Page 18 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Semiconductor theory-Active and passive components

Objectives: At the end of this lesson you shall be able to
• explain atom conductor, semiconductor, insulator and atomic structure
• state the function of N and P type semiconductor, PN junction, depletion region
• state the coding of semiconductor devices and its meaning
• explain active and passive components, symbols - uses.

Atom
The very tiny fundamental unit of an element which is capable of independent existence is the atom. An atom of
any element consists of a central core called Nucleus. A number of small particles called electrons move around
the central core.
The nucleus contains protons and neutrons. A proton in the nucleus possess a positive electrical charge. An
electron in an atom possess negative electrical charge. In normal state, the atom is electrically neutral, that is the
number of electrons is equal to the number of protons in the nucleus.
Difference between conductors insulators and semi conductors: We are familiar with conducting and insulating
materials. Conducting materials are good conductors of electricity. Insulating materials are bad conductors of
electricity. There is another group of materials called as semiconductors, such as germanium and silicon. These
are neither good conductors nor good insulators.
The conductors on valence electrons are always free. In an insulator the valence electrons are always bound.
Whereas in semi conductors the valence electrons are normally bound but can be set free by supplying a small
amount of energy. Several electronic devices are made using semi conductor materials.
Semi conductors - Atomic structure: Germanium (Ge) and silicon (Si) are examples of semi conductors. Fig 1a
shows a germanium atom. In the centre is a nucleus with 32 protons. 32 revolving electrons are distributed
themselves in different orbits. There are 2 electrons in the first orbit, 8 electrons in the second orbit, and 18
electrons in the third orbit. The fourth orbit is the outer or valence orbit which contains 4 electrons.
Fig 1b shows a silicon atom. It has 14 protons in the nucleus and 14 electrons in 3 orbits. There are 2 electrons
in the first orbit and 8 in the second orbit. The remaining 4 electrons are in the outer or valence orbit.

Fig 1

In semiconductor materials, the atoms are arranged in an orderly pattern called a crystal lattice structure. If a
pure silicon crystal is examined we find that the four electrons in the outer (valence) shell of an atom is shared
by the neighbouring atoms as in Fig 2.

The union of atoms sharing the valence electrons is called a covalent band. That means a valence electron being
shared by two adjacent atoms. Each atom appears to have a full outer shell of eight electrons.
Types of semiconductors : A pure semiconductor is called an intrinsic semiconductor. For example, a silicon
crystal is an intrinsic semiconductor because every atom in the crystal is a silicon atom. One way to increase
conductivity in a semiconductor is by ‘doping’. This means adding impurity atoms to an intrinsic semiconductor.
The doped semi-conductor is known as an extrinsic semiconductor.

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CITS : Power - Electrician & Wireman - Lesson 60-69