Realization of gigahertz-frequency impedance matching circuits for nano-scale devices
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(a) Schematic of the circuit (dashed box) and setup: The BJ is connected to a transmission line of length d, which is shunted by a line of length l, with parasitic inductances L from the T-junction and capacitances C1 and C2 at the ends of the transmission line. A bias-tee allows DC biasing of the junction using a SMU. A directional coupler allows the measurement of the reflection coefficient using a VNA. (b) Optical microscope image of the described device. The input port (left) splits into the upper transmission line to which the break junction (black rectangle) is connected and the open stub (lower line). Airbridges are used to connect the groundplanes at the T-junction. (c) Image of the break junction showing the end of the transmission line, the contact pads (gold rectangles), and the break junction (gold-red line). (d) SEM image of the break junction. The wire is 70 nm wide.
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(a) Reflectance spectra |S 11| taken at different resistances of the break junction, the spectra are offset by 1.5 dB each for better visibility. The fit extracting the electrical parameters of the circuit is shown by the solid line, the fits for determining the RF impedance of the break junction are shown as dashed lines. (b) The measured reflection coefficient |S 11| (blue dots) plotted vs. the measured DC resistance at ν r = 6.085 GHz. The dotted line represents the reflection coefficient calculated using the extracted parameters. (c) Extracted RF impedance ZBJ plotted vs. measured DC resistance RBJ , the dashed line indicates .
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