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Abstract

The Wide Field Imager (WFI), one of two complementary instruments on board ESA's next large X-ray mission Athena, combines state-of-the-art resolution spectroscopy with a large field of view and high count rate capability. Centerpiece of the WFI instrument are four DEPFET (Depleted p-channel field effect transistor) sensors with a size of 512_512 pixels each and one fast detector with a size of 64_64 pixels. They are planned to be operated in drain current readout mode, which enables fast readout rates but is sensitive to inhomogeneities of the drain currents. These inhomogeneities arise from the sheer size of the DEPFET sensor matrix and are originated in the spatial distribution of wafer properties and process parameters. We characterized the drain current distribution of a prototype device (64_64 pixels) utilizing the same layout and technology as specified for the pre-flight production of Athena's WFI DEPFET detectors. In order to better understand the origin of the current spread we measured I-V characteristics of all pixels and extracted threshold voltages and transconductance values of the detector in operational conditions. This is enabled by features of the VERITAS read-out ASIC.