Английский язык: Методические указания и задания к контрольным работам для студентов специальности «Техническая эксплуатация транспортного оборудования», страница 12

     However, it is impossible for the electron to gain energy from the electric field in insulators and to carry an electric current because of wide energy gap. This is only the case in the electrical breakdown of insulators when an external electric field is so strong that electrons in the filled band gain enough energy to jump the forbidden energy gap and move into the empty band.

     The band model of a semiconductor is similar to that of an insulator with the exception that the forbidden energy gap is quite narrow. This means that a few electrons in the filled band can be pushed through the forbidden energy gap by the thermal energy of the crystal at room temperature. Therefore, only a few electrons can carry an electric current; the material is named intrinsic semiconductor.

     Electrons  and holes. In the conduction band of semiconductors electrons are free to conduct electric current. Jumping of a valence electron into the conduction band is always accompanied by the appearance of a hole. Now let us consider the situation in the valence band after some electrons have left it. These electrons leave behind empty levels. Such empty levels are called holes because they represent empty energy levels in a nearly filled band. In other words, a hole is the vacancy caused by a valence electron breaking its covalent bond and becoming a conduction electron.

     As temperature increases, more heat energy is available and more valence electrons will acquire the necessary energy to jump the forbidden gap into the conduction band. Thus, the resistivity of the semiconductor decreases and more electrons become current carries.

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