Read the text once again and find examples of the Attributive Group

 

Translate the following word-combinations paying attention to the Attributive Group.

 

voltage equation; magnetic field properties; terminal voltage; electrical device operation; electric circuit characteristics; transmission line wire insulator; solar-energy conversion cell

Give English equivalents to the following word-combinations. Use them in sentences of your own.

 

концентрация носителей зарядов; содержание примесей; тепловое возбуждение; кремневый кристалл; дефицит электронов; проводимость полупроводника; зонная теория; при обычной температуре

Complete the following sentences according to the information given in the text.

 

1Electrons and holes are created by …

2Intrinsic semiconductors have equal …

3At the temperature above absolute zero electrons …

4The energy gap ranges from …

5The term ‘intrinsic semiconductor’ is defined as …

 

Look through the text and make a short summary of the important information.

9 Speaking task. Speak on intrinsic semiconductors using the summary made in Exercise 8.

 

Text III

Useful terms. Read and memorize the following words and word-combinations.

Collision столкновение

Crystal lattice кристаллическая решетка

Energy band энергетическая зона

Excite возбуждать

Hole conduction дырочная проводимость

Interact взаимодействовать

Majority carrier носитель основной

Minority carrier носитель неосновной

Overlap перекрывать

P-n junction электронно-дырочный переход (p-n переход)

Randomize разупорядочить

Regular array правильный ряд

Scattering рассеивание

Spread out распространяться

Velocity скорость

2 For questions (1-7) choose which of the sentences (A-H) fit into the numbered gaps in the text. There is one extra sentence which does not fit in any of the gaps.

Semiconductor Current

 

Both electrons and holes contribute to current flow in an intrinsic semiconductor.

The current which will flow in an intrinsic semiconductor consists of both electron and hole current.

In addition, other electrons can hop between lattice positions to fill the vacancies left by the freed electrons. This additional mechanism is called hole conduction because it is as if the holes are migrating across the material in the direction opposite to the free electron movement.

The flow of electric current depends upon the acceleration of charges by an externally applied electric field. Because of collisions, charged particles in a solid are not accelerated indefinitely by the applied field, but rather, after every scattering event, the velocity of a charged particle tends to be randomized. Thus the acceleration process must start anew after each scattering event and charged particles achieve only finite velocity along the electric field E, the average value of the velocity being denoted by νD, the drift velocity. The effectiveness of the charge transport by a particular charged particle is expressed by the mobility μ, which is defined as μ = νD/E. The electrical conductivity σ depends upon the mobility of the charged carriers as well as on their concentration n, and is simply written as σ = ne μ, where e is the charge of the carriers. The advantage of the expressing the conductivity in this form is the explicit separation into a factor n which is highly sensitive to external parameters, such as temperature, pressure, optical excitation, irradiation, and into another factor μ, which depends characteristically on scattering mechanisms and on the electronic structure of the semiconductor.

The electrical conductivity of semiconductors ranges from about 103 to 10-9 ohm-1 cm-1, as compared with a maximum conductivity of 10-17 ohm-1 cm-1 for good insulators.

The electric current is usually due only to the motion of electrons, although under some conditions, such as very high temperatures, the motion of ions may be important.

A crystalline solid consists of a large number of atoms brought together into a regular array called a crystal lattice. The electrons of an atom can each have certain energies, so-called energy levels, as predicted by quantum theory. Because the atoms of the crystal are in close proximity, the electron orbits around different atoms overlap to some extent, and the electrons interact with each other; consequently the sharp, well-separated energy levels of the individual electrons actually spread out into energy bands.

The type of charge carrier, electron or hole that is in largest concentration in a material is sometimes called the majority carrier and the type in smallest concentration the minority carrier. Although the minority carriers play a minor role in electrical conductivity, they can be important in rectification and transistor actions in a semiconductor.

In an intrinsic semiconductor like silicon at temperatures above absolute zero, there will be some electrons which are excited across the band gap into the conduction band and which can produce current. When the electron in pure silicon crosses the gap, it leaves behind an electron vacancy or "hole" in the regular silicon lattice. In an n-type semiconductor, the dopant contributes extra electrons, dramatically increasing the conductivity. In a p-type semiconductor, the dopant produces extra vacancies or holes, which likewise increase the conductivity. It is however the behavior of the p-n junction which is the key to the enormous variety of solid-state electronic devices.

A The mobility of a particle with charge e and mass m can be related directly to the mean time between scattering events (also called the relaxation time) by the expressed μ = e τ/m.