Impure or extrinsic semiconductor
Definition: By carefully mixing a few special impurities with a pure semiconductor, the semiconductor’s electrical conductivity increases manifold. This is called an impure or extrinsic semiconductor.
The process of mixing impurities is called doping and the mixed impurities are called dopant.
A current of about a few milliamperes or mA (10-3 A) can thus be passed through a Silicon or Germanium crystal. The reason for this increase in current is that due to doping, the energy gap between the valence band and the conduction band of the semiconductor is greatly reduced, thereby increasing the number of charge carriers in the crystal.
All of the semiconductors that are widely used around the world today are extrinsic semiconductor. These extrinsic semiconductors are made by carefully mixing special types of external materials to increase electrical conductivity. Extrinsic semiconductor are of two types: 1. n-type, and 2. p-type.
n-type Semiconductors
Composition: Completely pure silicon (Si) or germanium (Ge) crystals are doped with small amounts of arsenic (As) or phosphorus (P) elements in a well-controlled manner. They are both pentavalent and elements in the fifth group (nitrogen group) of the periodic table, both having 5 electrons in the outermost shell of the atom.
Principle of Operation: A silicon crystal doped with arsenic is shown in the figure. When an arsenic atom takes up space in this crystal, it is surrounded by a cluster of silicon atoms.
Arsenic has an extra electron left in its outer shell after forming 4 covalent bonds with 4 surrounding silicon atoms. This electron has no place in any bond, so it acts as a free electron i.e. conduction electron. It has been found that, on average, only 1 phosphorus or arsenic atom is mixed with about 10⁶ germanium or silicon atoms to obtain a sufficient number of conduction electrons. As a result, it is possible to take the electrical conductivity of the crystal to the desired value. The figure shows the energy band of an n-type extrinsic semiconductor. The dotted line in the figure indicates the energy level of the excess electrons from the pentavalent element due to doping. The relative atomic number of pentavalent atoms is very small, so the electrons stay at one energy level, forming no bonds. These electrons are excited and can easily be transported along the conduction band. This line is known as the Fermi level. The higher the doping level, the more the Fermi level shifts towards the conduction band.
Definition: A crystal obtained by mixing a small amount of a pentavalent element (e.g., arsenic or phosphorus) into a pure semiconductor crystal is called an n-type extrinsic semiconductor.
Discussion
- A complete n-type crystal is neutral, not negatively charged. This is because the arsenic or phosphorus atoms in the crystal are neutral even though some electrons are free.
- The value of the energy gap between the Fermi level and the conduction band is about 0.05 eV.
- In an n-type semiconductor the major (number) charge carriers are electrons and the minority charge carriers are holes.
- On average only 1 impurity atom needs to be doped with about 10⁶ core atoms. That is why the original silicon or germanium crystal needs to be completely pure. Most of the cost of making a semiconductor crystal is spent on refining the crystal. But the price of this crystal is very low.
- Phosphorus or arsenic elements donate free electrons to semiconductor crystals, so they are called donor elements.
Since the negatively charged electrons act as the majority charge carriers, this crystal is called n-type.
p-type Semiconductors
Composition: Completely pure silicon or germanium crystals are doped with small amounts of boron (B) or aluminum (Al) elements in a well-controlled manner. They are both trivalent and elements in the third group of the periodic table, both having 3 electrons in the outermost shell of the atom.
Principle of Operation: A silicon crystal doped with boron is shown in the figure. When a boron atom takes place within an assembly of silicon atoms, 3 bonds can be completed with the surrounding 4 silicon atoms. However, due to the lack of 1 electron in the outer shell of boron, the fourth bond is not complete, resulting in the creation of a hole. Effective positively charged holes can be transferred through the crystal by applying a suitable voltage. They act as charge carriers and the electrical conductivity increases many times compared to pure semiconductors.
The figure shows the energy band of a p-type extrinsic semiconductor. The dotted line shown in the figure indicates the energy level of the hole from the trivalent element due to doping. This is the Fermi level of a p-type semiconductor. The higher the doping level, the lower the Fermi level falls towards the valence band. Electrons in the conduction band can easily be excited to the Fermi level. This results in the formation of charge carrier holes in the conduction band.
Definition: A crystal obtained by mixing a small amount of a trivalent element (e.g., boron or aluminum) into a pure semiconductor crystal in a well-controlled manner is called a p-type extrinsic semiconductor.
Discussion
Given below are some facts about p-type semiconductors as compared to n-type semiconductors—
- A complete p-type crystal is sterile.
- The value of the energy gap between the valence band and the Fermi level is about 0.05 eV.
- In a p-type semiconductor the major (number) charge carriers are holes and the minority charge carriers are electrons.
- On average only 1 impurity atom needs to be doped with about 10⁶ core atoms.
- Adding boron or aluminum elements to a semiconductor crystal creates holes in their bond, so they can accept electrons from the crystal. That is why boron or aluminum is called an acceptor element.
Since the effective positively charged holes act as majority charge carriers, this crystal is called p-type.
Difference between n-type and p-type semiconductor
| n-type semiconductor | p-type semiconductor |
|---|---|
1. Produced by doping pentavalent elements with pure semiconductors. |
1. Produced by doping pure semiconductors with trivalent elements. |
2. Negatively charged electrons act as majority charge carriers and thermal electron-hole pairs act as minority charge carriers. |
2. Positively charged holes act as majority charge carriers and thermal electron-hole pairs act as minority charge carriers. |
3. These are also called as pentavalent semiconductors. |
3. These are also called as trivalent semiconductors. |
4. Number of electrons is greater than number of holes. |
4. Number of holes is greater than number of electrons. |
5. The impurities added to the n-type semiconductors are called as donor impurities. |
5. The impurities added to the p-type semiconductors are called as acceptor impurities. |
6. Example: boron, gallium, indium, aluminum etc. |
6. Example: phosphorus, arsenic, antimony etc. |