Performing reliability stress tests on Metal-Oxide-Semiconductor Field Effect Transistors (MOSFET) helps to determine and fulfill requested data-sheet conditions in the semiconductor industry. By heating the transistor and only biasing its gate (Bias Temperature Instability – BTI) the device parameters, e.g. the threshold voltage, degrade which finally limits the operating lifetime of the transistor. Not stressing or operating the transistor restores its original properties to a certain extent.
To determine how stress and recovery are linked, various stress tests with varying stress time, oxide electric field and oxide thickness are necessary. The figure illustrates the differences in the recovery behavior depending on the sort of stress. In contrast to Negative Bias Temperature Instability (NBTI) where the relaxation of the threshold voltage after stress is a straight line on a logarithmic timescale in most cases, PBTI stress reveals a kink or an S-shape of the recovery. Notable relaxation after PBTI stress instead appears to happen later but faster, compared to NBTI. This asymmetric behavior is more pronounced at harsher stress conditions. The long time recovery slope over the initial time recovery slope (b_l/b_i) increases with increasing stress time and the oxide electric field ranging from NBTI with e.g. -8 MV/cm up to PBTI with e.g. +8 MV/cm. A feasible explanation therefore would be a non homogeneous weight of the dispersion of the emission times of certain defects. Deeper analysis of the relaxation slope reveals steps (discrete emission times), that confirming our assumption and provides further opportunity to investigate the relation of stress and recovery of BTI in more detail.