The Gunn
diode is a
special diode used in
microwave oscillators and
detectors. They are made on a
gallium arsenide substrate.
Then an epitaxial layer of n-
doped gallium arsenide is
applied. Epitaxial means that the
crystaline structure of this
layer has the same
orientation as the
substrates.
The
substrate is placed against a
heatsink. This is also the
anode of the
diode. The
cathode is
connected to the epitaxial
layer.
When th
voltage over the
diode becomes so
high that
the
electric field in the epitaxial
layer rises above about 350V/mm2,
an
area with
high field strength is
formed. This
field moves towards the
cathode. The time it takes to reach
the
cathode dictates the
frequency of the
microwave pulses.
By using the gunn diode in a
resonating circuit you get a
sinusoidal signal.
One of the most
common uses for the gunn
diode is in
automatic
door openers at
shoping centers. Here there is used two gunn diodes
one to
transmit and one to
recive. By measuring the variations
in the
reflected microwave signal, the
device can 'see' if
someone is approaching.
This is similar to a
radar, but with much lower
effects(about 10 mW).