INTRODUCTION TOSEMICONDUCTORS MATERIAL

Chapter 1

Objectives

• Discuss basic structures of atoms

• Discuss properties of insulators, conductors, and

semiconductors

• Discuss covalent bonding

• Describe the conductions in semiconductor

• Discuss N-type and P-type semiconductor

• Discuss the diode

• Discuss the bias of a diode

EKT 102: Basic Electronic Engineering

Lecture’s Content

1.1 Atomic structure

1.2 Semiconductor, conductors and insulators

1.3 Covalent bonding

1.4 Conduction in semiconductors

1.5 N-type and P-type semiconductors

1.6 Diode

1.7 Biasing the diode

1.8 Voltage-current characteristic of a diode

1.9 Diode models

1.10 Testing a diodeEKT 102: Basic Electronic Engineering

1.1 Atomic Structure

ATOM

Basic structure

Atomic number

Electron shells

Valence electron

Ionization

EKT 102: Basic Electronic Engineering

1.1 Atomic Structure (cont.)

The Atom

1. Atom is the smallest particle of an element that retains the characteristics of that element.

2. An atom consists of the protons and neutrons that make up the nucleus (core) at the center and electrons that orbit about the nucleus.

The nucleus carries almost the total mass of the atom.

Figure 1: The Bohr model of an atom

EKT 102: Basic Electronic Engineering

1.1 Atomic Structure (cont.)

Carry negative charge

Carry positive charge

Neutral(carry no charge)

EKT 102: Basic Electronic Engineering

Figure 2:

1.1 Atomic Structure (cont.)

EKT 102: Basic Electronic Engineering

++ +

Proton+ Electron Neutron

NucleusNucleus

Figure 3: Bohr model of hydrogen and helium

(a) Hydrogen (b) Helium

Num. of protons = Num. of electrons Electrically balanced (neutral) atom

1.1 Atomic Structure (cont.)

EKT 102: Basic Electronic Engineering

Figure 4: The periodic table of the elements

1. Element in periodic table are arranged according to atomic number.

2. Atomic number equals number of protons in nucleus which is the same as the number of electron in an electrically balanced atom .

1.1 Atomic Structure (cont.)

EKT 102: Basic Electronic Engineering

Electrons and Shells

1. Electrons near the nucleus have less

energy than those in more distant

orbits.

2. Each distance (orbit) from the

nucleus corresponds to a certain

energy level.

3. In an atom, the orbits are grouped

into energy levels = shells.

4. A given atom has a fixed number of

shells and each shell has a fixed

maximum number of electrons.

Figure 5: Bohr model of the silicon atom

1.1 Atomic Structure (cont.)

EKT 102: Basic Electronic Engineering

Valence shell

+ -- -

-

--

-

-

-

--

--

-

-

-

-

-

-

-

--

--

-

-

-

-

-

29 n

29 p

Shell 1

Shells or orbital paths

Valence electron

Valence Electron1. Valence shell is the outermost

shell in an atom that determines the conductivity of an atom.

2. The electrons in valence shell are called valence electrons.

3. Valence electrons have higher energy and are less tightly bound to the atom.

Figure 6: Bohr model of copper atom (Cu)

Shell 2

Shell 4

Shell 3

1.1 Atomic Structure (cont.)

EKT 102: Basic Electronic Engineering

The Maximum Number of Electrons in Each Shell

1. The maximum number of electrons (Ne) in each shell is calculated using formula below:

where = number of shell.

2. Example for the copper atom (Cu) shell :

22nNe

Atomic number = 29 = 29 electrons

1st shell :

2nd shell :

3rd shell :

4th shell :

2+8+18=28

1.1 Atomic Structure (cont.)

EKT 102: Basic Electronic Engineering

Ionization

1. When atom absorb energy (e.g heat source) the energies of the electron are raised.

2. Valence electron obtain more energy and more loosely bound to the atom compared to the inner electron.

3. If a valence electron acquires sufficient energy – escape from the outer shell and the process of losing valence electron called ionization.

4. The resulting positively charged atom is called a positive ion.

5. The escape electron is called free electron.

6. On the other hand, the atom that has acquired the extra electron is called a negative ion.

7. This can occur in certain atoms when a free electron collides with the atom and is captured (e.g ).

+Nucleus

Figure 6: Ionization of hydrogen atom (H)

-+

Nucleus

1.2 Semiconductors, conductors and insulators

EKT 102: Basic Electronic Engineering

Material

Insulators Semiconductors Conductors

In terms of electrical properties

All materials are made up of atoms that contribute to its ability to conduct electrical current

1.2 Semiconductors, conductors and insulators

EKT 102: Basic Electronic Engineering

1. An atom can be represented by the valence shell and a core.

2. A core consists of all the inner shells and the nucleus.

+6 for nucleus, -2 for the two inner-shell electrons(net charge +4)

Valence shell = -4e

Inner-shell = -2e

Nucleus: 6 protons 6 neutrons

Figure 7: Diagram of a carbon atom

1.2 Semiconductors, conductors and insulators (cont.)

EKT 102: Basic Electronic Engineering

Insulators Material does not conduct electrical current (e.g rubber, plastic, glass). Valence electron are tightly bound to the atom – very few free electron.

Conductors Material that easily conducts electrical current. The best conductors are single-element material (e.g copper, silver, gold, aluminum). Only one valence electron very loosely bound to the atom - free electron.

Semiconductors Material between conductors and insulators in its ability to conduct electric current. In its pure (intrinsic) state is neither a good conductor nor a good insulator. Most common semiconductor - silicon, germanium, and carbon which contains four

valence electrons.

1.2 Semiconductors, conductors and insulators (cont.)

EKT 102: Basic Electronic Engineering

Band gap - the difference between the energy levels of any two orbital shells. Band - another name for an orbital shell (valence shell = valence band). Conduction band – the band outside the valence shell where it has free electrons.

Figure 8: Energy diagram for three types of materials

Band Gap

The amount of energy that a valence electron must have to jump from the valence band to conduction band.

1.2 Semiconductors, conductors and insulators (cont.)

EKT 102: Basic Electronic Engineering

Band Gap (cont.)

Figure 10: Energy band diagram for an unexcited (no external energy) atom in a pure (intrinsic) Si crystal.

Conduction band

Energy

E3 = 0.7eV

E1

E2

E4 = 1.8eVEnergy gap

E = energy level

Valence band

1.2 Semiconductors, conductors and insulators (cont.)

EKT 102: Basic Electronic Engineering

Comparison of a Semiconductor Atom to a Conductor Atom

1. A valence electron in Si atom feels an attractive

force of +4 compared to Cu atom which feels an

attractive force of +1.

2. Force holding valence electrons to the atom in

Si > in Cu.

3. The distance from its nucleus of Cu’s valence

electron (in 4th shell) > Si’s valence electron (in

3rd shell).

4. The valence electron in Cu has more energy than

the valence electron in Si.

5. Easier for valence electrons in Cu to acquire

enough additional energy to escape from their

atoms and become free electrons than in Si.

Figure 11: Bohr diagrams of the silicon and copper atoms.

14 protons – 10 electrons

29 protons – 28 electrons