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# 50

Conductors, resistance and semi-conductors

§1 Electrical current can be defined as a flow of electrons from on point to another through a material or even a vacuum (1). If the terminals of a battery are connected, the electrons flow from the negative to the positive end; depending on the nature of the material they are flowing through, the electrons travel more or less freely, i.e. each material has a different resistance to this flow.

§2 Conductors are substances which have a low resistance. Most metals and liquids, especially acid solutions, are good conductors. On the other hand, there are some materials which offer a high resistance so that practically no electrons are released (2): rubber, porcelain, plastic are classified as insulators. The case of gases is more complex: at normal temperature and pressure conditions, they are good insulators, yet, the example of neon gas shows that, when maintained at low pressure, the molecules become ionized, thus allowing a flow of electrons.

§3 Moreover other factors can affect the resistance: the length and cross-section of the conductor and particularly its temperature. Sometimes the higher the temperature of conductors is, the more resistance increases. However most insulators behave the other way round(3): when heated, their resistance decreases, so that insulation breakdowns and consequently short- circuit are likely to(4) happen.

§4 Semi-conductors can be considered as conductors or insulators according to conditions. Resistivity, which is the resistance offered by a certain material at a given temperature, may be altered by adding small amounts (5) of impurities. This characteristic explains the importance of the semi-conductor in electronics where it is the basic element for miniaturization.

§5 By introducing elements with a different number of valence (6), the conductivity of semi-conductors such as germanium or silicon can be enormously altered and, depending on the added element, a positive (P-type) or negative (N-type) semi-conductor can be manufactured.

§6 Take the example of germanium into which arsenic and indium have been incorporated: a PN junction is formed and where the positive and the negative areas meet, the transition area has a high resistance. However if an externally-provided voltage is applied (7) and a negative terminal is linked to the N-side and a positive terminal to the P-side, the positive and the negative charges within the crystal attract each other, thus creating a flow of current. On the other hand, if the polarity is reversed, as a result, the internal charges are drawn away from the limit area (called the blocking layer) - since only charges of different signs attract each other - consequently no current can flow. This feature enables to use semi-conductors as rectifiers.

Vocabulary notes:
1 - vacuum= le vide
2 - to release : to set free
3 - to behave the other way round : to act inversely
4 - to be likely to: to be possible
5 - amount : quantity
6 - number of valence: number of electrons that an atom may share with another, i.e. there may be free electrons (and gaps to attract free electrons)
7 - an externally-provided voltage: an external source of power is applied

UNDERSTANDING THE TEXT

1 a. Read the first sentence of each paragraph and decide what each paragraph probably discusses.

• 1 a/ General information about electrical current ? b/ General comments about electronics ? c/ Why can electrons flow in a vacuum.
• 2 a/The importance of conductors in electronics ? b/ What makes some substances good or bad conductors ? c/ How can resistance be altered ?
• 3a/ This paragraph adds a new information to the previous paragraph. b/ This paragraph discusses the drawbacks of resistance ? c/ Why length and cross-section can be big drawbacks ?
• 4 The paragraph deals with : a/ The properties of semi-conductors ? b/ the manufacturing of semi-conductors ? c/ the manufacturing of insulators ?
• 5 The paragraph deals with : a/ conductivity ? b/ germanium and silicon ? c/ semi-conductors ?
• 6 a/ What happens if germanium is incorporated into arsenic and indium ? b/ a detailed description of how a semi-conductor operates ? c/ Why indium forms a PN junction ?
  

b. Now read the whole text and check your answers.

2 - Linkwords.
a. Pick up linkwords used to :
(§2,3,6) express opposition and contrast ;
(§2,3,6) express consequence ;
(§5, 6) introduce an example.
b. Look for the English equivalents of
§1 and 4 en fonction de - §3 de plus - §5 en + participe présent exprimant le moyen - §6 parce que, puisque.

3 - Pick up the relative clauses in §1, 2, 4, 6. (Careful, the relative pronoun may be left unexpressed !)

4 - Pick up two contracted adverb clauses (formes elliptiques) in § 2, 3 and rewrite them as full sentences.

5 - Analyse the following sentences (§2)
"At normal temperature and pressure conditions, gases are good insulators, yet, the example of neon gas shows that, when maintained at low pressure, the molecules become ionized, thus allowing a flow of electrons."

note : that is to be placed in d.

