14.3 I NTRINSIC S EMICONDUCTOR

. I NTRINSIC S EMICONDUCTOR We shall take the most common case of Ge and Si whose lattice structure is shown in Fig. . . These structures are called the diamond-like structures. Each atom is surrounded by four nearest neighbours. We know that Si and Ge have four valence electrons. In its crystalline structure, every Si or Ge atom tends to share one of its four valence electrons with each of its four nearest neighbour atoms, and also to take share of one electron from each such neighbour. These shared electron pairs are referred to as forming a covalent bond or simply a valence bond . The two shared electrons can be assumed to shuttle back-and- forth between the associated atoms holding them together strongly. Figure . schematically shows the -dimensional representation of Si or Ge structure shown in Fig. . which overemphasises the covalent bond. It shows an idealised picture in which no bonds are broken (all bonds are intact). Such a situation arises at low temperatures. As the temperature increases, more thermal energy becomes available to these electrons and some of these electrons may break–away (becoming free electrons contributing to conduction). The thermal energy effectively ionises only a few atoms in the crystalline lattice and creates a vacancy in the bond as shown in Fig. . (a). The neighbourhood, from which the free electron (with charge – q ) has come out leaves a vacancy with an effective charge (+ q ). This vacancy with the effective positive electronic charge is called a hole . The hole behaves as an apparent free particle with effective positive charge. In intrinsic semiconductors, the number of free electrons, n e is equal to the number of holes, n h . That is n e = n h = n i ( . ) where n i is called intrinsic carrier concentration. Semiconductors posses the unique property in which, apart from electrons, the holes also move. FIGURE . Three-dimensional dia- mond-like crystal structure for Carbon, Silicon or Germanium with respective lattice spacing a equal to . , . and . Å.