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### Course: Modern Physics (Essentials) - Class 12th>Unit 5

Lesson 2: Basics of semiconductors

# Extrinsic semiconductors N-type

What happens if you add impurities to a pure semiconductor? Impure semiconductors are usually called extrinsic semiconductors. In this video, we are going to explore the N-type semiconductors. Created by Mahesh Shenoy.

## Want to join the conversation?

• At you draw all the fifth Phosphorus electron at the same energy level. But isn't this in contradiction to the Pauli exclusion principle? Or are the Phosphorus atoms so far apart in the solid that their electrons can be in the same state?
• From the author:That's a very good point. And you are absolutely right. Technically the electrons can't be at the same energy level, however, since we are keeping the doping concentration so low, the phosphorus atoms are so far apart, that we can totally neglect the energy level difference between the electrons of various phosphorus atoms.
• Why aren't we speaking about the holes of Phosphorous?
• He did mention that the phosphorous atoms get a positive charge by losing an electron.
• @ he said "is going to give you a lot more got more electrons than holes lot more electrons that's it that's the idea" when the free electrons of phosphorus excited , they will leave holes, right? how the number of electrons greater than the number of holes?
• Isn't the valency of Phosphorus 3..?
(1 vote)
• Q: Isn't the valency of Phosphorus 3?

Yes you're absolutely correct. Phosphorus has a valency of 3.

The electronic configuration of Phosphorus is 2,8,5 or 1s2 2s2 2p6 3s2 3p3

It has 5 valence electrons in its outermost shell and requires 3 electrons to complete its octet.

`Valency means the number of electrons donated or received to complete the octet.`

It is also a non-metal which tends to receive electrons. It is easy for an atom to receive 3 electrons than to donate 5.

Also, Phosphorous shows both the valencies 3 and 5.

Let me give you an example.

With three unpaired electrons in three p orbitals, phosphorous shows valency 3, like in case of compound PCl3.

Whereas, In case of PCl5, one of 3s electron gets promoted to 3d orbital as follows:

P: 1s2 2s2 2p6 3s1 3p3 3d1

Thus, phosphorous atom undergoes sp3d hybridization to form five equivalent sp3d orbitals each containing one unpaired electron.
• What happens to the Silicon atom that is displaced by a phosphorus atom?
• In donor level, isn't each phosphorus atom contributing 3 electrons as it has 3 electrons in highest 3p orbital? If it is, then there should be 2 electrons available from each phosphorus in the donor level because we've seen in the picture that 4 electrons from it's third shell making covalent bond with other silicon atoms. Or am I missing something?
• How do we know that the energy of electrons in donor level are closer to the energy of conduction band?