Researchers have demonstrated how noise in a microwave amplifier is limited by self-heating at very low temperatures (J Schleeh et al, ‘Phonon black-body radiation limit for heat dissipation in electronics’, Nature Materials, 10 November: doi:10.1038/nmat4126). The research has been performed as part of a collaboration between Chalmers University of Technology in Sweden and the California Institute of Technology, together with co-authors from the University of Salamanca and Chalmers spin-off Low Noise Factory.
Picture: Cross-sectional image of ultra-low-noise transistor. Electrons, accelerated in the high-mobility channel under the 100nm gate, collide and dissipate heat that fundamentally limits the noise performance of the transistor. Illustration: Lisa Kinnerud and Moa Carlsson, Krantz NanoArt.
The findings could be important for future discoveries in areas of science such as quantum computers and radio astronomy, it is reckoned.
Many significant discoveries in physics and astronomy are dependent on registering a barely detectable electrical signal in the microwave regime (e.g. the discovery of cosmic background radiation that helped confirm the Big Bang theory, or the detection of data from scientific instruments in space missions en route to distant planets, asteroids or comets).
Faint microwave signals are detected by transistor-based low-noise amplifiers. Researchers at Chalmers University of Technology have now optimized indium phosphide (InP) transistors using a special process for this purpose. Low Noise Factory designs and packages the amplifier circuits.
“Cooling the amplifier modules to -260°C enables them to operate with the highest signal-to-noise ratio possible today,” says Jan Grahn, professor of microwave technology at Chalmers. “These advanced cryogenic amplifiers are of tremendous significance for signal detection in many areas of science.”
Picture: Electron microscope image of InP HEMT. The region affected by the self-heating process is highlighted in the cross section.
Using a combination of measurements and simulations, the researchers investigated what happens when a microwave transistor is cooled to one tenth of a degree above absolute zero (-273°C). It was thought that noise in the transistor was limited by hot electrons at such extreme temperatures. However, the new study shows that the noise is actually limited by self-heating in the transistor.
The self-heating is associated with radiation from phonons (quantum particles that describe the thermal conductivity of a material) at very low temperatures. The results of the study are based on experimental noise measurements and simulations of phonons and electrons in the transistor at low temperatures.
“The study is important for the fundamental understanding of how a transistor operates close to absolute zero temperature, and also how we should design even more sensitive low-noise amplifiers for future detectors in physics and astronomy,” says Grahn.
The study was conducted at the Gigahertz Centre, a joint venture between Chalmers, research institutes, company partners, and the Swedish Governmental Agency for Innovation Systems (Vinnova).
