[time-nuts] quartz / liquid nitrogen

[Dana Whitlow]

Many years ago, circa 1977, I was moved to try some crude tests on a few
semiconductor devices at LN2 temperature (77K).

These tests were very crude, involving dunking the parts into the LN2 bath,
and
many failed outright.  Most of the devices tested were in plastic packages.

Here are the results as I remember them, applicable only for the survivors:

Silicon bipolar transistors:   The DC beta fell to very low values.
Junction
forward voltages rose considerably.

Silicon JFETs:  Seemed to continue working reasonably well.

Silicon MOSFETs:  Same as JFETs

Red LEDs:   The junction forward voltages rose considerably, to about  5V as
I recall.   The light output per unit current rose truly spectacularly.

My first experiences with seriously-cryogenic RF amplifiers were at the
Arecibo Observatory beginning about 11 years ago.  These were all either
GaAs- or InP-based and we cooled them to ~15K, generally leading to
input-referred amplifier noise temperatures of ~3K.  Many of the devices
needed continuous exposure to light to work properly when cold, and the
metal block amplifier packages had holes in the lid directly over the active
device chips. Small red LEDs in ordinary plastic packages were inserted
in the holes and were driven at a few mA, generally in a series string.
Since cool-down was fairly gradual over a span of at least a couple hours,
there was little problem with thermal shock and almost all LEDs survived
cooldown and warmup for the several cycles they experienced during
my 10 years at the observatory.

RF amplifier biasing was invariably done with opamp circuits to maintain
set drain currents and drain voltages, with said bias control circuits
outside
the dewar at room ambient temperature.   Failures were not too uncommon,
largely attributed to connector misbehavior at low temperature.  Formation
of "ice" (really frozen air) inside the dewars was suspected because fine
wires
inside the dewar were often found to have fairly sharp bends at improbable
locations upon warmup for diagnostic purposes (or due to cooling system
failure).

Cooling was done with a closed-cycle gaseous He system, using the
Gifford-McMahon cycle.  Note that He does not liquefy (at reasonable
pressures) until around 4K.  All dewars for this kind of work depend on
high vacuum inside for thermal insulation, with black body radiation
and direct conduction through wires and mounting structures being
the principal remaining heat leaks.

At these temperatures, maintenance of high vacuum inside the dewar was
essentially automatic because all components of the inward-leaking air
were known to freeze out.  This could lead to a hazard because over time,
months or years, enough air could freeze out to result in dangerously high
internal pressures upon "thawing" when the dewar was warmed for any
reason.  For this reason, all dewars were equipped with blowout plugs
to avoid high pressure's damaging the dewars themselves.

Dana


On Tue, Apr 3, 2018 at 12:26 AM, Mark Sims <holrum at hotmail.com> wrote:

> And you want your semiconductors to be in ceramic/lided packages with the
> bond wires flapping in free air.   Bond wires embedded in epoxy like to
> break...  don't ask how I found this out  ;-)   ... it brings back bad
> memories... and makes bad memories...  Quantum chips have very
> elaborate/specialized bonding to survive liquid helium... even with that,
> thermal cycling still breaks them.
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