Optimal choice of material for HEB superconducting mixers

We demonstrate that a potential distinction in ultimate performance of phon on-cooled and diffusion-cooled HEB mixers is not due to the cooling mechani sms but rather due to the different properties of available superconductors . The only available material for a phonon-cooled mixer with sufficiently l arge IF bandwidth (similar to 4 GHz) is NbN, whereas a variety of clean mat erials (e.g., Nb, NbC, Al) are suitable for a diffusion-cooled mixer. For a readily achievable device length of 0.1 mu m for example, the diffusion-co oled IF bandwidth can be greater than or equal to 10 GHz. The requirement o f low local oscillator (LO) power can also be more easily met in diffusion- cooled devices by selection of a material with lower critical temperature a nd low density of electron states. In contrast, the parameters in the NbN-b ased mixer cannot be widely varied because of the high resistivity and high transition temperature of the material and the necessity of using ultrathi n films. Given the limited availability of LO power from compact solid-stat e sources at frequencies above 1 THz, a diffusion-cooled mixer based on alu minum is a very attractive choice for low-background radioastronomy applica tions.