extrinsic semiconductor

Extrinsic semiconductor and their types

Our article on semiconductors describes its two types intrinsic and extrinsic semiconductors. As you know the semiconductor has conductivity in between of conductor and insulator. Intrinsic semiconductors have low conductivity at room temperature, which makes them unsuitable for use in electronic components.

To tackle this problem a small amount of impurity is doped into the pure semiconductor as a result of which extrinsic semiconductors were manufactured.

Doping in semiconductor

The process of adding impurity atoms to the pure/intrinsic semiconductor is known as doping. By doping some impurity into intrinsic semiconductors it is converted into an extrinsic semiconductor.

The purpose behind doping is to manufacture semiconductor which has good conduction capabilities.

What is an extrinsic semiconductor?

When some impurity is added into the pure semiconductor during its crystal growth to increase its conduction capabilities, then the developed material is known as an extrinsic semiconductor.

Pure semiconductor silicon (Si) and germanium (Ge) are tetravalent elements. According to the types of impurity added one is pentavalent element another is trivalent elements, the extrinsic semiconductor can be classified into 2 classes based on impurity added.

  1. N-type or donor extrinsic semiconductor
  2. P-types or acceptor extrinsic semiconductor
Two types of extrinsic semiconductor
1. p-type extrinsic semiconductor
2. n-type extrinsic semiconductor

Types of extrinsic semiconductor

N-type

When a pentavalent impurity is added to the pure/intrinsic semiconductor during crystal growth, the resulting material is an N-type extrinsic semiconductor. In N-type extrinsic semi-conductor ‘N’ stands for the negative charge of electrons.

When the pentavalent impurity is added to the silicon or germanium, they form covalent bonds with the silicon atom. But as the silicon atom has 4 electrons in the valence shell. 4 electrons of the pentavalent impurity combine with it and 1 electron remains spare. Which moves to the conduction band of the extrinsic semiconductor as a free electron.

The added pentavalent impurity is also called donor type impurity because of the reason that it donates 1 free electron to the conduction band.

In N-type, semiconductor electrons are majority charge carriers and holes are minority charge carriers.

P-type

When a trivalent impurity is doped into the pure semiconductor which increases its conduction capabilities, the developed material is a P-type semiconductor. Here ‘P’ refers to positive as the charge of a hole is positive.

When the tetravalent atom ( silicon or germanium ) is doped with the trivalent impurity like boron (B), aluminum (Al). These atoms form covalent with each other but as trivalent impurity has only 3 electrons to combine it leaves one incomplete bond considered as a hole.

These impurities accept electrons into the hole so they are also known as acceptor impurity and respective semiconductors in which this type of impurity is used known as acceptor extrinsic semi-conductor.

p-type semiconductors have holes as majority charge carriers and electrons as minority charge carriers.

Applications of extrinsic semiconductor

Due to its conduction capabilities at room temperature, it is used in many components of electrical and electronic devices.

Few components/devices where the extrinsic semi-conductor is used are listed below:

  • Semiconductor diode
  • Transistor
  • A field-effect transistor ( FET )
  • In optoelectronics as a light emitter

Leave a Comment

Your email address will not be published.