Classification of Metals, Conductors and Semiconductors

Solids can be classified as metals, semiconductors or insulators based on conductivity or resistivity and energy bands in electronics. This helps us understand the band theory and the importance of valence and conduction bands in solids. Let us also study the types of semiconductor.

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Classification Based on Conductivity

The relative values of electrical conductivity (σ) and resistivity (ρ=1/σ) help classify solids as:

• Metals – These are solids which have very low resistivity or very high conductivity). Hence,
  σ ~ 10² – 10⁸ S/m
  ρ ~ 10^-2 – 10^-8 Ωm

• Insulators – These are solids which have very high resistivity or very low conductivity. Hence,
  σ ~ 10^-11 – 10^-19 S/m
  ρ ~ 10¹¹ – 10¹⁹ Ωm

• Semiconductors – These are solids which have resistivity or conductivity values between those of metals and insulators. Hence,
  σ ~ 10⁵ – 10^-6 S/m
  ρ ~ 10^-5 – 10⁶ Ωm

It is important to note that these values are indicative and could go outside the range as well. Let’s shift our focus to semiconductors. These can be of the following types:

Types of Semiconductor

 

1. Elemental semiconductors – available naturally like Silicon (Si) and Germanium (Ge)
2. Compound semiconductors – made by compounding two or more metals together. They are sub-divided into these categories:
   1. Inorganic; like CdS, GaAs, CdSe, etc.
   2. Organic; like anthracene, doped phthalocyanine, etc.
   3. Organic polymers; like polypyrrole, polyaniline, polythiophene, etc.

Most semiconductor devices available are based on elemental and compound inorganic semiconductors. Eventually, organic semiconductors were used and semiconducting polymers were developed.

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Classification Based on Energy Bands

Bohr’s model of the atom states that the energy of an electron is determined by the orbit in which it revolves. This is true for an isolated atom. In a solid, atoms are close together. Therefore, electrons from neighbouring atoms come very close even overlap at times. Hence, the motion of an electron is different in a solid as compared to an isolated atom.

In an atom, electrons in the innermost orbits, which are filled are called Valence electrons. On the other hand, electrons in the outer orbits that do not fill the shell completely are called Conduction electrons.

In a crystal, every electron has a unique position and different pattern of surrounding charges. Hence, each electron has a different energy level. These energy levels with a continuous variation of energy form the Energy bands. The energy band which includes the energy levels of the valence electrons is called Valence band. Also, the energy band above it is called Conduction band. If there is no external energy supplied, then the valence electrons stay in the valence band.

Now, electrons move from the valence band to the conduction band when the lowest level in the conduction band is lower than the highest level in the valence band. Usually, the conduction band is empty.

Classification of Metals, Semiconductors, and Insulators

• In case of metallic conductors, conduction band overlaps on the electrons in the valence band.
• In insulators, there is a large gap between both these bands. Hence, the electrons in the valence band remain bound and no free electrons are available in the conduction band.
• Semiconductors have a small gap between both these bands. Some valence electrons gain energy from external sources and cross the gap between the valence and conduction bands. By this movement, they create a free electron in the conduction band and a vacant energy level in the valence band for other valence electrons to move. This creates the possibility of conduction.

Let’s understand this better with the following diagrams and explanations:

Case 1.

E_c denotes the lowest energy level of the conduction band and E_v denotes the highest energy level of the valence band. The above diagram describes a solid where the valence band is partially empty. Hence, electrons from the lower energy levels can move to the higher levels making conduction possible. Also, the resistivity of such solids is low or the conductivity is high.

Case 2.

E_c and E_v are the same as case 1 and E_g is the energy gap. This diagram describes a solid where the conduction and valence bands overlap each other. Hence, electrons can easily move from the valence to the conduction band. This makes a large number of electrons available for conduction. Also, the resistivity of such solids is low or the conductivity is high.

Case 3.

This diagram describes a solid where the energy gap (E_g) is very large (>3 eV). Due to this large gap, electrons cannot be excited to move from the valence to the conduction band by thermal excitation. Hence, there are no free electrons in the conduction band and no conductivity. These are insulators.

Case 4.

This diagram describes a solid where the energy gap is small (< 3 eV). Since the gap is small, some electrons acquire enough energy even at room temperature and enter the conduction band. These electrons can move in the conduction band increasing the conductivity of the solid. These are semiconductors. The resistivity of semiconductors is lower than that of insulators but higher than that of metals.

Solved Examples for You

Question: Name the different types of semiconductors with examples.

Solution: Semiconductors are of the following types:

• The ones available naturally like Silicon and Germanium – Elemental Semiconductors. Also, the ones made by compounding two or more metals together – Compound Semiconductors.
• Further, compound semiconductors are of three types – inorganic (CdS, GaAs, etc.), organic (anthracene, etc.) and organic polymers (polypyrrole, polyaniline, etc.).

Question: What are valence and conduction bands?

Solution: In a crystal, every electron has a different energy level. These different energy levels along with the continuous variation of energy are called Energy bands. The energy band which includes the energy levels of the valence electrons is called the valence band. And the band above it is called the conduction band.