Offset

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.

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.

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