Defects in the semiconductor bulk material are of minor concern for the operation of MOS devices, but are mentioned here for the sake of completeness. As the name suggests, bulk defects are located within the crystalline semiconductor material. They are in part intentionally introduced during device manufacturing in the form of dopands, to control the type, the conductivity and other parameters of a layer or volume. Apart from the introduction of dopands, deep level impurities are unintentionally introduced due to contamination. As bulk defects mainly affect the lifetime of minorities, they play a secondary role in inversion layer devices. However, in devices where a certain minority lifetime is required for proper operation, e.g. CMOS optical sensors, random access memory (RAM) or photodiodes, bulk defects are of significant importance. For the characterization of bulk defects electrical methods, e.g. DLTS, and various physical or chemical methods can be used to quantify their presence and concentration. This kind of defects will not be considered more closely throughout this thesis, as the main focus is on the reliability of MOS devices.
