  Transition Edge Sensors are ultra-sensitive superconducting detectors with
applications in many areas of research, including astrophysics. The device
consists of a superconducting thin film, often with additional normal metal
features, held close to its transition temperature and connected to two
superconducting leads of a higher transition temperature. There is currently no
way to reliably assess the performance of a particular device geometry or
material composition without making and testing the device. We have developed a
proximity effect model based on the Usadel equations to predict the effects of
device geometry and material composition on sensor performance. The model is
successful in reproducing I-V curves for two devices currently under study. We
use the model to suggest the optimal size and geometry for TESs, considering
how small the devices can be made before their performance is compromised. In
the future, device modelling prior to manufacture will reduce the need for
time-consuming and expensive testing.
