A semiconductor to which a suitable impurity in a controlled rate is added to increase its conductivity is called an extrinsic semiconductor.
Based on the type of impurity added they are classified into two types. They are,
- n-type semiconductor
- p-type semiconductor
N-type semiconductors
Definition: When a small amount of pentavalent impurity or element with five outermost electrons (such as phosphorus or arsenic) is added to a pure semiconductor crystal to get extra electron in conduction band, the resulting extrinsic semiconductor is called as N type semiconductor. (Here N stands for negative)
Example: As shown in Fig. (a) the four electrons of the Phosphorous atom form covalent bonds with the valence electrons of the Ge atoms. The fifth electron of the Phosphorous atom is not covalently bonded but it loosely bound to the Phosphorous atom. By increasing the thermal energy or by applying electric field, this electron can easily be excited from the valence band to the conduction band and is available for electrical conduction.
Every Phosphorous atom added to the semiconductor contributes one conduction, electron without creating a positive hole. Hence Phosphorous is called a donor atom, since it donates free electrons. The material as a whole has excess of electrons which are the majority current carriers and they form donor energy level (Ed) which lies close to the conduction band.
Since the majority current carriers are negatively charged particles, this type of semiconductor is called an n-type semiconductor. Fig. (b) gives the band diagram of the n-type semiconductor.
It is seen that the Fermi level is shifted towards the conduction band and lies in between the conduction band and donor level, indicative of the presence of excess of electrons.
Characteristics
- In N type semiconductors the electrons are the majority carriers.
- It is electrically neutral, no addition of either positive charges or negative charges.
- Donar energy level appears just below the conduction band.
P-type semiconductors
Definition: When a small amount of trivalent impurity or element with three outermost electrons (such as aluminum, boron, indium gallium) is added to a pure semiconductor crystal to get extra holes in valance band, the resulting extrinsic semiconductor is called as P type semiconductor.
Example: As shown in Fig. (a) the three valence electrons of the Aluminum atom form covalent bond with the valence electrons of Ge atom. One more electron is needed for bonding which is not available. This deficiency of one electron becomes a hole.
For every Aluminum atom added, a hole is introduced in the semiconductor. The holes which are in excess in a p-type semiconductor are treated as positively charged particles. They take part in electrical conduction and the majority carriers in the p-type semiconductor.
Due to thermal excitation, a hole in one location may be occupied by an electron from the neighbouring atom in which case the hole is shifted to a new location. This process goes on as a result of which the hole moves about in random. Thus the holes are free to conduct electricity.
The impurity atoms which contribute holes to the semiconductor are called acceptor impurities and they form acceptor level (Ea) which lies close to the valence band. Since the current carriers are positively charged holes, this type of semiconductor is called p-type.
Fig. (b) gives the band diagram of a p-type semiconductor. As shown in Fig. (b), since the holes are in excess, the Fermi level is shifted towards the valence band and lies in between the valence band and acceptor level.
Characteristics
- In P type semiconductors the holes are the majority carriers.
- It is electrically neutral, no addition of either positive charges or negative charges.
- Accepter energy level appears just above the valance band.
| Read More Topics |
| Elemental and compound semiconductors |
| Quantum theory |
| Success of classical free electron theory |
