Semiconductors are neither good conductors nor good insulators, but belong to a class of material between the two categories.
The semiconductor materials are:
- Germanium (Ge)
- Silicon (Si)
2.) Intrinsic Semiconductor
Intrinsic semiconductors are pure Germanium or Silicon crystal.
- A stable needs 8 electron on the outermost shell.
- For Germanium or silicon atoms, each atom has 4 valence electrons on its outermost shell (valence shell).
- Hence by forming covalent bond with other atoms of the same material, it results in a stable crystalline structure.
3.) Covalent
It is the sharing of electrons with neighboring atoms in order to fill its outer shell with 8 electrons.
4.) Holes
Each time, an electron breaks away from covalent bond, a hole is created.
- A hole represents the absence of an electron in a covalent bond.
- The hole acts like a positive chare because it will attract and capture any free electron that happens to come near it.
- A hole (positive charge) can attract adjacent bound electron move to fill up the hole, the hole at the original position disappears.
- But a new hole is created at that adjacent bound electron when it moves out of its shell.
- In this way, a hole can move from one atom to another atom.
5.) Doping
It is the process of adding controlled amount of impurities to pure semiconductor to improve conductivity.
6.) Extrinsic Semiconductor
It is the process of adding controlled amount of impurities to pure semiconductor to improve conductivity,
There are two types of extrinsic semiconductor, namely:
- N-type semiconductor/material
- P-type semiconductor/material
7.) N-type Semiconductor
By doping the pure crystal with pentavalent (donors) impurities, that is, a material with 5 valance electron, e get N-type semiconductor.
Examples of pentavalent impurities are:
- Antimony (Sb)
- Arsenic (As)
- Phosphorus (P)
Only free electrons are produced by doping with pentavalent impurities.
As a result, for N-type semiconductor, there are more free electrons than holes.
Hence, the majority carriers are the free electrons and the minority carriers are the holes.
Illustration:
8.) P-type Semiconductor
By doping the pure crystal with trivalent (acceptors) imurities, that is, material having 3 valence electrons, we get P-type semiconductor.
Examples of trivalent impurities are:
- Aluminium (Al)
- Boron (B)
- Gallium (Ga)
- Indium (In)
Only the holes are produced by doping with trivalent impurities.
As a result, for P-type semiconductor, there are more holes than free electrons.
Hence, the majority carriers are holes.
The minority carriers are the free electrons.
Illustration:
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