5 - Right or wrong? (§1)

• Electricity can be defined as negative electrons flowing to a positive pole.
• Conducting or resisting properties era due to the electronic nature of materials.
• Electrons can travel in a vacuum.
• The speed of electrons depends on the material.

7 - First fill in the left column and the top line, then write I or C to indicate that the material acts (or tends to act) as an insulator or a conductor. (§2 and 3)

• How can resistivity be defined?
• What effect have small amounts of impurities when added to some materials?
• What is the chemical difference between an impurity and the material to which it is added?

LANGUAGE STUDY

1 - Use the phrases below to give definitions:
Rubber, porcelain and plastic can be classified as insulators.
Electrical current can be defined as a flow of electrons.
Semi-conductors can be considered as conductors or insulators.
The materials used in electricity can be divided into three categories: conducting, magnetic, insulating.

• A resistor is a device to add resistance to a circuit
• Celluloid, plastic and asbestos (= amiante) are examples of insulators
• There are two main groups of transistors : bipolar and field effect transistors.
• One can say the thermo ionic valve is the ancestor of transistors.
• AC motors, alternators and generators are examples of rotating machines.
• One can say transistors have a practically unlimited life.
• The effects of an electrical current are thermal, luminous, chemical and magnetic.
• A motor is a machine which transforms electrical power into mechanical energy.
• There are two kinds of semi-conductors: the P-type and the N-type.
• One can say the wavelength of UHF ranges from 10 to 1 m.
• Primary cells, lead-acid accumulators and solar cells are examples of electric cells.
• A DC is a current that flows in one direction.

2 - Link words: cause
Copper is currently used because / since it offers little resistance
The reason why copper is currently used is ( that) it offers little resistance.
Copper is currently used because of 1- its low resistance 2 - it(s) offering little resistance.
Link the following sentences as in the examples above.

• Plastic is used to coat wires: it's a good insulator.
• Most equipment is protected by fuses: shirt-circuits can damage it.
• One can use a diode as a rectifier: the current can flow in only one direction.
• A bulb can emit light: the filament is heated to white heat.
• You must never touch electrical equipment with wet hands: water is a good conductor.

3- Linkwords: consequence
Copper is an excellent conductor consequently (= as a result - so that - for this reason - thus - therefore - so ) it is used for electrical wires.
Use the sentences of the previous exercise and express consequence.

4 - Express cause and consequence
Use the following phrases to talk about the diagram

 

	A	B
Fig. 2	dirty contacts blown fuse switchid off faulty switch power failure	bulb lights up current flows caurrent can't flow circuit is interrupted

5 - Transform as in the example
When molecules are maintained at low pressure, they become ionized.
When maintained at low pressure, molecules become ionize.

• When an insulator is heated; it loses resistance.
• When a resistor is inserted into a circuit, it modifies the intensity.
• If a CRT is hit by an electron, it fluoresces.
• When they were first introduced, transistors were not very reliable.

6 - Modal verbs: passive form
A temperature rise may decrease the resistivity of a material.
The resistivity of a material may be decreased by a temperature rise.
Turn into the passive

• Other factors can affect the resistance.
• We can consider semi-conductors as insulators or conductors.
• One can transform AC voltages into DC.
• You may insert a lamp into the circuit.
• You must keep electrical appliances away from water.

7 - Comparisons
The higher the temperature (is), the more resistance increases
Transform as in the example above:

• The temperature gets lower, resistance decreases.
• Many electrons are released, resistance is low.
• According to Ohm's law; when the voltage increases, the current is stronger;
• If current is weak, it's less dangerous.
• In a transformer, you reduce the voltage but you step-up the intensity.
• An insulator is thin, its dielectric strength is less high.

8 - Which insulating material would you choose ? Justify.
a- A heating element must be manufactured: peak temperature 170 °C - peak voltage 250 V - manufacturing method: moulding or machining - no special mechanical requirements.
b- Wires must be installed in a bathroom : peak voltage 250V

material	peak temperature (°C)	dielectric strength (KV/mn)
porcelain	> 180	16	easy moulding
synthetic rubber	60	14	oil and solvent resistant
cotton	100	5	cheap - good resistance to drawing
asbestos	> 180	3	fire resistant - water absorbing
glass	> 180	25	high mechanical resistance
PVC	60	35	easy processing
teflon	250	18	expensive - excellent mechnaical properties

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The following text can be used as a complementary reading / comprehension exercise.

Origin of the document: http://www.lucent.com/ideas2/heritage/transistor/info.html

Bell Labs celebrates 50 years of the Transistor What Is A Transistor?

