Thermal properties of charge noise sources

Measurements of the temperature and bias dependence of Single Electron Transistors (SETs) in a dilution refrigerator show that charge noise increases linearly with refrigerator temperature above a voltage-dependent threshold temperature, and that its low temperature saturation is due to SET self-heating. We show further that the two-level fluctuators responsible for charge noise are in strong thermal contact with the electrons in the SET, which can be at a much higher temperature than the substrate. We suggest that the noise is caused by electrons tunneling between the SET metal and nearby potential wells

Influence of Spacer Thickness on the Noise Performance in InP HEMTs for Cryogenic LNAs

InP high electron mobility transistors (InP HEMTs) with different spacer thickness 1 to 7 nm in the InAlAs-InGaAs heterostructure have been fabricated and characterized at 5 K with respect to electrical dc and rf properties. The InP HEMT noise performance was extracted from gain and noise measurements of a hybrid low-noise amplifier (LNA) at 5 K equipped with discrete transistors. When biased for optimal noise operation, the LNA using 5 nm spacer thickness InP HEMTs achieved the lowest average noise temperature of 1.4 K at 4-8 GHz. The InP HEMT channel noise was estimated from the drain noise temperature which confirmed the minimum in noise temperature for the 5 nm spacer thickness InP HEMT. It is suggested that the spacer thickness acts to control the degree of real-space transfer of electrons from the channel to the barrier responsible for the observed noise variation in the cryogenic InP HEMTs

On the relation between rf noise and subthreshold swing in InP HEMTs for cryogenic LNAs

4 - 8 GHz low-noise amplifiers (LNAs) based on InP high electron mobility transistors (InP HEMTs) with different spacer thickness in the InAlAs-InGaAs heterostructure were fabricated and characterized at 5 K. A variation in the lowest average noise temperature of the LNA was observed with spacer thickness. We here report that the subthreshold swing (SS) at 5 K for the HEMT exhibited similar dependence with spacer thickness as the lowest average noise temperature of the LNA. This suggests that low-temperature characterization of SS for the HEMT can be used as a rapid assessment of anticipated noise performance in the cryogenic HEMT LNA

On the Angular Dependence of InP High Electron Mobility Transistors for Cryogenic Low Noise Amplifiers in a Magnetic Field

The InGaAs-InAlAs-InP high electron mobility transistor (InP HEMT) is the preferred active device used in a cryogenic low noise amplifier (LNA) for sensitive detection of microwave signals. We observed that an InP HEMT 0.3-14GHz LNA at 2K, where the in-going transistors were oriented perpendicular to a magnetic field, heavily degraded in gain and average noise temperature already up to 1.5T. Dc measurements for InP HEMTs at 2K revealed a strong reduction in the transistor output current as a function of static magnetic field up to 14T. In contrast, the current reduction was insignificant when the InP HEMT was oriented parallel to the magnetic field. Given the transistor layout with large gate width/gate length ratio, the results suggest a strong geometrical magnetoresistance effect occurring in the InP HEMT. This was confirmed in the angular dependence of the transistor output current with respect to the magnetic field. Key device parameters such as transconductance and on-resistance were significantly affected at small angles and magnetic fields. The strong angular dependence of the InP HEMT output current in a magnetic field has important implications for the alignment of cryogenic LNAs in microwave detection experiments involving magnetic fields

On the relation between rf noise and subthreshold swing in InP HEMTs for cryogenic LNAs

4 - 8 GHz low-noise amplifiers (LNAs) based on InP high electron mobility transistors (InP HEMTs) with different spacer thickness in the InAlAs-InGaAs heterostructure were fabricated and characterized at 5 K. A variation in the lowest average noise temperature of the LNA was observed with spacer thickness. We here report that the subthreshold swing (SS) at 5 K for the HEMT exhibited similar dependence with spacer thickness as the lowest average noise temperature of the LNA. This suggests that low-temperature characterization of SS for the HEMT can be used as a rapid assessment of anticipated noise performance in the cryogenic HEMT LNA

Nonequilibrium probing of two-level charge fluctuators using the step response of a single-electron transistor

We report a new method to study two-level fluctuators (TLFs) by measuring the offset charge induced after applying a sudden step voltage to the gate electrode of a single-electron transistor. The offset charge is measured for more than 20 h for samples made on three different substrates. We find that the offset charge drift follows a logarithmic increase over 4 orders of magnitude in time and that the logarithmic slope increases linearly with the step voltage. The charge drift is independent of temperature, ruling out thermally activated TLFs and demonstrating that the charge fluctuations involve tunneling. These observations are in agreement with expectations for an ensemble of TLFs driven out of equilibrium. From our model, we extract the density of TLFs assuming either a volume density or a surface density

Angular Dependence of InP High Electron Mobility Transistors for Cryogenic Low Noise Amplifiers under a magnetic field

This work addresses the angular dependence of DC properties in 100nm InP HEMT devices under the influence of applied static magnetic field at 2 K. When kept at an angle 90o towards a magnetic field of 14 T, the maximum output drain current Ids was reduced more than 99 %. A rotation sweep of the transistor revealed a strong angular and B-field dependence on Ids. This was correlated with a reduction in dc transconductance and increase in on-resistance of the transistor. The RF properties of the transistor were tested by measuring an 0.3-14 GHz InP HEMT MMIC low-noise amplifier (LNA) at 2 K kept at an angle 90o towards a magnetic field up to 10 T. The gain and noise temperature were strongly decreased and increased, respectively, already below 1 T. The results show that precise alignment of the cryogenic InP HEMT LNA is crucial in a magnetic field. Even a slight mis-orientation of a few degrees leads to a strong degradation of the gain and noise temperature

Reduction of Noise Temperature in Cryogenic InP HEMT Low Noise Amplifiers with Increased Spacer Thickness in InAlAs-InGaAs-InP Heterostructures

The impact of InP HEMT spacer thickness on cryogenic performance in low noise amplifiers (LNAs) has been investigated. 100 nm gate-length InP HEMTs based on InAlAs-InGaAs-InP heterostructures with different spacer thickness (1 nm, 3 nm and 5 nm) were fabricated. The Hall measurements, simulated band structures and dc characteristics of InP HEMTs were compared for all the three different epitaxial structures at 5 K. The noise performance of the InP HEMT was studied using a three-stage 4–8 GHz hybrid LNA at 5 K. All LNAs yielded an average gain above 30 dB across the whole band. When biased for optimal low noise operation, the LNA with 5 nm spacer thickness InP HEMTs achieved an average noise temperature of 1.3 K. The LNAs with spacer thickness of 1 nm and 3 nm InP HEMTs exhibited a higher average noise temperature of 1.9 K and 1.7 K, respectively. The reduction in LNA noise temperature with increased spacer thickness was observed to correlate with a strongly enhanced electron mobility in the InP HEMT structure at 5 K