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1. Experimental Characterization of Temperature-Dependent Microwave Noise of Discrete HEMTs: Drain Noise and Real-Space Transfer

   https://doi.org/10.1109/IMS37962.2022.9865505  

   Gabritchidze, Bekari; Cleary, Kieran; Kooi, Jacob; Esho, Iretomiwa; Readhead, Anthony C.; Minnich, Austin J. (June 2022, 2022 IEEE/MTT-S International Microwave Symposium - IMS 2022)

   We report wafer characterization of the S-parameters and microwave noise temperature of discrete GaAs and GaN HEMTs over a temperature range of 20 - 300 K. The measured noise temperature (T50) exhibits a dependence on physical temperature that is inconsistent with a constant drain temperature, with Td for the GaAs and GaN devices changing from ~ 2000 K and ~2800 K at room temperature to ~ 700 K and ~ 1800 K at cryogenic temperatures, respectively. The observed temperature dependence is qualitatively consistent with that predicted from a theory of drain noise based on real-space transfer of electrons from the channel to the barrier. 

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2. Characterization of GaN-on-SiC Wafers using a Multi-Method Laser-Based Pump-Probe Technique

   https://doi.org/10.1109/ITherm55368.2023.10177642  

   Song, Yiwen; Shoemaker, Daniel; Kang, Kyuhwe; Schuette, Michael; Tweedie, James S.; Sheppard, Scott T.; Choi, Sukwon (May 2023, 2023 22nd IEEE Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm))

   Gallium nitride (GaN) high electron mobility transistors (HEMTs) are key components of modern radio frequency (RF) power amplifiers. However, device self-heating negatively impacts both the performance and reliability of GaN HEMTs. Accordingly, laser-based pump-probe methods have been used to characterize the thermal resistance network of epitaxial material stacks that are used to fabricate HEMT structures. However, validation studies of these measurement results at the device level are lacking. In the present work, a GaN-on-SiC wafer was characterized using frequency-domain thermoreflectance and steady-state thermoreflectance techniques. The thermal conductivity of the GaN channel/buffer layer, SiC substrate, and the interfacial thermal boundary resistance at the GaN/SiC interface were determined. Results were validated by performing thermal imaging and modeling of a transmission line measurement (TLM) structure fabricated on the GaN-on-SiC wafer. 

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3. A cryogenic torsion balance using a liquid-cryogen free, ultra-low vibration cryostat

   https://doi.org/10.1063/5.0089933  

   Fleischer, S. M.; Ross, M. P.; Venkateswara, K.; Hagedorn, C. A.; Shaw, E. A.; Swanson, E.; Heckel, B. R.; Gundlach, J. H. (June 2022, Review of Scientific Instruments)

   We describe a liquid-cryogen free cryostat with ultra-low vibration levels, which allows for continuous operation of a torsion balance at cryogenic temperatures. The apparatus uses a commercially available two-stage pulse-tube cooler and passive vibration isolation. The torsion balance exhibits torque noise levels lower than room temperature thermal noise by a factor of about four in the frequency range of 3–10 mHz, limited by residual seismic motion and by radiative heating of the pendulum body. In addition to lowering thermal noise below room-temperature limits, the low-temperature environment enables novel torsion balance experiments. Currently, the maximum duration of a continuous measurement run is limited by accumulation of cryogenic surface contamination on the optical elements inside the cryostat. 

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4. Dispersive readout of a high-Q encapsulated micromechanical resonator

   https://doi.org/10.1063/5.0101402  

   Bousse, Nicholas E.; Kuenstner, Stephen E.; Miller, James M. L.; Kwon, Hyun-Keun; Vukasin, Gabrielle D.; Teufel, John D.; Kenny, Thomas W. (August 2022, Applied Physics Letters)

   Encapsulated bulk mode microresonators in the megahertz range are used in commercial timekeeping and sensing applications, but their performance is limited by the current state of the art of readout methods. We demonstrate a readout using dispersive coupling between a high-Q encapsulated bulk mode micromechanical resonator and a lumped element microwave resonator that is implemented with commercially available components and standard printed circuit board fabrication methods and operates at room temperature and pressure. A frequency domain measurement of the microwave readout system yields a displacement resolution of 522 fm/Hz, which demonstrates an improvement over the state of the art of displacement measurement in bulk-mode encapsulated microresonators. This approach can readily be implemented in cryogenic measurements, allowing for future work characterizing the thermomechanical noise of encapsulated bulk mode resonators at cryogenic temperatures. 

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5. Thermal testing for cryogenic CMB instrument optical design

   https://doi.org/10.1117/12.2629490  

   Goldfinger, David C.; Ade, Peter A.; Ahmed, Zeeshan; Amiri, Mandana; Barkats, Denis; Basu Thakur, Ritoban; Beck, Dominic; Bischoff, Colin A.; Bock, James J.; Buza, Victor; et al (August 2022, Proceedings of the SPIE)

   Zmuidzinas, Jonas; Gao, Jian-Rong (Ed.)

   Observations of the Cosmic Microwave Background rely on cryogenic instrumentation with cold detectors, readout, and optics providing the low noise performance and instrumental stability required to make more sensitive measurements. It is therefore critical to optimize all aspects of the cryogenic design to achieve the necessary performance, with low temperature components and acceptable system cooling requirements. In particular, we will focus on our use of thermal filters and cold optics, which reduce the thermal load passed along to the cryogenic stages. To test their performance, we have made a series of in situ measurements while integrating the third receiver for the BICEP Array telescope. In addition to characterizing the behavior of this receiver, these measurements continue to refine the models that are being used to inform design choices being made for future instruments. 

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