The Invention
This unprecedented flow of innovations began in the mid-40s when a team of scientists at Bell Labs set out to find a solution that would replace the vacuum tube and the mechanical relays with something better, something more reliable, more efficient, less costly to maintain. On December 16, 1947, Walter Brattain, backed by his team and the entire pool of Bell Labs science, made another adjustment to his odd-looking contraption consisting of germanium, gold strips, insulators and wires, and observed, for the first time, an amplification of the input signal. The transistor was born and unknowingly, the information age. Nobel Prize winners John Bardeen, Walter Brattain, and William Shockley subsequently developed the techniques to make the technology practical, effectively teaching the industrial community how to use it to create applications from hearing aids to telephone switches, from portable radios to television sets.

Invented at Bell Laboratories in 1947, the transistor resulted from efforts to find a better amplifier and a replacement for mechanical relays. The vacuum tube had amplified music and voice during the first half of the 20th century, and it had made long-distance calling practical. But it consumed lots of power, operated hot and burned out rapidly. The telephone network required hundreds of thousands of relays to connect circuits together to complete calls. Network relays were mechanical devices, requiring regular maintenance to clean and adjust.
Cheaper to make than the vacuum tube and far more reliable, the transistor cut the cost and improved the quality of phone service and, seemingly overnight, spawned countless new products and whole new industries.

How a Transistor Works
The transistor has many applications, but only two basic functions: switching and modulation -- the latter often used to achieve amplification.
In the simplest sense, the transistor works like the dimmer in your living room. Push the knob of the dimmer, the light comes on; push it again, the light goes out. Voila! A switch. Rotate the knob back and forth, and the light grows brighter, dimmer, brighter, dimmer. Voila! A modulator. To understand amplification, think of this: a relatively effortless action by you to turn the knob from its low to high setting translates into a much more impressive reaction by the light - the whole room beams with light! Voila! An amplifier!
Both the dimmer and the transistor control current flow, be it through a lamp or a device to be activated. Both act as a switch--on/off--and as a modulator/amplifier-- high/low. The important difference is that the "hand" operating the transistor is millions of times faster. And it's attached to another electrical source--a radio signal in an antenna, for example, a voice in a microphone, or data signal in a computer system, or even another transistor.
Transistors are made of semi-conductors such as silicon and gallium arsenide. These materials carry electricity moderately well--not well enough to be called a conductor, like copper wires; not badly enough to be called an insulator, like a piece of glass. Hence their name: semi-conductor.
The 'magic' a transistor performs is in its ability to control its own semi-conductance, namely acting like a conductor when needed, or as an insulator (non-conductor) when that is needed.
Semi-conductors differ in the way they act electrically. Putting a thin piece of semi-conductor of one type between two slices of another type has startling results: a little current in the central slice is able to control the flow of the current between the other two. That little current in the middle slice is the juice that is supplied by an antenna or another transistor for example. Even when the input current is weak, as from a radio signal that's traveled a great distance, the transistor can control a strong current from another circuit through itself. In effect, the current through the 'output side' of the transistor mimics the behavior of the current through the 'input side'. The result is a strong, amplified version of the weak radio signal.

What Transistors Do
In microchips today, which contain millions of transistors 'integrated' together in a particular pattern or 'design', the amplified output of one transistor drives other transistors that, in turn, drive others, and so on. Build the sequence one way and the chip can be made to amplify weak antenna signals into rich quadraphonic hi-fidelity sound. Build the chip differently, and the transistors interact to create timers to control watches or microwave oven, or sensors to monitor temperatures, detect intruders, or control car wheels from locking (ABS systems). Arrange the transistors in a different array and create arithmetic and logic processors that drive calculators to calculate, computers to compute, 'process' words, search complex data bases for information, networks to 'talk' to each other, or systems that transmit voice, data, graphics and video to make our communications networks.
It may take a score of transistors, interconnected in teams called logic gates, to accomplish a task as simple as adding one and one. But put enough transistors together in appropriate patterns and transistors end up knock off big jobs by working fast switching on and off 100 million times per second or more--and by working in huge teams.
As discrete components as in the old days, a thousand transistors would occupy dozens of printed circuit boards the size of postcards. But thanks to such techniques as photolithography and computer-aided design, millions of transistors and other electronic components, complete with wiring, can be compactly organized on an integrated circuit smaller than a cornflake.

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