Shrinking device sizes and the requirements to operate at higher temperatures in up-to-date applications of MOS transistors result in the accelerated degradation of crucial device parameters, such as the threshold voltage and the mobility. Of particular importance is the Negative Bias Temperature Instability (NBTI), which is observed when a negative bias is applied to the gate of pMOS transistors with the other contacts grounded. NBTI is considerably enhanced at higher temperatures but the degradation shows similar features also at room temperature.
The initial degradation is commonly interpreted assuming elastic hole trapping due to tunneling carrier exchange with the substrate. Long-term degradation, on the other hand, is often supposed to be due to the creation of interface states. From an experimental point, elastic tunneling is usually identified as a temperature-independent process following a logarithmic time dependence and experimental evidence for some thermally nitrided oxides has been given. In contrast, long-term degradation is frequently reported to follow a power-law with exponents in the order of 0.12 to 0.15.
Previous experiments using conventional parameter analyzers with a time resolution in milli-seconds have indicated that, at least for up to medium stresses, a logarithmic time dependence is observed during the first three decades (1ms up to 1s). This logarithmic short-term degradation shows a strong temperature activation similar to the long-term degradation (Ea = 0.1eV) and a super-linear stress field dependence (Eox²). For longer stress times and higher stress fields, degradation starts to deviate from the logarithmic behavior.
Studies using different ultra-fast measurement techniques operating in the micro-second range are performed to investigate short-term degradation. Thereby a strong field- and temperature-dependence of the initial degradation is observed, which is incompatible with the frequently assumed elastic hole trapping mechanism but favors a thermally activated hole trapping process.