 |
|
Give Russian equivalents of the following phrases.
Good conductors, unfilled space, in the valence energy band, travelling in all directions, many different velocities, mobile electrons, plenty of nearby unfilled states, in simple metals, metal because of the scattering of electrons, semiclassical theory, an effective mass.
Complete the sentences.
1.In the absence of an electric field, there exist electrons travelling in all directions and many... 2. When an electric field is applied, a slight imbalance develops and …3. Resistance comes about in a metal because of the scattering of ...4. A theory of conduction in simple metals is the Drude model, in which scattering is characterized…
Put questions to the following sentences.
1. Metals are good conductors because they have unfilled space in the valence energy band. 2. Electrons in the band can be accelerated by the field because there are plenty of nearby unfilled states in the band. 3. A better model is the so-called semi classical theory, in which the effect of the periodic potential of the lattice on the electrons gives them an effective mass.
Retell the text.
Text 18
Semiconductors. Superconductors
Vocabulary
1. fraction – преломление
2. band gap – частотный интервал
3. hole – отверстие
4. impurity – примесь
5. acceptor – акцептант
6. capture,v – увлечь
7. formation – образование
8. state – состояние
9. type of charge – тип зпряда
10. property – свойство
11. interaction – взаимодействие
12. mediate – служить связующим звеном
13. superfluid – избыточный
14. concentration – сосредоточение
A solid with filled bands is an insulator, but at finite temperature, electrons can be thermally excited from the valence band to the next highest, the conduction band. The fraction of electrons excited in this way depends on the temperature and the band gap, the energy difference between the two bands. Exciting these electrons into the conduction band leaves behind positively charged holes in the valence band, which can also conduct electricity.
In semiconductors, impurities greatly affect the concentration and type of charge carriers. Donor (n-type) impurities have extra valence electrons with energies very close to the conduction band which can be easily thermally excited to the conduction band. Acceptor (p-type) impurities capture electrons from the valence band, allowing the easy formation of holes. If an insulator is doped with enough impurities, a Mott transition can occur, and the insulator turns into a conductor.
Superconductors are those materials which are neither conductors nor insulators. They are totally dependent on the temperature. As the temperature changes their properties also change. In metals and certain other materials, a transition to the superconducting state occurs at low (sub-cryogenic) temperature. By an interaction mediated by some other part of the system (in metals, phonons), the electrons pair up into Cooper pairs. The Cooper pairs form a superfluid which has zero resistance.