Near Quantum Limited Nb-Al-AlO x -Nb Mixers on 9 μm Thick Silicon Substrates around 350 GHz

We present design, performance measurements and simulations of Nb-Al-AlO _x -Nb superconductor-insulator-superconductor (SIS) mixers on silicon membranes mounted into waveguide mixerblocks with beam leads. The mixers are designed for use in astronomy for radio frequencies (RF) between 290 and 390 GHz and a 4-12 GHz intermediate frequency (IF) output band. Different mixer designs are discussed. Each design includes a pair of SIS junctions, connected in series or in parallel. Measurements confirm that silicon membrane substrates with beam lead contact technology support excellent cooling of the devices and show noise temperatures that can be limited to values between once and twice the quantum limit in the RF bandwidth. Measurement results are compared to simulations based on the Quantum Theory of Mixing (QTM). For the parallel junctions we use an adaption of the QTM to calculate the single-junction DC IV-characteristics separately. The resulting simulated DC IV-characteristics are in excellent agreement compared to 3D electromagnetic (EM) simulations and measurements.