Conduction Band

A conduction band is a delocalized band of energy levels in a crystalline solid that is partially filled with electrons. These electrons are highly mobile and are responsible for electrical conductivity. However, before we learn about conduction band we will understand the band theory.

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Band Theory

A useful way to visualize the distinction between conductors (metal), insulators and semiconductors is to plot out their energies for electrons within the material. Rather than having distinct energies, as within the case of free atoms, the energy states which are available form bands.

Crucial to the conductivity method is whether or not or not there ar electrons inside the conductivity band. In insulators, the electrons inside the valence band are separated by a large gap from the conduction band, in conductors like metals the valence band overlaps the conduction band, and in semiconductors, there is a very little enough gap between the valence and conduction bands that thermal or totally different excitations can bridge the gap.

With such a small gap, the presence of a less proportion of a doping material can increase conduction dramatically. A vital parameter inside the band theory is that of the Fermi level, the highest of the accessible electron energy levels at lower temperatures. The position of the Fermi level in relevance to the conductivity band might be important to place confidence in crucial electrical properties.

Energies for electrons

What is Conduction Band?

The conduction band is the band of electron orbitals that electrons can bounce up into from the valence band when energized. At the point when the electrons are in these orbitals, they have enough energy to move freely in the material. This movement of electrons makes an electric current to flow. The valence band is the furthest electron orbital of a particle of a particular material that electrons involve.

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The energy distinction between the highest occupied energy state of the valence band and the least abandoned condition of the conduction band is known as the bandgap and is demonstrative of the electrical conductivity of a material. An enormous bandgap implies that a great deal of vitality is required to energize valence electrons to the conduction band. Then again, when the valence band and conduction band cover as they do in metals, electrons can promptly bounce between the two groups (see Figure 1) which means the material is profoundly conductive.

The electrons in this energy band can expand their energies by going to higher energy levels inside the band when an electric field is applied to quicken them or when the temperature of the crystal is increased. These electrons are called conduction electrons, as particular from the electrons in filled energy bands, which, all in all, don’t add to electrical and thermal conduction.

Conduction Band in Semiconductors and Metallic Conduits

In metallic conduits, the conduction electrons compare to the valence electrons (or a bit of the valence electrons) of the constituent molecules.

In semiconductors and inductors at adequately low temperatures, the conduction band has no electrons. Conduction electrons originate from thermal excitation of electrons from a lower energy band or from impurity atoms in the crystal.

Valence Band and Conduction Band

Here are some of the differences between a valence band and conduction band.

Conduction Band Valence Band
Higher energy level band Energy band formed by a series of energy levels containing valence electrons
Partially filled by the electrons. Always filled with electrons
Empty band of minimum energy. Band of maximum energy
Electrons can gain energy from the external electric field. Electrons are not capable of gaining energy from the external electric field.
The free-electron is able to move anywhere within the volume of the solid. No flow of current due to electrons present in this band.
Current flows due to such electrons. The highest energy level which can be occupied by an electron in the valence band at 0 K is called the Fermi level.

Forbidden Band

The forbidden band is the energy gap between a conduction band and valence band. Some of its characteristics include;

  • No free electron is present.
  • No Energy.
  • ΔEg=(C.B.)min−(V.B.)max
  • The Width of the forbidden energy gap depends upon the nature of the substance.
  • As temperature increases (↑), the forbidden energy gap decreases (↓) very slightly.

Understanding Related Terms

Conduction Electrons

Electrons are allowed to move inside a solid. The movement of these electrons can offer ascent to the conduction of power by making an electric flow through the solid. The energy diagram of an intrinsic semiconductor found to follow this author’s convention of representing conduction electrons by green circles. The blue circles speak to firmly bound electrons that don’t altogether add to conduction through a strong.

