The bias temperature instability (BTI) is a serious reliability concern in metal-oxide-semiconductor field-effect transistors (MOSFETs). It is observed when a large voltage is applied to the gate contact of the MOSFET while all other terminals are grounded. The effect is considerably stronger when a negative bias voltage is applied to a p-channel MOSFET, which is referred to negative BTI (NBTI). Its counterpart for positive bias in an n-channel transistor is referred to as PBTI. It is suspected that both effects arise from a similar fundamental origin. Even though BTI was described for the first time over half a century ago, its underlying cause is still disputed. The effect is suspected to be caused by point-defects in the oxide which are able to capture and emit charges under operating conditions. The present work focuses on NBTI in silicon (Si) based devices using silicon dioxide (SiO2) as gate oxide. Due to the continuous downsizing, devices have reached dimensions where they merely contain very few defects each, which, during the last years, has allowed to study the electric response of single defects in experiments. Research indicates that NBTI consists of two components dominantly contributing to the device degradation. In measurements, the signal often does not recover fully to the initial “unstressed” level. One, therefore, distinguishes between the recoverable component (RC) and a more or less permanent component (PC). Theoretical models have been previously developed for both components. These models are presented in this work and are subsequently used as a basis for the investigations performed in this thesis. The parameters of the models can be determined by measurements, but the same parameters can also be theoretically calculated for several defect candidates. For the theoretical calculations, density functional theory (DFT) is used. A comparison of the obtained data is used to judge whether a suggested defect candidate is suitable to explain NBTI. Because of the gate oxide not being crystalline but rather amorphous, an additional layer of complexity is added. This results in a broad distribution of parameters, as seen in the measurements. In order to obtain theoretical defect parameter distributions to compare with, a large number of DFT calculations has to be performed in different amorphous structures, consuming considerable computational resources. In the present work the results of such calculations for the three most promising defect candidates the oxygen vacancy (OV), the hydrogen bridge (HB) and the hydroxyl-E0 (H-E0) center, are presented. The present work focuses on narrowing down the possible number of defect candidates for NBTI. It also addresses an additional feature observed in measurements, the so-called volatility (defects frequently disappearing and reappearing in the measurements). A possible explanation for this effect involves hydrogen relocating within the oxide, which has led to a more detailed investigation of hydrogen migration barriers in SiO2 in this work. Lastly, all the mentioned models rely on the assumption of potential energy surfaces (PESs), which describes the energy of a system of atoms in terms of their position. The PESs are usually assumed to be of a parabolic shape around the energy minimum. This assumption is subjected to a closer examination in this work, thereby also investigating possible explanations for double charge capture and emission processes as seen in experiments. The results of this work provide a better understanding of the parameters needed for the models describing the RC and PC of NBTI, also giving new insights into the possible link of the two models.