1927 and 1942, but he was killed during the Second World War and the details
of his work were never discovered.
In fact, it wasn’t until 195 1, following the discovery of the bipolar transistor,
that researchers really started to investigate this effect in earnest. They found
that by creating a semiconductor diode from a compound semiconductor
formed from two or more elements-such as gallium arsenide (GaAs)-light
is emitted from the PN junction, that is, the junction between the P-Type and
N-type doped materials.
As for a standard diode, a LED conducts electricity in only one direction
(and it emits light only when it’s conducting). Thus, the symbol for an LED is
similar to that for a normal diode, but with
two arrows to indicate light being emitted J Q
(Figure 4- 12). J
A LED formed from pure gallium arsenide
emits infrared light, which is useful for sensors, Figure 4-1 2. Symbol
for a LED
but which is invisible to the human eye. It was
discovered that adding aluminum to the semiconductor to give aluminum
gallium arsenide (AlGaAs) resulted in red light humans could see. Thus, after

9 In conversation, the term LED may be spelled out as “L-E-D’ or pronounced as a single word
to rhyme with “bed”.