no means8 simple and have some rather unique properties. It was not until after this theory had been applied to the motion of electrons in crystalline solids that a satisfactory theory of semiconductors emerged*. It was not until this theory was available that a satisfactory definition of a semiconductor could be given.

A brief and elementary account is given here of the mechanism for conducting various kinds of matter. The fundamental ideas underlying this process will be discussed only at some 1 e n g t h 0.

It is known that in electrolytes and in gases conduction occurs owing to** the motion of ions, that in metal conduction takes place on account of the motion of electrons, and as for insulators there is no conduction at all, but only a slight displacementof the charges within " the atoms themselves. There is still another kind of matter in which conduction does take place by virtue of electrons just as in metals, but, as distinct from** the behaviour of metals, a substance of this kind exhibits an increa.se of resistance as the temperature falls. Such a substance is called a semiconductor and at the absolute zero" of temperature it would be an insulator.

On the whole", the variation of resistance with temperature is explained as follows: in a metal only a very few electrons are free to move upon the application of a potential difference, and as the temperature of the metal is lowered, the thermal vibration of its atoms is reduced, with the consequence that the atoms interfere" less with the motion of the electrons, and as a result the resistance is lowered. These electrons free to move in a metal are in semiconductors bound loosely** to the atoms. At absolute zero a semiconductor has no current carriers; as the temperature is raised, more and more of the loosely bound electrons are released by thermal energy and conduction is improved, that is, the resistance is lowered as the temperature rises.

The current carriers in semiconductors may be supplied by an impurity *. For example, an arsenic impurity in silicon supplies one loosely bound excess electron for each atom of arsenic dissolved 2°, and, hence, conduction is due to a transfer" of excess electrons; such a semiconductor is said to be of n-type, because the carriers are negative. A trace 22-of boron in silicon, on the contrary", removes" one electron for each atom of boron dissolved and the "hole"" left in the electronic structure of a silicon atom provides a type of conduction called hole or defect 2в conduction. This type of conduction occurs due t 0 the transfer from atom to atom of electrons into available holes. A semiconductor in which the conduction is due to holes is said to be of p-type, because the carriers act like positive charges.

For the time being7 two semiconductors, germanium and silicon, are of special interest as they form the basis of the

transistor - an electronic amplifiers-jp wtiich ttie amplified current is emitted and tiansported witliin the semiconducting solid. As long as transistors are constructed of solid material, tiie> can be produced in smaller size as compared with tubes of corresponding electrical performance, are more rugged and reliable, and are freer from microphonics *.

УПРАЖНЕНИЯ

Грамматические упражнения

1. Переведите предложения, обращая внимание на перевод словпсометаний со словом as:

as to, as for «что касается (чсчо-л )», «в отношении (чего л.) >, as far as, so far as «насколько», «поскольку»; as long as, so long as «поскопьк> , «пока»; as follows «следующим образом», «следуюище»; as yet «еше ; as soon as «как только»; as well as «как... так h»;as well «также»; as if, as thougfi «как если бы» (§ 60). Запомните эти словосочетания.

as to, as for

1. Substances when classified as to their magnetic behaviour form three groups.

2. This"text will treat electronic devices as to their properties.

3. As for the resistance of connecting wires, we may consider it later.

4. As for the size of the device, it was given in the first chapter, as far as, so far as

5. As far as we know the rocket is tlie only means of propulsion for space ships.

6. So far as we know this planet has no atmosphere, as long as, so long as

7. Alpha particles continue to exist only as long as they move fast enough to avoid collecting electrons.

8. Boyles law states that the product of the pressure by the volume of a given mass of gas is constant as long as the temperature does not change.

9. The ramjet provides a very satisfactory engine as long as the missile remains at an altitude where there is sufficient oxygen.

10. So long as there is relative motion between a conductor and a magnetic, field, a voltage will be generated in the conductor.

11. So long as a nucleus remains whole, the surface tension forces are obviously holding the forces of electrical repulsion under control. "

as follows

12. Characteristics required from a rocket engine are as follows; a tremendous liberation of energy, high operating pressures and temperatures and unusual propellant flow rates.

13. The preceding discussion may be summed up as follows: the same atoms that occur on the Earth are to be found in the remotebf parts of the Universe.

as yet

14. As yet we have not spoken of the speeds at which the spaceships will travel in the interplanetary orbits.

as soon as

15. As soon as the iron is removed from the magnetic field, It loses its magnetism.

16. The space vehicle and the cosmonaut become weightless, as soon as the lift and drag vanish

as well as

17. Angular velocity and angular acceleration have direction as well as magnitude.

as well

18. Neutrons not only cause fission; they produce other effects as well.

as if, as though

19. It was found that many substances, when placed in the path of cathode, would fluorescence as if they were being exposed to invisible, ultra-violet light.

20. Gamma rays behave as though they were minute particles of energy, called photons.

2. Переведите предложения, обращая внимание на перевод словосочетаний со словом so:

so as to «так чтобы», «с тем чтобы»; so that «гак что», «так чтобы»; so tar «до снх пор»; ог so «или около этого»; and so on, and so forth «и так далее» (§61). Запомните эти словосочетания.

so as to

1. The fuel used in the internal combustion engine is gasoline, which is mi.xed with air in a carburator so as to form an explosive mixture.

2. Our definition of vertical flight will be elastic so as to include aircraft that can operate from small ground areas even if they do not rise or descend quite vertically.

so that

3. The object of physics is to determine exact relations between physical phenomena so that the sequence of events can be clearly understood and definitely predicted.

4. Every particle of a body possesses weight, so that the pull of the earth on the body is made up of a large number of forces directed toward the centre of the earth.

so far

5. All of the propellants so far discussed have one characteristic in common - a low boiling point.

vanish - исчезать