Basics of Semiconductors

A conductor is a material that offers very low resistance to the flow of electricity, when voltage is applied to it. An insulator is a material, that offers very high resistance to the flow of electricity, when voltage is applied to it.

Later, this device replaced it in the form of transistors, and reduced the size of the bulky computers and converted it into a low consumption device. This transistor is called silicon transistor and it was invented by Michael Faraday, in 1954.

As mentioned earlier, this material has conductivity level somewhere in between the extremes of an insulator and a conductor. The energy gap between the conduction band and valence band in an insulator is 6eV, which means that the electron in this energy gap, must attain 6eV energy to conduct electric current. This energy gap is called forbidden gap.

But this forbidden gap is not much in semiconducting materials as compared to the insulators and the value of this energy gap in semiconducting materials is 1eV. The conductors have an overlapping of the conduction band and valence band and that is the reason why conductors like copper conduct electricity easily.

The atoms of silicon and germanium have a definite pattern called crystal which is periodic in nature. The periodic arrangement of atoms in the materials is called lattice. The semiconducting materials like Germanium (Ge) and Silicon (Si) belong to 'group 4 elements' and the number of valence electrons in their outer shell is 4.

In order to complete the structure of inert gas, they will always try to complete the 8 electrons in the outermost orbit. So the four valence electrons in the semiconductors, in order to attain noble gas configuration, bonds with the four electrons from the neighboring atoms. This type of bonding is called covalent bonding.

This is a pure form of a semiconductor crystal. The valence electrons in the intrinsic devices, absorb heat energy, due to which the covalent bond breaks down, and the electrons become free.

Semiconducting materials have 'negative temperature coefficient', as their resistance decreases with increase in temperature, which is a unique phenomenon that occurs in semiconducting materials and not in conductors.

Semiconducting materials are not very good conductors but, when the valence electrons absorb energy from the natural sources like thermal energy, the kinetic energy of the valence electrons increases and they conduct well. In simple words, when we increase the temperature of such a material, its conductivity also increases.

When the properties of intrinsic semiconducting materials are altered by adding impurities to it, it becomes an extrinsic semiconducting material. These impurities can be pentavalent materials or trivalent materials.

The examples of pentavalent impurities are antimony, phosphorus, arsenic, etc., which are 'group 5 elements' and have 5 valence electrons in the outermost orbit.

Trivalent impurities such as boron, gallium and indium are 'group 3 elements' and have 3 electrons in the outermost orbit. These impurities are added in the ratio of 1:10⁶, i.e., in 10 million atoms of intrinsic semiconductors, we add 1 impure atom.

The extrinsic semiconducting material is the material which is formed after the addition of these impurities and the process of adding impurities to a pure semiconducting material like Si or Ge is called Doping.

Depending upon the type of impurity (pentavalent or trivalent) we add to the pure semiconducting material, the extrinsic semiconducting material is further classified into:

• N-type
• P-type

N-type: It is produced when a pentavalent impurity such as antimony or phosphorus is added to a pure semiconducting material like silicon or germanium. How is such a semiconducting material formed? This concept will be clear from the following example.

Antimony, a pentavalent impurity, is doped with an intrinsic semiconducting material, Silicon. Antimony is a group 5 element and has 5 electrons in its outermost shell and silicon is a group 4 element and has 4 electrons in its outermost orbit.

Covalent bonding is formed between the antimony and silicon, when the 4 valence electrons of silicon bond with 4 valence electrons of antimony. But, an excess electron of antimony (5 valence electrons), remains in the doped material, and takes part in the process of conduction and as it donates the electrons for current flow, it is called donor atom.

In N-type semiconducting materials, the majority carriers are electrons and the minority carriers are holes (positively charged).

P-type: It is produced when a trivalent impurity such as boron or indium is added to a pure semiconducting material like germanium. Let us take an example of how these are formed. Boron, a trivalent impurity, is doped with silicon to form an extrinsic P-type. Boron is a group 3 element and has 3 valence electrons in its outermost orbit.

After doping, these three valence electrons of boron bond with three valence electrons of silicon (4 valence electrons). In this case, there is a hole formed in boron atom, because boron has only 3 electrons and a positive charge is created in the atom, which is otherwise called lack of electron.

So, to fill this hole, an electron from the neighboring atom, moves to occupy this hole, and thereby current flow is produced. We know that flow of electrons causes electric current.

As the boron atom accepts the electron from the neighboring atom, it is called acceptor atom. In P-type semiconducting materials, holes are the majority carriers and electrons are the minority carriers.

Conventional current flows in the direction opposite to the flow of electrons. The direction of flow of holes is opposite to the direction of electrons. That is, the direction of the conventional current is same as the direction of flow of holes.

The mass-action law is the fundamental law of semiconductors which states that, "Under equilibrium, the product of the electron concentration and the hole concentration is always equal to the square of the intrinsic carrier concentration at a given temperature".

np = n_i²

where,
n = concentration of free electrons in thermal equilibrium
p = concentration of holes in thermal equilibrium
ni = intrinsic carrier concentration

★ These are the important devices used to produce basic electronic components like diodes, transistors, rectifiers, logic gates, MOSFETs (metal-oxide-semiconductor field-effect transistor), capacitors, thermistors, light-emitting diodes, etc.

★ Industrial Control System (ICS) uses such devices.

★ Conversion of electric power for electric rail roads is done by rectifier diodes, which is made of semiconducting materials.

★ All digital and analog circuits use such devices.

Distribution of sustainable electrical energy plays an important role in this technology. This technology is expanding its applications in biomedical electronics also.

Such devices are used in many fields like electronics and communication, automobile and industrial engineering, computers, etc. Without such devices even solid state devices would be not possible.