At this transition an abrupt change of the direction of the solution is observed: the anode has flipped from the singular point I to the point III.
The transition can be followed in an enlarged diagram of the center part of Fig. 37.12. Here with increased bias, a minute increase of the drift current n2(F) moves the intersection of ni (F) and П2(F) from below to above the singular point II (Fig. 37.12).
At the transition point, the original high-field domain fills the entire crystal, the solution has now degenerated to be contained within the singular point II. When the drift field curve moves with further increased bias very slightly above II, the direction of the solution flips, but the position П2(F) remains essentially "pinned” near II. It can only move up when the crystal is again entirely filled by the anode – adjacent domain.
Fig. 37.13 CdS crystal with larger donor density showing the steeply increasing branch in the pre-breakdown range that is stabilized and measured sequentially with anode-adjacent high-field domains (Hadley et al. 1972)