Conduction Electrons

Conductor

A material with low resistivity utilized for contacts and interconnects in semiconductor preparing. Conductors have a partially filled valence band, through which electrons can move freely, as appeared in the energy diagram to the left. In this way, in a conductor, the conduction band is equivalent to the valence band, and the charge bearers are fundamentally electrons. In keeping with this author’s convention, the electrons in this semi filled conduction band are shown in green circles.

Conductor

Conductivity (s)

The measure of how freely current can course through a material. Copper, with its high conductivity of 5.95 x 107 W-1m-1, conveys electric flow more freely than does aluminium, with its marginally lower conductivity of 3.77 x 107 W-1m-1. Conductivity is the converse of resistivity r:

s = 1/r.

Effective Density

The measurement of conductivity involves two parameters: carrier concentration and mobility of charge carriers involved.

Carrier concentration

The carrier concentration is usually determined by the effective density of states N∗. It represents the density of states located at conduction band edge Ec or valence band edge Ev. The conduction band electron concentration is, therefore the N*c at Ec times the Fermi-Dirac distribution (probability of occupancy).

Here m*c is the density of states effective mass. For Si m*c=1.1mo.Here the effective mass m*c is different from the conductivity effective mass m*n. Both are not the same one uses the geometric mean and the other uses harmonic mean of band curvature effective masses.

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Mobility

It describes the ease with which charge carriers drift in the material

Where m*n is the conductivity effective mass and is equal to 0.26mo for Si, and τ is the mean time between scattering events.

Conductivity depends on the above two quantities, for electrons it can be expressed as

The derivation is straight forward. But the key point is here the effective mass and is not the same for the density of states and charge transport. The following are the effective masses for reference

The Density of states effective mass: 1.1mo for Si; 0.067mo for GaAs

Conductivity effective mass: 0.26mo for Si; 0.067mo for GaAs

Conduction Band Questions

1. What is the value of the effective density of states function in the conduction band at 300k?

a) 3*1019 cm-3

b) 0.4*10-19 cm-3

c) 2.5*1019 cm-3

d) 2.5*10-19 cm-3

Answer: c

Explanation: Substituting the values of mn=m0, h=6.626*10-34J/s, k=1.38*10-23 and T=300K, we get

Nc =2.5*1019 cm-3.

2. Electrons from valence band rise to conduction band when the temperature is greater than 0 k. Is it True or False?

a) True

b) False

Answer: a

Explanation: As the temperature rises above 0 k, the electrons gain energy and rises to the conduction band from the valence band.

3. The thermal equilibrium concentration of the electrons in the conduction band and the holes in the valence band depend upon?

a) Effective density of states

b) Fermi energy level

c) Both A and B

d) Neither A nor B

Answer: c

Explanation: The electrons and holes depend upon the effective density of the states and the Fermi energy level.

4. What is the SI unit of conductivity?

a) Ωm

b) (Ωm)-1

c) Ω

d) m

Answer: b

Explanation: The formula of the conductivity is the σ=1/ρ.

So, the unit of resistivity is Ωm.

Now, the unit of conductivity becomes the inverse of resistivity.

5. In an insulator, the forbidden energy gap between the valence band and conduction band is of the order of:

A) 1MeV

B) 0.1MeV

C) 1eV

D) 5eV

Answer: D

Explanation: In insulators, the forbidden energy gap is largest and it is of the order of 6 eV.

6. The forbidden energy bandgap in conductors, semiconductors, and insulators are EG1, EG2, and EG3 respectively. The relation among them is:

A) EG1=EG2=EG3

B) EG1<EG2<EG3

C) EG1>EG2>EG3

D) EG1<EG2>EG3

Answer: B

Explanation: In insulators, the forbidden energy gap is very large, in the case of semiconductor it is moderate and in conductors, the energy gap is zero.

7. The forbidden gap in the energy bands of germanium at room temperature is about:

A) 1.1eV

B) 0.1eV

C) 0.67eV

D) 6.7eV

Answer: C

Explanation: ΔEg (Germanium) = 0.67eV