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.

1.Give Russian equivalents of the following phrases.

The fraction of electrons, at finite temperature, positively charged holes, thermally excited, from the valence band, neither conductors nor insulators, are totally dependent on the temperature, a transition to the superconducting state, zero resistance.

2.Complete the sentences

1.The fraction of electrons excited in this way depends on …2.Impurities greatly affect the concentration and…3. Donor (n-type) impurities have extra valence electrons with…4. In metals and certain other materials, a transition to the superconducting state occurs… 5. The Cooper pairs form a superfluid which has….

3.Put questions to the following sentences.

1.Acceptor (p-type) impurities capture electrons from the valence band, allowing the easy formation of holes. 2. Superconductors are those materials which are neither conductors nor insulators. 3. By an interaction mediated by some other part of the system (in metals, phonons), the electrons pair up into Cooper pairs. 4. The Cooper pairs form a superfluid which has zero resistance.

4.Give the definitions.

Superconductors, the conduction band, acceptor (p-type) impurities.

5.Retell the text.