Microwave Design and on-Wafer Characterization of Cryogenic Quantum-Well Infrared Photodetectors

We demonstrate the first cryogenic heterodyne photodetectors that possess a 30 GHz bandwidth at the far-infrared <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10 \mu \mathrm{m}$</tex> (30 THz) and are embedded in an optical waveguide. The mixers are based on Quantum-well Infrared Photodetectors (QWIPs) with intrinsic quantum-limited conversion efficiency that are embedded in gallium arsenide (GaAs) waveguide. In particular, we report on microwave design and measurements of a QWIP that is grown on a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${6 2 5} \mu {m}$</tex> GaAs substrate. For the design, we have minimized the mesa layer area to the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$40 \times 20 \mu \mathrm{m}^{2}$</tex> active part of the QWIP, connected to the test GSG pads through wide airbridges - all to minimize the parasitic reactances of the device. To validate this design on-wafer at the nominal operating temperature of 40 K, we designed a custom multiline-thru-reflect-line (mTRL) calibration kit and performed two-tier S-parameter as well as source/receiver power calibrations. This allowed us to measure the roll-off in the QWIP responsivity and its input impedance that validated method-of-moments EM-simulations and were used to quantify its intrinsic bandwidth that exceeded 30 GHz.