11. Make a plan and render the text.

LESSON TWENTY-ONE

Grammar: 1. The Subjective Infinitive Construc­tion.

2. The Absolute Participle Construction (Practice of Translation).

3. Composite Prepositions and Conjunc­tions.

4. Word-building and Word-combina­tions.

READING EXERCISES

1. Two-syllable words: >

current, solver, rubber, compounds, alloys, oxide, sul­phides, possess, cooling, dozen, quantum, motion, voltage, ex­treme, atom, thermal, normal, crystal, framework, meaning, either, being, proper, offer, enrich.

2. Many-syllable words:

semiconductor, semiconduction, excellent, electricity, material, insulator, variety, various, property, phenomenon, sensitive, luminous, mechanics, crystalline, satisfactory, rela­tive, available, nucleus, electron, impurity, absolute, as­sociate, intrinsic, specimen, particular, material, desirable.

3. 'Practise reading the following word-combinations:

quartz insulator, semiconductor materials, quantum the­ory, wave mechanics, electron flow, heat motion, charge car­rier, electron vacancy, zinc oxide, conductivity type, impu­rity content, impurity semiconductor.

4. Memorize the spelling and pronunciation of the following words:

quartz f'kwD:tsl — кварц, silicon Psilikanl — кремній, germanium Idsa.'meinjoml — германій, selenium [si'lirnjom] — селен, zero I'zioroul — нуль, zinc [zirjk] — цинк, exhibit lig'zibitl — показувати, проявляти.

5. Practise connected reading:

semiconductors and semiconductivity, electric current, excellent insulators, a great variety, chemical compounds, alloys of metals, metallic oxides and sulphides, some unex­pected properties, they are sensitive, can be detected, at a dis­tance, to explain the phenomenon, theory of semiconductors, relatively free electrons, charge carriers, are also bound, as­sociated atoms, electron vacancy, possible intrinsic conduc­tion, do the charge-carrying, zinc oxide, proper impurity content, compounds of several materials, new ideas and laws.

Text a. Semiconductors and semiconductivity

All substances have some ability to conduct electric cur­rent, however, they differ in the ease with which the current can pass through them. Metals such as copper, gold, silver or iron are excellent conductors of electric current whereas ma­terials such as glass, porcelain, polystyrene, rubber, or quartz are considered to be excellent insulators. Intermediate between the two groups are semiconductor materials, i. e. those which have a conductivity intermediate between that of an insulator and that of a metal. They include all minerals, many chemical elements, a great variety of chemical com­pounds, alloys of metals, and a number of organic compounds.

The semiconductors such as silicon, germanium, sele­nium, phosphorum, boron and various other metallic oxides and sulphides appear to possess some unexpected properties. For instance, their conductivity increases with heating and falls with cooling. However, heat is not the only phenomenon influencing semiconductors. They are sensitive to light, ra­diation, too.

Some of the properties which are now associated with semiconductors have been known for a century or more. But a great deal of experimental work had to be carried out before scientists were able to explain the phenomenon of semiconduc- tivity. It was only after the quantum theory in the form of wave mechanics was applied to the motion of electrons in crystalline solids that a satisfactory theory of semiconduc- tivity emerged.

It is clear now that the electric current (eletricity) is a flow of electrons — the basic electrical charge present in all elements. Electricity flows when electrons move from one atom of a substance to the next. Now it became obvious that the high conductivity of the good conductor is due to a great number of relatively free electrons in the metallic crystal. They provide a great number of charge carriers to give high currents under low voltage (low resistance). The extremely low conductivity of the insulators is due to the lack of avail­able free electrons. The electrons in this case are tightly bound to the nucleus.

In semiconductors electrons are also bound, but the con­nection is so weak that the heat motion of the atoms of a sub­stance pulls them away and sets them free. Therefore the number of free charge carriers in semiconductors is a func­tion of temperature or of impurities present. For example, at absolute zero temperature pure germanium would be an insulator. As the temperature is raised some of the electrons are broken loose from their associated atoms by thermal vibration and become available as charge carriers. In addi­tion, wherever an electron has left its normal position in the interatomic bond, there is an electron vacancy or hole in the crystal framework which is a positive charge carrier. Thus when an electron is released from a bond by thermal energy

/ ^ Я ^ ,, J) І/ * * J 4

there is created an electron-hole pair, making possible in­trinsic conduction by both positive and negative carriers.

~~k semiconductor in which the conduction is due to holes is said to be of p-type, because the carriers act like positive charges. Semiconductors in which electrons do the charge- carrying are called n-type semiconductors, "A" meaning negative carriers.

The copper oxide and selenium are always p-type. Zinc oxide is always n-type. Silicon and germanium can be made either p-type or n-type, the conductivity type being depend ent on the proper impurity content. When the semiconductor conducts because of impurities, it is called an impurity semiconductor. Many semiconductors, particularly at high temperatures, conduct because both electrons and holes are thermally excited. These are called intrinsic semiconductors.

Semiconductors in which the carriers of current are elec­trons are called electronic semiconductors. When the ions of a solid are the charge carriers, as in silver chloride, the solid is an ionic semiconductor.

Now much work is being done to develop compounds of several materials which may offer similar or more desirable properties..

The study of semiconductors by our scientists not only enriched physics with new ideas and laws and played an im­portant role in the advance of knowledge of the electrical, optical and mechanical properties of solids, but also showed the continuously increasing technical impor tance of semi con ductor devices.

VOCABULARY NOTES

1. a great variety (of) — велика різноманітність

2. their conductivity increases with heating and falls with cooling — їх

провідність збільшується при нагріванні, знижується при охоло­дженні

3. the only (phenomenon) — єдине (явище)

4. unexpected properties — непередбачені властивості

5. a great deal of (a great number of, many) — багато

6. satisfactory theory — прийнятна теорія

7. Iffilgh, low) conductivity is due to — (висока, низька) провідність по­яснюється

8. free electron — вільний електрон

9. to set (smb., smth.) free — звільняти (кого-н., що-н.)

10. charge carrier — носій заряду (струму)

11. lack of available free electrons — відсутність вільних електронів

12. some of the electrons are broken loose (from) — деякі електрони відриваються (від)

13. 225

    wherever an electron has left its normal position — як тільки електрон залишає своє нормальне положення