Journal of Low Temperature Physics manuscript No. (will be inserted by the editor) N. Iyomoto, S.R. Bandler, R.P. Brekosky, A.-D. Brown, J.A. Chervenak, E. Figueroa-Feliciano, F.M. Finkbeiner, R.L. Kelley, C.A. Kilbourne, M.A. Lindeman, F.S. Porter,T. Saab, J.E. Sadleir, S.J. Smith Modeling of TES X-ray Microcalorimeters with a Novel Absorber Design Received July 23, 2007, Accepted September 15, 2007 Keywords Microcalorimeters, Transition Edge Sensors, X-ray spectroscopy Abstract We have successfully modeled our new TES (transition-edge sensor) x-ray microcalorimeters with a novel x-ray absorber design that is suitable for close-packed array with high quantum efficiency. We have determined device pa- rameters that reproduce complex impedance curves and noise spectra throughout the transition. Observed pulse height, decay time and baseline energy resolution were in good agreement with simulated results using the same parameters. PACS numbers: 29.30.Kv, 29.40.Vj 1 Introduction We have achieved breakthrough energy resolution ∆ E in TES (Transition-Edge Sensor) x-ray microcalorimeters with an absorber design suitable for dense arrays in which the absorbers contact the TES’s only at normal metal features on the TES’s 1,2 . We have successfully used solid Au and BiAu x-ray absorbers made in this new design. In Iyomoto et al. 2007 1 , we showed results of modeling one of the Au-absorber devices that achieved 2.4±0.2 eV resolution with MnK α lines. In this paper, we describe more details and also show modeling results of a BiAu- absorber device that achieved 1.8±0.2 eV resolution with MnK α lines 2 . We have modeled devices in which the absorbers contact the TES in a “stem” region that is shaped like the letter “T” 1,2 which we have currently selected as our standard design. 2 Complex impedance curves In order to determine the physical parameters of our TES sensors, we measured their complex impedance Z at several bias points within the transition, as shown Code 662, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA E-mail: Naoko.Iyomoto@nasa.gov