Drain current characteristics of Athena WFI flight-like DEPFETs

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 is the large detector assembly consisting of four DEPFET (Depleted p-channel field effect transistor) sensors with a size of 512x512 pixels each, and one fast detector with a size of 64x64 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 spacial distribution of wafer properties and process parameters. We characterized the drain current distribution of a large detector lab module (one quadrant of the large detector assembly, 512x512 pixels) out of 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 the channel length modulation parameters, threshold voltages and transconductance values of the external (g m) and internal (g q) gate of the individual pixels of the detector in operational conditions. This is enabled by features of the VERITAS readout ASIC.