Twin boundaries are special angle boundaries between two identical crystallites. The least disturbed twin boundary is that of a stacking fault, as discussed in Sect. 4.5.1, by proceeding from an AB sequence to a BC sequence.
Other twin boundaries are under an angle at which each second (or third) atom falls onto a lattice site as shown in Fig. 4.13. It can therefore be regarded as an array of vacancy lines similar to an array of edge dislocations in low-angle grain boundaries.
Other crystallite boundaries may occur under a wide variety of angles and may incorporate at the interface a variety of disorders, including vacancies, dislocations, and liquid-like structures with a high degree of local stress.
Grain boundaries can have a major influence on semiconducting properties by trapping carriers and creating compensating space charge layers. Some of the effects related to carrier mobility will be discussed later in this book.