Studies of prototype DEPFET sensors for the wide field imager of Athena

The Wide Field Imager of the Athena telescope will combine an excellent spectroscopic performance and high count rate capability with a large field of view. For these purposes, its focal plane consists of two complementary detectors, using DEPFET active pixel sensors. One is the high count rate detector with a small field of view, which has to be operated with a readout speed of 80 mu s per frame. In contrast, the large area detector will cover a large field of view and has to be read out with a frame rate <= 5 ms. Its sensitive area is covered by four identical active pixel arrays, consisting of 512 x 512 pixels, each. Since a column parallel readout will be used, 512 pixels are connected to one single channel of a readout ASIC. The readout will be accomplished by either sensing a voltage step on the source node or a change of the transistor drain current. The former so-called source follower mode requires long settling times - proportional to the load capacitances - but can cope with local inhomogeneities. Alternatively, the latter so-called drain current mode provides a fast readout - independent to the load capacitance - but implicates a higher sensitivity on local variations of the DEPFETs bias currents. Both modes are implemented in the VERITAS 2.1 readout ASIC and were studied with 64 x 64 pixels arrays. Drain current devices could be operated with significantly smaller settling times but suffer from a slightly increased noise at similar shaping times in comparison to the source follower ones. By using an optimized timing with dedicated settling and shaping times, the devices of both modes feature a comparable spectral performance.