EEC 124 Electronics I Lecture note Pdf Download

Concept of Semiconductors Materials Week 1

1.1 MATTER
All matter is made of atoms, an atom has two parts, nucleus and the electron cloud (see fig 1.1.)
Electrons are the smallest and lightest particles. They carry a negative electric charge. The electron
cloud contains one or more electrons which are moving at high speed around the nucleus. Each
electron carries the same amount of negative charge. The nucleus consists of one or more particles.
The particles in the nucleus are of two kinds.
Protons: A proton is about 1840 times more massive than an electron. It carries a positive
electric charge that is equal in size but opposite in sign to the charge carried by an electron.
Therefore all protons carry the same amount of positive charge.
Neutrons: A neutron has about the same mass as a proton but carries no electric charge. Modern
physics has shown that there are other kinds of participle. It also tells us that protons, neutrons,
electrons and the other particles may really be made of even smaller particles. To understand
electronics we need to know about protons, neutrons and electrons, but not about the other kinds of particle.

1.2 Fundamental of Electricity
1.2.1 Electron and Protons
All solids, liquids and gases are principally made up of two basic types of participles known as
electrons and protons. The electron is the smaller of the two; the proton is 1840 times more massive
than the electron. The electron carries a negative electrical charge. The proton carries an equal and
opposite positive charge.
When a material is in an uncharged state, it contains as many protons as it does electrons. However,
if we remove some electrons from the material, the net positive charge on it exceeds the remaining
negative charge so that the material exhibits a net positive charge. This phenomenon can be
experienced by anyone wearing clothes manufactured from man-made fiber; while the garment is
being worn, some electrons transfer to the wearer, and the static charge built up in this way may
cause the wearer to experience an electrical shock during removal of the garment.

1.2.2 Basic Atomic Structure EEC 124 Electronics I Lecture note Pdf Download

All atoms have broadly the same type of structure, with the heavier protons forming the nucleus,
around which the electrons orbit (see fig 1.2). The electrons orbit in distinct layers or shells.
The radius of the orbit depends on the balance between two forces: the mechanical outward force
on the electron due to its motion and the inward electrostatic pull between the positive charge on the
nucleus and the negative charge on the electron. The shell in which an electron finds itself depends
on its energy; a high –energy electron orbits in a shell further away from the nucleus than does a low-energy electron.

1.3 Fermi Energy Levels

The outer electrons of a single atom can jump from one energy level to another as they receive or
give off quantities of energy. Unexcited electrons are in the valance band. Excited electrons are in
one of the conduction bands. Only one such band is shown in fig 1.3 but atoms of all elements have
several higher conduction bands. Electrons gaining energy jump from a conduction band to one of
even higher energy. If they drop from one conduction band to one at a lower level, they lose energy
as they do so.
Fig. 1.3 Electron Energy Levels: (a) in Non-Conductors; (b) and (c) in Conductors; (d) in ntype Semiconductors; (e) in p-type Semiconductors
In a block of conducting material we can imagine many millions of atoms with electrons continually
gaining or losing energy. They are continuously jumping up or down from one band to another. If
the atoms are mainly unexcited, most electrons will be in the valence band and a few in the lower
conduction bands. As the atoms become more excited – by heating them for example – we find
fewer electrons in the valence band, most in the lower conduction bands and a few in the higher
conduction bands.
The greater the average energy of the electrons, the more we find the higher conduction bands. We
need some way of saying where most of the electrons are and some way of saying what their
average energy is. We can show where electrons are by adding to the energy level diagrams, as in
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EEC 124 Electronics I Lecture note Pdf Download