Figure 2. Diagrams to show different cases of Fermi-level pinning, taking into account band-edge “defects” at T = 0.

(a) Type-I: both donors and acceptors are dominant over band-edge “defects.” (b) Type-II: only acceptors (or donors) are dominant over band-edge “acceptors” (or “donors”). (c) Band-edge “defects” are dominant. Note that in real systems, the dominant defects can be any defects beyond vacancies and only the relative positions of defect formation energy lines, and determines which scenario should be applied.