Investigations on radiation hardness of DEPFET sensors for the Belle II detector

In the upgrade of the Belle detector at KEK (Tsukuba, Japan) the two innermost layers of the vertex detector will be realized by a pixel detector (PXD) consisting of DEPFET (DEpleted P-channel Field Effect Transistor) matrices. As the position of the detector will be very close to the beam pipe, it will suffer from intense radiation levels. The main radiation background is the luminosity related 4-fermion final state radiation, which damages the silicon bulk material and the silicon dioxide from the gate contacts. With the dose expected at Belle II, the DEPFET suffers mainly from additional leakage current and increase in noise. In addition, defects in the silicon dioxide change transistor parameters, e.g. the threshold voltage. We will show results on the hardness factor of electrons after a 10MeV electron irradiation which was performed in the dose and energy range relevant for the PXD. In addition, we present X-ray irradiations of DEPFET equivalent test structures and compare radiation hardness for different oxide parameters in the prototype production.