Irradiation studies of DEPFET-like devices with X-rays

The upcoming upgrade of the Belle-Experiment at KEK will impose new challenges in radiation hardness for the utilized DEPFET-devices (Depleted p-channel Field Effect Transistor). The upgrade in Belle II will result in an increased luminosity and therefore in a significantly higher radiation dose up to 1 Mrad (10 kGy) per year which the DEPFET-devices need to withstand. Radiation damage through ionizing and non-ionizing radiation is possible. In the case of ionizing radiation positive charge carriers are created that will be collected at the interface between silicon and silicon dioxide. The creation of these charge carriers will result in a threshold voltage shift of the deployed transistors. In order for the detector to remain functional, the operating voltage needs to be adjusted depending on the threshold voltage shift. Therefore a detailed characterization of the voltage shift due to the radiation damage of ionizing radiation is crucial for the correct predicament of the detector operation. The test devices employed for the irradiation studies simulated the behavior at the Si-SiO2 interface of the DEPFET. The irradiation was executed at the X-ray facility at KIT (Karlsruhe Institute of Technology) with maximum photon energy of 60 keV.