Introduction of Semiconductor Devices

SEMICONDUCTOR DEVICES

INTRODUCTION

An electronic device is essentially a device in which electrons flow through a vacuum or gas or semiconductor. An atom is composed of charged particles namely electrons and protons. The nucleus consists of a number of neutral particles called neutrons and a number of positively charged called protons. The electron spirals towards the nucleus and when an electron jumps from higher orbits to lower orbits, the radiation of energy takes place. The electrons in the innermost shells are very strongly attached to the atom and have the minimum energy. Hence, these electrons cannot be easily separated.

ENERGY-BAND THEORY

An electron revolving around the nucleus of an atom has potential energy, boller centrifugal energy, magnetic energy and rotational energy. These energies determine the total energy or energy level of an electron, measured as electron volt.

An electron volt can be defined as the amount of energy required to move an electron through a potential of one volt i.e., the amount of energy gained or lost when an electron falls through or moves against the potential difference of one volt.

1 eV = 1.6 x 10^-19 Joules

Each shell has an energy level. An electron in the outermost orbit is loosely bound to the nucleus and has higher energy. Similarly, the electron revolving in the inner shell very close to the nucleus is very tightly bound and has only a small amount of energy. Thus, it cannot be easily isolated from an individual atom.

Hence, the valence electrons in the outermost shell can be easily extracted from the atom. These isolated electrons involve in chemical reactions and bond the atoms together.

Covalent Bond

The valence electrons form a bond with the valence electrons of an adjacent atom. This bond is called covalent bond.

Energy Band

The valence electrons having highest energy level form the covalent bonds. The energy levels of the corresponding valence electrons merge into each other due to the coupling between valence electron. This creates an energy band i.e., closely spaced energy levels.

Energy-Gap

The difference between the energy levels of any orbits is called as energy gap.

In practice, only two bands of energy levels are considered. They are valence band and conduction band.

Valence Band

The valence electrons combine together to form the covalent bonds. The energy band formed due to merging of energy levels related with the free electrons is called valence band.

Conduction Band

When energy is applied to the valence electrons in the covalent bond, these valence electrons escape from the bond and the electrons become free to conduct. The energy band due to merging of energy levels associated with the free electrons is called conduction band.

Forbidden Energy Gap

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

ENERGY BAND DIAGRAM

The graphical representation of the energy bands in a solid is called energy band diagram.

The materials are classified into three types depending on the energy band diagram.

Insulator

In insulator, the forbidden energy gap is very large between the conduction band and the valence band. Hence large electric field is required to remove the electrons from valence band to conduction band. Practically, it is impossible for an electron to jump from valence band to conduction band. The number of free electrons in the insulator is very small in the order of 107 electrons/m³. The forbidden energy gap is very large about 7 eV. For example, in diamond, the energy gap is 6 eV. Fig. 1.1 shows the energy band diagram of an insulator.

In general, Insulator has

i. Full valence band

ii. Empty conduction band

iii. Large energy gap

iv. Negative temperature co-efficient i.e when temperature is increased some valence band electrons acquire energy and moves to the conduction band.

Properties

i. Conductivity is poor

ii. Rigid, crystalline nature

iii. High melting and boiling temperature

iv. Unidirectional

Conductors

A conductor is a material that easily conduct or pas the current. In conductor, the valence band and conduction band, overlap together. Example: copper, silver, gold, Aluminium.

The atoms of the above material consists of only one valence electron which is loosely bound to the atom.

These valence electrons can be easily broken away from their atom and become free electrons ready for conduction, and enter into the conduction band.

Thus the valence and conduction bands overlap with each other and there is no forbidden energy gap.

Fig.1.2 shows the energy band diagram of conductor.

Properties

i. Good Conductivity

ii. Rigid, crystalline nature

iii. Low melting and boiling temperature

iv. Non-directional

Semiconductor

Semiconductor is a material whose electrical properties lies in between that of insulators and conductors.

They have partially filled conduction band and valence band with narrow energy between the two bands, Ex. Silicon, Germanium, Gallium Arsenide etc.

At OK there are no electrons in the conduction band and the valence band is completely filled. Thus it behaves as an insulator.

When the temperature is increased, some of the electrons in the valence band enter into the conduction band. Thus it behaves as conductor.

The electrical conductivity of the semiconductor lies in the range of 10-3 to 10-6 per ohm per cm.

Fig.1.3 shows the energy band diagram of semiconductors.

Properties

i. Conductivity can be increased by doping.

ii. Low melting and boiling temperature.

iii. Rigid, crystalline nature.

iv. Directional.

Semiconductors are classified into two types