Trap sites in the gate dielectrics of Metal-Oxide-Semiconductor Field Effect Transistors (MOSFETs) play a crucial role in a number of effects not immediately related to each other. By assuming a suitable energetic and spatial distribution of trap sites, a model for excess noise in the drain current of MOSFETs was presented in 1957. More recently, oxide traps were found to be involved in reliability issues such as Negative Bias Temperature Instability (NBTI). While with large-area transistors, the drain current excess noise takes the ubiquitous 1/f spectral shape and NBTI degradation and -recovery proceed as continuous processes, small-area transistors containing just a handful of traps show discrete levels in their drain current. In steady-state, these levels and the abrupt transitions between them are commonly referred to as Random Telegraph Noise (RTN). With NBTI degradation and recovery, although the device is then in a pronounced non-equilibrium, similar transitions between discrete levels can be observed.
By carrying out a number of carefully designed stress/relaxation experiments, it is possible to extract the statistical parameters of the individual traps of the transistor, provided that the latter is small enough such that the drain current steps can be assigned unambiguously to the traps. This is aided by the non-uniform drain current distribution in the channel of a real MOSFET, causing individual traps to cause distinctive drain current steps. Binning of the emission times and step amplitudes into a two-dimensional histogram called a spectral map reveals marked clusters of events. Since the underlying physical process is poissonian, the temporal probability distribution of events is exponential, while the step height is distributed as a narrow gaussian. Unfortunately, trapping a carrier at one site perturbes the current distribution in the channel, modulating the characteristic step amplitudes of other traps. This effect can be observed in a number of measurements, and has to be taken into account when extracting the statistical parameters of the traps.
Varying the stress time shows that the clusters do not move within the spectral map, i.e. step amplitude and emission time are independent of stress time, ruling out, for example, diffusive processes. On the other hand, the amplitude of the probability distribution of a particular trap increases with stress time, consistent with the picture that for a trap to show an emission event it has to capture a carrier within the preceding stress phase, which becomes more probable when this stress phase is longer. From a row of experiments with different stress times and the resulting probability amplitudes, it is therefore possible to extract the capture time constants of the traps. Repeating this block of experiments at different temperatures and/or different stress voltages and/or different relaxation voltages, the dependencies of the emission- and capture times on these parameters can be investigated.