6.2.1 Offset

Acquisition of 25kSamples  yields 3 to 4 usable decades in time for each sequence. The combined sequences result in 5 to 6 decades in time, with a possibly too large deviation of VG,str   from the reference VG,strref   set at the DSO during the first decade1 . In the remaining decades the data can be either fit by a logarithmic time-dependence

ΔI  (t  )   I (t  )− I
--D--str--= -D--str----D,0 = − B log 10(tstr∕t0,ref)
  ID,0          ID,0
(6.1)

with ID,0 = ID (t0,ref)  , or a power-law − A(tstr∕t0,ref)n  with a very small exponent n ≈ 0.04  . ID,0   is obtained at stress-level with a delay t0,ref   and thus not equal to I (0)
 D  [40] and results in an offset of the relative degradation, see Fig. 6.6.


PIC


Figure 6.6: Different reference times t0,ref   result in different degradation. It can be seen that for t0,ref = 50μs  about 25%  of the ΔID ∕ID,0   are missed. On the other hand, too short t0,ref   are not reasonable and result in a spurious shift by a not-yet steady measurement signal (t0,ref = 0.2μs,1μs  ). Compare with Fig. 6.7 and Fig. 6.10.