Conductivity of Materials: Conductors, Semiconductors, Insulators

Solid materials are used primarily in electronic devices to achieve electrical conductivity. These materials can be divided into insulators, semiconductors, and conductors, which differ in their potential to achieve electrical conductivity because they vary in resistance. The resistance of these materials is related to the tendency of their electrons to leave the valence band and go into conduction. Materials that make a good conductor are those that are easily influenced by outside energy and, therefore, can go into conduction with a minimal amount of energy. Materials that make a good insulator are not easily influenced by outside energy and, therefore, need a greater amount of energy to go into conduction. The energy needed for a semiconductor falls somewhere in between this range. This article discusses the differences between conductors, semiconductors, and insulators and the energy needed to place them into conduction.

Assume that we have a cubic centimeter block of an insulator, semiconductor, and conductor. If resistance is measured between opposite faces of each block, some very unique differences will be revealed. The insulator cube will measure several million ohms. The conductor cube, by comparison, will measure only 0.000001 Ω. The semiconductor cube will measure approximately 300 Ω. In effect, this shows a rather wide range of resistance differences in the three materials. All three materials can conduct, but only when a specific amount of energy is applied.

Energy-Level Diagrams

A convenient way to evaluate the relationship of conductors, semiconductors, and insulators is through the use of energy-level diagrams. This takes into account the amount of energy needed to cause an electron to leave its valence band and go into conduction. An energy-level diagram represents a composite of all atoms within the material. The valence band is located at the bottom and the conduction band is at the top of each diagram. The valence band represents the highest energy level that electrons can attain and still be influenced by the nucleus. Electrons in this band normally combine with valence-band electrons of other atoms to form molecules or compounds. A number of other electrons exist below the valence energy band. As a rule, we are only concerned with the response of valence and conduction band electrons in the operation of semiconductor devices. Electrons in the conduction band are not specifically bound to the nucleus and are free to move. Conduction band electrons have a higher energy level than the valence band electrons.

Figure 1. Energy-level diagrams for (a) insulators, (b) semiconductors, and (c) conductors

The valence band represents a composite energy level of the valence electrons of each atom. A specific amount of outside energy must be added to valence electrons to cause them to go into conduction. The area that separates the valence and conduction bands is referred to as the forbidden region. A certain amount of energy is needed to cause valence electrons to cross the forbidden region. If the energy is insufficient, electrons are not released for conduction. They will remain in the valence band.

The width of the forbidden region indicates the conduction status of a particular material. In atomic theory, the width of the forbidden gap is expressed in electron volts (eV). An electron volt is defined as the amount of energy gained or lost when an electron is subjected to a potential difference of 1 V. The atoms of each element have a specific energy-level value that permits conduction.

Insulators

The energy-level diagram of an insulating material has a very wide forbidden gap. A carbon crystal or natural diamond is an excellent insulator. Crystallized carbon has a forbidden gap of approximately 6 eV. The large width of this region keeps valence electrons from crossing the forbidden gap and going into conduction. For valence electrons to travel through the forbidden gap, they must acquire additional energy. The amount of energy needed by good insulators is generally very high. The energy level value that causes an insulator to go into conduction is often called breakdown voltage. 

Figure 2. A typical Application of an Insulating Material is to Provide Separation Between Conductors and Conductive Surfaces or Structures

Even the best insulators will go into conduction if sufficient energy is applied. Thyrite is an example of this condition. Thyrite is commonly used as a lightning arrester. At normal voltages, it is an ideal insulator. When it is subjected to high voltage, electrons cross the forbidden gap and go into conduction. This shunts the high voltage to ground, thus protecting a device from lightning. When good insulators are operated at high temperatures, the increased heat energy causes valence electrons to go into conduction.

Semiconductors

The forbidden gap of a semiconductor is much smaller than that of an insulator. Silicon, for example, needs to gain 1.21 eV of energy at absolute zero ($−273^{o}C$) to go into conduction. Energies of this magnitude are not easily acquired. As a result, the valence band remains full, the conduction band remains empty, and these materials respond as insulators. However, an increase in temperature causes the conductivity of this material to change. At normal room temperature or $−25^{o}C$, the valence electrons acquire thermal energy that is greater than the normal eV value. This essentially reduces the width of the forbidden gap and causes a semiconductor to be a conductor. This particular characteristic is extremely important in solid-state electronic devices.

Figure 3. Semiconductors are Commonly used to make Chips.

Conductors

The energy-level diagram of a conductor is quite unusual compared with other materials. In a sense, the valence band and conduction band are one and the same. Conductivity is explained as having an interaction of different energy levels of the valence band.

Figure 4. Conductors are used to Provide Electrical Paths

Studies show that the atoms of most metals and semiconductors are in the form of a crystal lattice structure. A crystal consists of a space array of atoms or molecules built up by regular repetition in a three-dimensional pattern. The energy levels of electrons in the crystal do not respond in the same manner as those of an individual atom. When atoms form crystals, the energy levels of the inner-shell electrons are not affected by the presence of neighboring atoms. However, the valence electrons of individual atoms are often shared by more than one atom.

The new energy level of valence electrons is found in a distinct band. The spacing between the energy levels of this band is very small compared with that of isolated atoms. Thus, electrons are free to absorb energy and to move from one point to another, conducting heat and electricity. In good conductors, the energy-level bands of valence electrons tend to overlap. This lowers the energy level of valence electrons and increases the electrical conductivity of the material.

Review Questions

1.              An energy level diagram shows the _____ band at the bottom and the _____ band at the top.

2.              The _____ band represents the highest energy level that electrons can attain and still be influenced by the nucleus.

3.              The _____ gap or region separates the valence band and conduction bands of an energy-level diagram.

4.              The width of the forbidden region of an energy-level diagram is expressed in _____.

5.              The energy-level diagram of a(n) _____shows a wide forbidden gap.

6.              The energy-level diagram of a(n) _____shows a narrow-forbidden gap.

7.              In the energy-level diagram of a(n) _____, the valence band and conduction band overlap.

8.              The energy that causes an insulator to go into conduction is called _____ voltage.

Answers

1.              valence, conduction

2.              valence

3.              forbidden

4.              electron volts

5.              insulator

6.              semiconductor

7.              conductor

8.              breakdown