FCE Reading- Part 6 Gapped Text

FCE Reading- Part 6 –Gapped text

You are going to read an article about PN junction. Seven sentences have been removed from the article. Choose from the sentences A – H the one which fits each gap (1-7). There is one extra sentence you do not need to use. Suggested time: 15 minutes.

PN Junction

The schematic symbol for a diode is:

As the symbols are oriented above, the wire on the left goes to the anode, and the wireonthe right goes to the cathode.(1)…………………

How does it work?

Materials are often broadly classified as conductors or insulators. For a substance to be a conductor, its electronic structure must be such that there are unoccupied energy levels close enough in energy to where the valence electrons normally reside that the electrons can easily be excited to those levels. (2)………………… These energy levels comprise what is commonly called theconduction band. The levels occupied by the unexcited valence electrons are known as thevalence band. In an insulator, the conduction band lies at sufficiently high energy above the valence band that the valence electrons cannot easily reach the levels that lie there, and the material does not conduct electricity. (3)………………… In some materials, the band gap is much smaller than that of an insulator, and the electrical conductivity is intermediate between that of a conductor and that of an insulator.(4)…………………

The most commonly used elements in semiconductor devices are probably silicon and germanium.(5)…………………that have either one more or one fewer valence electron than does the semiconductor in question. For example, silicon, which is in qroup IV, (6)…………………. If an arsenic atom, which has five valence electrons, replaces a silicon atom, it then has one electron left over, which cannot participate in bonding and is thus available for conduction. (7)………………… If, instead, one substitutes some boron atoms, which have only three valence electrons, this results in electron vacancies in the crystal lattice, orholes. When an electron moves to fill one of these holes, it leaves behind another hole. This type of semiconductor is calledp-type. Thus, we speak of the charge carriers in n-type material as being electrons, and in p-type material as being holes.

(Abridged from http://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/64.56)

FCE Open close – Part 2

For questions 1-20, read the text below and think of the word which best fits each gap. Use only one word in each gap.

The simplest semiconductor (1)………………... is a p-n junction. That is, a piece of p-type material and a piece of n-type material placed in contact with each other. If no external potential is applied, some of the electrons in the n-type material can cross the junction to fill holes in the p-type material. This leaves a (2)………………... charge on the n-type section and a (3)………………... charge on the p-type section, which prevents further flow of charge carriers across the junction. The resulting potential is (4)………………... thecontact potentialorjunction potential. We will see that we must overcome this junction potential in order to make the diode (5)………………... electricity.

If we (6)………………... bias the junction, that is, place a potential across it so that the n-type section is positive with respect to the p-type section, any (7)………………... in the p-type region cross back over the junction into the n-type region, where all the electrons are drawn (8)………………... from the junction towards the positive supply terminal. Similarly, any (9)………………... that were in the n-type region are filled, and the holes in the p-type region are drawn towards the negative terminal. The junction is thus (10)………………... of charge carriers, and current (11)………………... flow.

If we bias the junction in the (12)………………... direction, that is, put a potential across it so that the p-type section is now positive with respect to the n-type section, the (13)………………... in the n-type material are repelled from the negative power (14)………………... terminal, as are the holes in the p-type region repelled from the positive terminal. The electrons in the n-type region are drawn towards the (15)………………... , which they can now cross to fill the holes in the p-type region, leaving more holes behind in the n-type region. Current can now (16)………………... , and the diode conducts. Since the above mentioned junction potential is opposite to the forward bias potential, whatever potential we (17)………………... must exceed this junction potential in order for the (18)………………... to conduct. The (19)………………... is, of course, the n-type region, and the (20)………………... is the p-type region.

(Abridged from http://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/64.56)

KEY

FCE Reading- Part 6 –Gapped text

You are going to read an article about PN junction. Seven sentences have been removed from the article. Choose from the sentences A – H the one which fits each gap (1-7). There is one extra sentence you do not need to use. Suggested time: 15 minutes.

The schematic symbol for a diode is:

As the symbols are oriented above, the wire on the left goes to the anode, and the wire G Most LEDs that have a round case have a longer lead on the anode than on the cathode, and a flat near the cathode lead.

How does it work?

Materials are often broadly classified as conductors or insulators. For a substance to be a conductor, its electronic structure must be such that there are unoccupied energy levels close enough in energy to where the valence electrons normally reside that the electrons can easily be excited to those levels. D In doing so, they leave vacancies (holes), which may then be filled by other electrons, thus allowing the electrons to move about the crystal lattice of the material, and a current to flow. These energy levels comprise what is commonly called theconduction band. The levels occupied by the unexcited valence electrons are known as thevalence band. In an insulator, the conduction band lies at sufficiently high energy above the valence band that the valence electrons cannot easily reach the levels that lie there, and the material does not conduct electricity. A This energy difference is called aband gap. In some materials, the band gap is much smaller than that of an insulator, and the electrical conductivity is intermediate between that of a conductor and that of an insulator.HSuch materials are calledsemiconductors.

The most commonly used elements in semiconductor devices are probably silicon and germanium. EOne can increase the electrical conductivity of a semiconductor bydopingit with a small amount of certain other elements that have either one more or one fewer valence electron than does the semiconductor in question. For example, silicon, which is in qroup IV, C has four valence electrons, each of which participates in a bond in the crystal lattice. If an arsenic atom, which has five valence electrons, replaces a silicon atom, it then has one electron left over, which cannot participate in bonding and is thus available for conduction. F Thus, doping silicon with a small amount of arsenic produces ann-typesemiconductor. If, instead, one substitutes some boron atoms, which have only three valence electrons, this results in electron vacancies in the crystal lattice, orholes. When an electron moves to fill one of these holes, it leaves behind another hole. This type of semiconductor is calledp-type. Thus, we speak of the charge carriers in n-type material as being electrons, and in p-type material as being holes.

FCE Open close – Part 2

For questions 1-20, read the text below and think of the word which best fits each gap. Use only one word in each gap.

The simplest semiconductor device is a p-n junction. That is, a piece of p-type material and a piece of n-type material placed in contact with each other. If no external potential is applied, some of the electrons in the n-type material can cross the junction to fill holes in the p-type material. This leaves a positive charge on the n-type section and a negative charge on the p-type section, which prevents further flow of charge carriers across the junction. The resulting potential is called thecontact potentialorjunction potential. We will see that we must overcome this junction potential in order to make the diode conduct electricity.

If we reverse bias the junction, that is, place a potential across it so that the n-type section is positive with respect to the p-type section, any electrons in the p-type region cross back over the junction into the n-type region, where all the electrons are drawn away from the junction towards the positive supply terminal. Similarly, any holes that were in the n-type region are filled, and the holes in the p-type region are drawn towards the negative terminal. The junction is thus depleted of charge carriers, and current cannot flow.

If we bias the junction in the forward direction, that is, put a potential across it so that the p-type section is now positive with respect to the n-type section, the electrons in the n-type material are repelled from the negative power supply terminal, as are the holes in the p-type region repelled from the positive terminal. The electrons in the n-type region are drawn towards the junction, which they can now cross to fill the holes in the p-type region, leaving more holes behind in the n-type region. Current can now flow, and the diode conducts. Since the above mentioned junction potential is opposite to the forward bias potential, whatever potential we apply must exceed this junction potential in order for the diode to conduct. The cathode is, of course, the n-type region, and the anode is the p-type region.

Problem solving activitiesKEY

1. Determine the diode voltage, the current and the potential difference across the resistor for the following circuits.

a. (1 mA) b.(86mA)

c.(16mA)

2.Which kind of output do you expect? Try to draw the output voltage across the diode.

Solution