Download FCE Reading- Part 6 –Gapped text - E

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.
A
This energy difference is called a band gap.
B
A voltage or current applied to one pair of the transistor's terminals changes the
current through another pair of terminals.
C
has four valence electrons, each of which participates in a bond in the crystal
lattice.
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.
E
One can increase the electrical conductivity of a semiconductor by doping it with a
small amount of certain other elements
F
Thus, doping silicon with a small amount of arsenic produces an ntype semiconductor.
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.
H
Such materials are called semiconductors.
PN Junction
The schematic symbol for a diode is:
diode
light-emitting diode (LED)
As the symbols are oriented above, the wire on the left goes to the anode, and the wire
on the 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 the conduction band. The levels occupied by the unexcited
valence electrons are known as the valence 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, or holes. When an electron moves to fill one of these holes, it leaves
behind another hole. This type of semiconductor is called p-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 ptype 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)………………... the contact potential or junction 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)