Electronic thermometer

By R. Parthasarathy
The Hindu
Friday, September 5, 2003

The primary thermometer presented here uses the physical
law of a tunnel junction, just as the constant-volume gas
thermometer is based on the ideal gas equation (PV=RT).

Temperature is a prime parameter to be measured in every
field of science and technology.

There is no simple solution for all applications of
thermometry from room temperature to the base temperature
of a refrigerator.

Classification

Thermometer which relates temperature to other measurable
quantities based on a known physical law is classified as
primary. The need for initial calibration and subsequent
drift in the sensor do not arise here.

All of the thermometers in commercial use are secondary;
they require calibration with reference to a primary
thermometer.

In a report published in Science 20 June 2003, Lafe
Spietz et. al. (Yale University, New Haven, USA) have
presented a new primary electronic thermometer which
relates temperature to voltage-dependent electrical noise
from a tunnel junction.

The form of this signal involves only the ratio of
electronic charge to Boltzmann constant, and the Fermi-
Dirac statistic governing electrons in a metal.

Primary thermometers

There are two types: Coulomb Blockade Thermometer (CBT)
and Shot Noise Thermometer (SNT). Both have a common
feature, namely they employ the phenomenon of electron
tunnelling in a junction.

The operation of the CBT utilises the non-linearity in
electron-electron interactions and requires high-
resistance ultra-small junction.

The latter limits the range of measurement to two orders
of magnitude in temperature and the maximum temperature
to about 20 k.

The operation of SNT is based on the physics of the shot
noise in large junctions which extends the temperature
range to over four decades from 50 mk to 25 k. The
accuracy achieved is 0.1 per cent as compared to 0.5 per
cent of CBD.

The proof-of-concept operation of this primary
thermometer and experimental results can be found in the
report .

It will find application in meteorology and in cryogenic
systems.
- R. Parthasarathy

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