As pointed out in Section 1.2, all tested variants of the RD model have been ruled out. Alternatively, the NBTI phenomenon may be ascribed to charge trapping into the oxide — a process that also involves tunneling of electrons and holes, respectively [57, 22]. However, the underlying microscopic mechanism has only been vaguely understood so far and the developed models are often oversimplified [70, 96, 97].
This chapter provides a brief overview of miscellaneous charge trapping mechanisms at a microscopic level. On the one hand, these mechanisms involve electrons and holes located in the inversion layer of a MOSFET. Therefore, it will be necessary to address the electrostatics of such devices considering quantum mechanical effects, such as the formation of subbands and the penetration of the charge carriers into the dielectric. On the other hand, there are traps, whose properties are highly sensitive to the exact defect configuration. As a consequence, the defects must be addressed using an atomistic theory, which considers the structural relaxation of the defects and the thermal lattice vibrations for instance. The above considerations will lead to three basic charge trapping processes, which will be tested as a possible explanation for NBTI in this thesis.
These processes allow for a vast number of different transitions from the conduction or the valence band into one trap.. In the second part of this chapter, all possible transitions will be incorporated in compact rate expressions, which can be used for device simulation.