Semiconductor Electronics: Materials, Devices and Simple Circuits

Notes

1. Electric Current

I = neAv_d

This current is possible only due to the flow of free electrons.

Resistance: R

Conductance: G

Resistivity: ρ

Conductivity: σ

2. Classification of Materials

According to Resistivity

• Metals: 10⁻² – 10⁻⁸ Ωm
• Semiconductors: 10⁵ – 10⁻⁶ Ωm
• Insulators: 10¹¹ – 10¹⁹ Ωm

3. Semiconductor Materials

1. Elemental:

Si and Ge

1. Compound:
   • Organic – Anthracene
   • Inorganic – CdS, GaAs, CdSe, InP, etc.
   • Organic polymers – Polypyrrole, polyaniline, etc.

4. Concept of Energy Bands

For silicon:

Si → 2, 8, 4

For germanium:

Ge → 2, 8, 18, 4

The electrons present in the outermost shell are called valence electrons.

Energy Level

For a number of atoms, electrons have unequal energy due to unequal distance from the nucleus.

Electrons are bound by centripetal force and Coulomb force.

Electrons have unequal potential energy due to unequal distance from the nucleus in their orbits.

Electrons have unequal kinetic energy due to unequal velocity.

For a number of atoms:

5. Energy Bands in Solids

For one atom → 1 electron.

For n atoms → n electrons.

After making a covalent bond:

Conduction Band

After applying heat as a source to increase the energy of valence electrons, the valence electrons are excited to a set of bands which is called the conduction band.

Valence Band

The band of different energy levels of valence electrons is called the valence band.

Energy Gap

The region between the conduction band and valence band is called the energy gap.

In the energy gap, electrons are not allowed.

In a metal, the conduction band is equal to the valence band.

6. Types of Semiconductors

1. Intrinsic semiconductor
2. Extrinsic semiconductor

7. Intrinsic Semiconductor

A pure semiconductor is called an intrinsic semiconductor, i.e. no kind of atoms are present.

At room temperature, when temperature increases, electrons present in atoms are given thermal energy.

The associated temperature and higher energy states are gained by the electrons.

It is called the valence level or band. At this stage, it is said to be the valence band completely filled.

The electrons jump to another band after gaining energy. It is called the conduction band.

After jumping from the valence band to the conduction band, a vacant place is left behind. This vacant place is called a hole.