Updated July 19,
to go to our main page on microwave diodes
What's so great about a Gunn
diode? They are used for cheap sources of microwave power. By the
way, when we say "source", it means "oscillator".
Just wanted to clear that up. John Battiscombe (J. B.) Gunn is in
our Microwave Hall of Fame, check
Another thing you should know:
a Gunn device is not actually a diode, even though everyone uses
this term! Why isn't it a "real" diode? Because it only
uses N-type semiconductor (actually a three-layer stack). Its more
accurate name is a transferred electron device.
But wait, here's a second opinion
Your statement "a Gunn
device is not actually a diode" isn't true. They ARE diodes,
because the word diode simply means it has two (active) electrodes.
It doesn't have to have a P-N junction. In fact the term diode
was coined for vacuum tube diodes (these actually may have three
or four connections, as they require a heater supply, which may
or may not form one of the active electrodes) which were made
before commercially produced P-N diodes. Semiconductor diodes
did exist before this but they were point contact diodes, but
weren't called this. They were of course crystals and cat's whiskers,
as used in crystal sets.
Good point John, we misspoke
in haste! Let's agree that a Gunn diode is like no other semiconductor
diode in that it doesn't use a PN junction... or a Schottky metal-semiconductor
contact! (Thanks, Chin-Leong!)
Gunn diodes have been around
since John Gunn discovered that bulk N-type GaAs can be made to
have a negative resistance effect. Gunn diodes have been a cheap
source of microwaves ever since! They are used in many commercial
applications for high frequency sources, including police radar,
and even K-mart door openers. Ever wonder why your radar detector
goes off when you pass a K-mart?
The I-V curves of a Gunn diode
will help explain the effect. For low voltages (up to 1 volt perhaps),
the Gunn diode behaves nearly as a linear resistor. Then at some
point the current stops increasing with increasing voltage. This
is known as the threshold voltage. Above this point the diode has
negative resistance (curve slopes downward), which mean that it
is just itching to oscillate! The operating point is usually about
4X the threshold voltage.
Below is a picture of a Gunn
diode oscillator for W-band. Note the WR-10 waveguide, and the cheap
heat sink. This bad boy must oscillate somewhere between 75 and
110 GHz, because that is the full extent of W-band. It is something
we found in a lab drawer, for all we know it is a blown device.
Nice use of a C-clamp to attach a heat sink!
How do you know if this Gunn
diode is OK, without a spectrum analyzer that goes to 110 GHz? Put
it onto a curve tracer! What's that?
negative resistance above 1 volt? Yes, this is a good device!