Difference Between Donor and Acceptor Impurities

The addition of impurities to a semiconductor material leads to cause variation in the conducting nature of the material. The fundamental factor of difference between donor and acceptor impurities is that a donor impurity donates charges to the semiconductor. As against an acceptor impurity accepts the charges from the semiconductor material.

Basically, the phenomenon of adding impurity to a semiconductor is known as doping. Doping contributes to the conductivity of the material. There are various factors of differentiation between donor and acceptor impurities which we will see in this content.

Content: Donor Vs Acceptor Impurities

1. Comparison Chart
2. Definition
3. Key Differences
4. Conclusion

Comparison Chart

Basis for Comparison	Donor Impurities	Acceptor Impurities
Basic	The impurities that increases conductivity by donating charge is known as donor impurities.	Those impurities that accepts the charge for increasing conductivity is known as acceptor impurities.
Also referred as	pentavalent impurities	trivalent impurities
Number of valence electrons	5	3
Forms	n-type semiconductor	p-type semiconductor
Group position in periodic table	Group V	Group III
Examples	Phosphorus, Bismuth.	Aluminium, Boron.

Definition of Donor Impurity

A dopant having 5 electrons in its valence shell when doped with a semiconductor to increase its conductivity is known as a donor impurity. It holds the ability to donate an extra electron present in its valence shell to the neighbouring atom. Thus is given the name ‘donor’. Due to the presence of excess negative charge, it forms the n-type region. Thus donor impurity is used to form n-type semiconductors.

Group V elements are said to be donor impurity because they consist of 5 electrons in the outermost shell. Thus it is also known as pentavalent impurity.

Consider that a pentavalent impurity Arsenic (As) is doped into a pure silicon structure.

As we know that arsenic has 5 electrons present in its valence shell. So, 4 electrons of arsenic form 4 covalent bonds with 4 electrons of neighbouring silicon atom as shown below:

But as we can see that here an extra electron is present. This extra weakly bonded electron freely flows around the crystal even at room temperature. The movement of this free charge inside the crystal generates current. Thus the excess electron here is known as charge carrier.

Definition of Acceptor Impurity

A dopant with 3 electrons in its valence shell, when doped with a semiconductor to raise its conductivity, is known as an acceptor impurity. It has the ability by which it can accept an electron from neighbouring atom as it has a vacancy of electron. Thus is called acceptor impurity. So, the presence of excess positive charge forms the p-type region. Thus acceptor impurity is used to form p-type semiconductors.

Group III elements are known as donor impurity because these elements consist of 3 electrons in the valence shell. Thus is known as trivalent impurity. Elements like boron, aluminium, indium and gallium are examples of trivalent impurity.

Consider an aluminium atom is doped in a pure crystal of silicon:

We know that aluminium atom consists of 3 electrons in the outermost shell. So, in this case, 3 electrons of aluminium form 3 covalent bonds with neighbouring silicon atom. But an incomplete bond exists because a vacancy of electron is present in the structure. This indicates the presence of excess positive charge (i.e., hole).

So, to fill the vacancy of that electron and complete the covalent bond, an electron is freed from neighbouring silicon atom at room temperature. This electron occupies the vacant place in the crystal thereby leaving the vacancy of an electron at the other place. In this way here a positive charge is moving in order to fill the vacant place.

Thus it is said that the movement of free charge creates current.

Conclusion

So, from this discussion, we can conclude that impurity is added in order to enhance the conductivity of the semiconductor. However, the type of added impurity leads to cause variation in the type of charge carriers responsible for conduction.