Impact of plasma effects on the performance of silicon sensors at an X-ray FEL

The impact of electron hole plasmas on silicon sensor performance was studied with a multi-channel Transient Current Technique (mTCT) setup. Electron hole densities of up to 1016 cm-3 (equivalent to 105 focused 12 keV photons) were created with sub-ns lasers (660 and 1015 nm) and the time resolved current pulses of segmented sensors (4 channels simultaneously) were read out by a 2.5 GHz oscilloscope. Measurements for strip sensors of 280μm thickness and 80μm pitch as well as 450μm thickness and 50μm pitch were carried out showing an increase of the charge collection time and an increase of the charge spread (charge cloud explosion) with increased charge carrier density. These effects were studied as a function of the applied bias voltage and electron hole density. It was shown that for the current AGIPD sensor design plasma effects in p-in-n sensors of 450μm thickness are negligible if at least 500 V bias is applied.