We are going to be discussing metals, semiconductors and Insulators using the Energy Band Theory in such a way that you will understand every bit of it. Let's get right into it.
The Electronic theory that can be used to distinguish Insulators, Semiconductors and conductors is called Energy Band Theory. It has three part; The Valence band, Conduction band and the Forbidden band. The Lowest available energy band where electrons can occupy is called the valence band. Forbidden band is the region where electrons can not occupy. Conduction band is the band where there are presence of free electrons that are responsible for conduction. Let's break them one after the other.
In an Insulating material, the forbidden band is very wide, as a result the free electrons in the valence band will not be able to jump into the conduction band thereby making it difficult to conduct electricity. It can be seen from the diagram above.
These are a class of materials with a narrow forbidden band between the valence and Conduction band. At normal temperature, some of the free electrons in the valence band attain sufficient energy to jump to the Conduction band where they may be called Conduction electrons. The gap left in the valence band by the movement of electrons is called a hole. It can be seen from the diagram above.In metals, the valence band and Conduction band can overlap. There is no forbidden band and that is why metal can conduct electricity easily. This can be seen from the diagram below.The electrons in the overlapping region are Conduction electrons. Since there is a large number of conduction electrons, metals are good conductors.
Semiconductors
Pure semiconductors are called intrinsic semiconductors. Silicon and germanium are most common examples of intrinsic semiconductors. These semiconductors are most frequently used in the manufacturing of transistors, diode and other electronic components.
Intrinsic semiconductors are also called undoped semiconductors. In an intrinsic semiconductor, the number of electrons in the Conduction band is equal to the number of holes in the valence.
Let's take a look at silicon and germanium.
Silicon
Silicon is an element whose atomic number is 14 i.e (2 8 8 2). The number of electrons in the outermost shell is 4.
Germanium
Germanium is an element whose atomic number is 32 i.e (2 8 18 4). It also has 4 electrons in its outermost shell.
Extrinsic Semiconductors
Extrinsic Semiconductors are formed when impurities are added to semiconductors. The process of adding impurities to semiconductors is called Doping. Adding impurities to semiconductors increases thier electrical conductivity,
Things to note
- Extrinsic Semiconductors have high electrical conductivity than intrinsic semiconductors.
- Extrinsic Semiconductors are used for the manufacturing of electronic devices such as diodes, transistors etc
- The number of electrons in the conduction band is not equal to the number of holes in the valence band.
Types of impurities
- Trivalent impurity
- Pentavalent impurity
