Those semiconductors in which some impurity atoms are embedded are known as extrinsic semiconductors.
Extrinsic semi conductors are basically of two types:
1. P-type semi conductors
2. N-type semi conductors
N-type Semi conductors: Let’s take an example of the silicon crystal to understand the concept of N-type semi conductor. We have studied the electronic configuration of the silicon atom. It has four electrons in its outermost shell. In N-type semi conductors, the silicon atoms are replaced with the pentavalent atoms like phosphorous, bismuth, antimony etc. So, as a result the four of the electrons of the pentavalent atoms will form the covalent bonds with the silicon atoms and the one electron will revolve around the nucleus of the impurity atoms with less binding energy. These electrons are almost free to move. In other words we can say that these electrons are donated by the impure atoms. So, these are also known as donor atoms. So, the conduction inside the conductor will take place with the help of the negatively charged electrons. Electrons are negatively charged. Due to this negative charge these semiconductors are known as N-type semiconductors.
Each donor atom has denoted an electron from its valence shell. So, as a result due to loss of the negative charge these atoms will become positively charged. The single valence electron revolves around the nucleus of the impure atom. Some experiments were performed. It was found that .01eV and .05eV energy is required to make the electron free from the nuclear forces.
When the semi conductors are placed at room temperature then the covalent bond breakage will take place. So, more free electrons will be generated. As a result, same no of holes generation will take place. But as compared to the free electrons the no of holes are comparatively less due to the presence of donor electrons.
We can say that major conduction of n-type semi conductors is due to electrons. So, electrons are known as majority carriers and the holes are known as the minority carriers.
P-type semi conductors: In a p-type semi conductor doping is done with trivalent atoms .Trivalent atoms are those which have three valence electrons in their valence shell. Some examples of trivalent atoms are Aluminum, boron etc. So, the three valence electrons of the doped impure atoms will form the covalent bonds between silicon atoms. But silicon atoms have four electrons in its valence shell. So, one covalent bond will be improper. So, one more electron is needed for the proper covalent bonding. This need of one electron is fulfilled from any of the bond between two silicon atoms. So, the bond between the silicon and indium atom will be completed. After bond formation the indium will get ionized. As we know that ions are negatively charged. So, indium will also get negative charge. A hole was created when the electron come from silicon-silicon bond to complete the bond between indium and silicon. Now, an electron will move from any one of the covalent bond to fill the empty hole. This will result in a new holes formation. So, in p-type semi conductor the holes movement results in the formation of the current. Holes are positively charged. Hence these conductors are known as p-type semiconductors or acceptor type semi conductors.
When these conductors are placed at room temperature then the covalent bond breakage will take place. In this type of semi conductors the electrons are very less as compared to the holes. So, in p-type semi conductors holes are the majority carriers and electrons are the minority carriers.