Achievable noise performance of spectroscopic prototype DEPFET detectors

A new generation of spectroscopic X-ray DEPFET detectors has been produced in the course of the detector development for the Wide Field Imager (WFI) of Athena. These devices served to perform a detailed analysis of the noise composition, which was enabled by the development of appropriate test algorithms. A result of the analysis is the distinction of different components, which sum up to the total noise. In particular the contribution of shot noise, white noise and 1/f noise to the readout noise is determined as well as the signal noise caused by the generation of charge carriers. The resulting parametrization enables the adaptation and optimization of operation modes to given purposes. The studied prototype detectors included 64x64 pixels with a linear gate design and provided an excellent noise, below 1.6 e(-)ENC at a readout time faster than 10 mu s/pixel and a temperature of -80 degrees C in rolling-shutter operation. This performance is enabled by an extended signal integration time. A further noise reduction is prevented by the signal noise, caused by charge carrier generation. In order to demonstrate the low noise properties of the DEPFET transistor, the measurement conditions were adapted and a noise of 0.95 e(-)ENC was measured at the expense of the sensor size - by operating only parts of the sensor in window mode - and dynamic range - by using the Al K alpha line at about 1.49 keV for calibration.