12th chemistry solid state-Electrical Properties

Electrical Properties

Conduction of Electricity in Metals

• Metals conduct electricity in molten state.
• The conductivity of metals depends upon the number of valence electrons.
• In metals, the valence shell is partially filled, so this valence band overlaps with a higher energy unoccupied conduction band so that electrons can flow easily under an applied electric field.
• In the case of insulators, the gap between filled valence shell and the next higher unoccupied band is large so that electrons cannot jump from the valence band to the conduction band.

Conduction of Electricity in Semiconductors

• The gap between the valence band and conduction band is so small that some electrons may jump to the conduction band.

• Electrical conductivity of semiconductors increases with increase in temperature.
• Substances like Si, Ge show this type of behaviour, and are called intrinsic semiconductors.
• Doping − Process of adding an appropriate amount of suitable impurity to increase conductivity

• Doping is done with either electron-rich or electron-deficient impurity as compared to the intrinsic semiconductor Si or Ge.

• There are two types of semiconductors:

1. n − type semiconductor
2. p − type semiconductor

• n − type semiconductor

• Conductivity increases due to negatively charged electrons
• Generated due to the doping of the crystal of a group 14 element such as Si or Ge, with a group 15 element such as P or As

• p − type semiconductor

• Conductivity increases as a result of electron hole
• Generated due to the doping of the crystal of a group 14 element such as Si or Ge, with a group 13 element such as B, Al or Ga

• Applications of n − type and p − type semiconductors

• In making a diode, which is used as a rectifier
• In making transistors, which are used for detecting or amplifying radio or audio signals
• In making a solar cell, which is a photo diode used for converting light energy into electrical energy

• A large number of compounds (solid) have been prepared by the combination of groups 13 and 15 or 12 and 16 to stimulate average valence of four as in Si or Ge.

• Examples of compounds of groups 13 − 15 are InSb, AlP, GaAs
• Examples of compounds of groups 12 − 16 are ZnS, CdS, CdSe, HgTe

• Some transition metal oxides like TiO, CrO₂, ReO₃ behave like metals.

• For example, ReO₃ resembles metallic copper in its conductivity and appearance
• Some oxides like VO, VO₂, VO₃, TiO₃ show metallic or insulating properties depending on temperature.

Do you know?

Polyacetylene, an organic compound shows conductivity when exposed to iodine vapours.