1. Nature and the Characteristics Of
Semi-conductors(diodes and doped)
By: Amah Philip

2. You have learned in chemistry that atoms
follow the octet rule. An atom is most stable
when there are eight electrons in its
outermost shell. An atom with only one or
two electrons in its outermost shell tends to
give away these outer electrons to gain
stability (i.e, it now has eight outermost
electrons). This means that one or two
outermost electrons are not tightly held to the
atom and are fairly free to travel.

3. If you take a look at the periodic table of
elements, you will observe that the group number
indicates the number of outermost, or valence
electrons, of an element. Thus, Groups III, IV and V
have three, four, five valence electrons, respectively.
Elements in Group IV share their valence electrons
with the nearest neighbouring atoms (figure
below), They form covalent bonds to attain a stable
filled orbital. Recall that the ability of any material to
conduct electricity depends on the behaviour of the
electrons in the outermost shells.

4. This property makes Group IV elements
relatively poor conductors of electricity. They
are called semiconductors. Semiconductors
are substances which have a resistance in
between that of conductors and insulators.
Electronic components made mainly of
semiconductors are called solid-state
electronic devices. Such components include
diodes and transistors.

5. Covalent bonds formed between Group IV atoms

6. Doped Semiconductor
The importance of semiconductors in today’s
electronic technology atoms from the fact that
their electrical properties are very sensitive to
small amounts of impurities. The process of
deliberately adding very small amounts of
impurities or foreign substances to an otherwise
pure substance is called doping, and the
impurities are referred to as dopants. Doping
results in an extrinsic semiconductor, or that
whose electrical properties depend upon the
presence of certain impurities.

7. Doping silicon with phosphorus. Phosphorus has one extra electron in its outermost
shell. Each of the four atoms of P participates in the bonding with nearby Si
atoms, leaving the extra electron weakly bounded. This electron is easily excited and
contributes to electrical conduction.

8. we increase the number of holes in intrinsic silicon, trivalent impurity atoms are
added. These are those atoms with three valence electrons such as Boron (B), indium
(in), and gallium (Ga). Each trivalent atom forms covalent bonds with four adjacent
silicon atoms. All three of the boron atom’s valence electrons are used in the
covalent bonds; and, since four electrons are required, a hole results when each
trivalent atom is added. Because the trivalent atom can taken an electron, it is often
referred to as an acceptor tom. The number of holes can be carefully controlled by
the number of trivalent impurity atoms added to the silicon. A hole created by this
doping process in not accompanied by a conduction free electron.

9. The purpose of doping is to increase the number of free
charges that can be moved by an applied voltage.
Silicon and germanium crystals are widely used in the
manufacture of semiconductor devices. They are
intrinsic semiconductors, or pure semiconductors
without doping. They belong to Group IV. Each element
has four outer electrons per atom and doping them
with an element with five outer electrons, such as
phosphorus (P), frees the fifth electron so that the
semiconductor has an excess electron. It is then known
as an n-type (negative-type) semiconductor, because
the major charge carriers are negative electrons.

10. Mixing the dopants of Group IV produces n-type
semiconductors. Take for example phosphorus and
silicon (Figure above).
Doping with an element that has only three outer
electrons, such as boron (B) or aluminium (AI),
produces a crystal lattice with spaces, known as holes,
which electrons from nearby atoms readily fill. The
type of semiconductor does not have free electrons,
which is equivalent to an excess of positive charges.
This is known as a p-type (positive-type)
semiconductor. Combining the dopants of Group III
elements with elements from Group IV results in p-
type semiconductors. Figure above shows silicon
doped with aluminium.

11. Doping an insulator like silicon into a viable
(although not so great) conductor is the basic
technique in the production of
semiconductors.
The n-type and p-type semiconductors can
be put together to form another important
electronic component- the diode.

12. Diodes
The diode is the simplest semiconductor
device. It allows a current to pass through it in
only one direction. It is produced when
crystals of pure silicon are doped so that a
junction is formed between p-type and n-type
regions. The p-type material meets an n-type
material across a narrow layer depleted of
charge carriers, known as the depletion layer.
Material in this layer conducts very poorly.

13. If the diode is connected to a battery so that
the negative terminal is joined to the n-type
semiconductor and the positive terminal to the p-
type semiconductor, electrons and holes can
cross the junction and produce current. Such a
junction is said to be forward-biased.
On the other hand, if the battery is connected
the other way around, so the its positive terminal
is connected to the n-type semiconductor and its
negative terminal to the p-type
semiconductor, the free electrons and holes are
forced away from the junction.

14. Practically no current results from this
connection. This is known as inverse-biased. A
diode oriented in this manner acts like a very
large resistor that is almost an insulator.
Diodes can therefore act as automatic
switches that can be turned on and off
whenever current. One major use of diodes is
to convert alternating current (AC) to direct
current (DC). Diodes built for this specific
purpose are called rectifiers.

15. A diode is also used to separate information from
transmitted radio signals or carrier waves. It
demodulates and detects the audio signals. Such a
diode is called a detector.
Diodes that detect and emit light are called light-
emitting diodes (LEDs). LEDs are often used as indicator
lights on videos and cassette players. Diode lasers that
emit narrow beams of coherent, monochromatic light
or infrared radiation are used in CD players and
supermarket bar-code scanners. They are compact and
powerful light sources.

16. Diagram of the forward and reverse-
biased
Forward-Biased: Positive end of the Reverse-Biased: Negative end of the
battery meets the positive end (the anode) battery meets the negative end (the
of the diode, causing current flow. cathode) of the diode, no current flow
occurs.