characterization, modeling, and design of esd protection circuits

5.3. Analysis 157
are flat, a direct result of the independence of these terms on width and number of fingers
as dictated by Eqs. (5.51) and (5.52). As expected, Vsb and Rsb increase linearly with SGS
and DGS. However, Rsb has a stronger dependence on DGS than on SGS, which may
reflect the fact that all stress current flows through the drain but then is split between
source and substrate paths. The snapback voltage appears to have a greater dependence on
SGS than on DGS, but the large error bars indicate that this difference is within
experimental error.
In the withstand current plots, the quadratic model terms for DGS and 1/W result in
curved response lines (the negative ITLP,ws vs. DGS curvature agrees with the HBM
withstand data in Fig. 5.56), while the interaction terms between DGS, 1/n, and 1/W result
in a pair of lines for each of these responses. For each factor the response curve is drawn
for the most positive and most negative influence the factor can have on the response as
determined by its interaction with other terms. As expected, in all cases ITLP,ws increases
as 1/n and 1/W increase. However, for some values of 1/n and 1/W, the model predicts that
ITLP,ws will decrease to negative values for large DGS. Although it cannot be directly seen
Vsb
Rsb*Wn
If
SGS DGS 1 / n 1 / W
Fig. 5.60 Catalyst model graph for Lot 1 V sb , R sb (multiplied by structure width),
and normalized I TLP,ws (If) as a function of SGS, DGS, 1/n, and 1/W.