Next: 1.3.2 HCD in High-Voltage Up: 1.3 Characteristic Features of HCD Previous: 1.3 Characteristic Features of HCD
In reality, however, even ultra-scaled modern MOSFETs can show severe HCD [30,31,41]. This was first demonstrated for gate lengths less than than 0.2um and supply voltages below 1.0V by Mizuno et al. [42]. The authors related this finding to an energy exchange mechanism populating the "hot" fraction of carrier ensemble. Here "hot" means that these carriers have energies substantially higher than the lattice temperature. Possible mechanisms responsible for such an energy gain include impact ionization [43], Auger recombination [44], electron-phonon [45], and electron-electron scattering [46,17,18].
Note that electron-electron scattering is of particular importance for nano-scale devices [30,41]. Particularly for these devices the situation is even more complicated because the dominant mechanism for Si-H bond-breakage changes from a single-carrier to a multiple-carrier mechanism [24,40,30,41]. For example, in a long-channel or high-voltage device carriers striking on the interface are already rather hot and are able to trigger silicon-hydrogen bond rupture by a single collision, which is referred to as the single-particle mechanism. In contrast, such extremely hot carriers do not exist in a sufficient quantity for scaled devices. Rather, several particles subsequently bombard a bond, thereby exciting and eventually rupturing it, which is referred to as the multiple-particle process. However, these two scenarios are just limiting cases and in a particular device geometry under certain operating/stress conditions a superposition of these two mechanisms has to be expected.
The most important consequence of the interplay between SP- and MP-carrier processes is the change of the worst-case condition of hot-carrier degradation: traditionally, the worst-case of HCD occurred at Vgs≅(0.4-0.5)Vds, corresponding to the maximum substrate current or - in other words - to the largest impact ionization rate [8,47,48,49]. However, this is not always the case even for long-channel devices; for example, in high-voltage p-MOSFETs the worst-case conditions are observed at the maximum gate current and no empirical law exists for this case [50,51,52]. This regime corresponds to the situation where the average carrier energy is maximum, that is, the carrier ensemble includes a substantial fraction of particles with energies high enough to induce the bond dissociation following a single impact to launch the SP-mechanism.