Superconducting-insulating-superconducting mixers incorporating a novel design were fabricated and evaluated for heterodyne detection in the frequency range of 600 to 720 GHz (ALMA Band 9). The improved design consists in tapering the corners of the RF transformer and a careful optimization of the dimensions to obtain a flat response over the full band. We demonstrate that this new design together with state-of- the-art technology lead to an improved noise temperature of T_N=100K. Since the working frequency of our devices crosses the superconducting gap frequency, we had to include the Mattis-Bardeen theory to simulate the response of this particular design. The results of the simulation are in excellent agreement with the experimental data.