Contrary to bias temperature instability, hot-carrier stress becomes less detrimental with increasing device temperature (cf. Figure 7.5) in longer channel devices. This relation is inverted for short channel devices, where hot-carrier stress becomes more severe at elevated temperatures [179].
| Figure 7.5: | Hot-carrier degradation expressed as ΔId at room temperature and for device stress temperatures of 25°C and 75°C. The data have been recorded for n-channel MOSFETs with an EOT of 2.5nm at various gate and drain voltages, where Vg = Vd. With increasing drain voltage and thus channel electric field, the hot-carrier induced damage increases. However, the induced damage decreases with increasing temperature. |
This channel length dependent influence of temperature on the degradation can be explained in terms of carrier kinetic energy distribution and scattering. As laid out in Chapter 2, charge carriers are accelerated by the electric field in the semiconductor and undergo scattering events. These events lead to changes in momentum and possibly in a loss/gain of particle energy. At higher temperatures the intensity of all scattering processes increases, which results in depopulation of the high energetical fraction of the carrier ensemble. As a consequence, increasing lattice temperature causes weakening of hot-carrier degradation.