What are Intrinsic and extrinsic semiconductors?

This is the second article on Semiconductor Materials. You can read previous and Next articles on Semiconductors by clicking on the Previous Article and Next Article buttons respectively at the end of this article. In the first article, we discussed the basic properties, examples and applications of Semiconductors. Different types of semiconductor materials like intrinsic and extrinsic semiconductors are mentioned there, but not in detail. In this article, I’m going to discuss What are Intrinsic and Extrinsic Semiconductors, how are they produced, their examples and their differences.

Contents of this article:

  • Intrinsic Semiconductors
  • Extrinsic Semiconductors
  • Difference between intrinsic and extrinsic semiconductors
  • n type semiconductor
  • p type of semiconductor
  • Differences between n-type and p-type semiconductors
  • Some questions and answers

What is an Intrinsic semiconductor?

One can define the intrinsic semiconductor as a pure semiconductor i.e. a semiconductor without any impurity.

  1. Intrinsic semiconductors are pure semiconductors. There is no doping in intrinsic semiconductors.
  2. The concentration of electrons and the concentration of holes are the same in an intrinsic semiconductor.
  3. The conductivity of an intrinsic semiconductor is lower than that of an extrinsic semiconductor.

What is an Extrinsic semiconductor?

One can produce an extrinsic semiconductor by doping the intrinsic semiconductor with impure atoms. The process of mixing impure atoms in an intrinsic semiconductor is known as doping. Extrinsic semiconductors have some specific properties that are listed below.

  1. Extrinsic semiconductors are impure semiconductors. This type of semiconductor is formed by doping impure atoms in a pure or intrinsic semiconductor.
  2. Electron and hole concentrations differ depending upon the nature of impure atoms.
  3. The conductivity of extrinsic semiconductors is much higher than that of intrinsic ones.

Suggested Article: What is Doping and Why do we need doping in Semiconductors?

Extrinsic semiconductor examples

Aluminum-doped Silicon, Arsenic doped Silicon, Boron doped Silicon, Aluminum doped Germanium, Boron doped Germanium, etc. are examples of extrinsic semiconductors.

Difference between intrinsic and extrinsic semiconductors

Intrinsic SemiconductorExtrinsic Semiconductor
1. Intrinsic semiconductors are pure semiconductors.1. Extrinsic semiconductors are doped impure semiconductors.
2. Electron concentration and hole concentration are equal.2. Electron concentration and hole concentration are not equal.
3. Conductivity of intrinsic semiconductors is relatively low.3. Conductivity of extrinsic semiconductors is higher than that of intrinsic ones.
Difference between intrinsic and extrinsic semiconductors

n-type and p-type semiconductors

On the basis of doping impurities, there are two types of Extrinsic Semiconductors –

  • n-type extrinsic semiconductor
  • p-type extrinsic semiconductor

n-type extrinsic semiconductors

n-type semiconductor (or donor-type semiconductor) can be formed by doping pentavalent atoms like Arsenic (As), Phosphorus (P), Antimony (Sb) etc. in an intrinsic semiconductor crystal.

Pentavalent atoms have four valence electrons. At the time of doping of semiconductors with pentavalent atoms, four valence electrons of each pentavalent atom form four covalent bonds with four valence electrons of the four nearest neighboring silicon atoms. Now, there will be an extra unpaired electron (fifth electron) of the pentavalent atom in the semiconductor crystal. This fifth electron helps to increase the concentration of electrons in semiconductor crystals even at room temperature. The doping concentration is about one impurity atom per ten lakhs (106) semiconductor atoms. Therefore, electron concentration becomes very high and thus the conductivity of the crystal becomes very high, even at room temperature.

n-type semiconductor examples: Arsenic-doped Silicon, Phosphorus doped Silicon, Arsenic doped Germanium, etc. are examples of n-type semiconductors.

p-type of extrinsic semiconductors

p-type semiconductor (or acceptor-type semiconductor) can be formed by doping trivalent atoms like Boron (B), Aluminum (Al), Gallium (Ga), Indium (In) etc. in an intrinsic semiconductor crystal.

Trivalent atoms have three valence electrons. At the time of doping trivalent atoms in a pure semiconductor (like Si), three valence electrons of the trivalent atom form three covalent bonds with three valence electrons of the three nearest neighboring silicon atoms. Now, there is an absence of electron in the trivalent atom to form the fourth covalent bond with the electron of its fourth neighboring silicon atom. So, there will be an extra hole (absence of electron) in the semiconductor crystal. This hole will increase the concentration of positive charge in the crystal. Therefore, a large number of impure trivalent atoms will give a large number of extra holes in the crystal to increase the hole concentration to a high value. In this way, we can get higher conductivity of semiconductors by doping trivalent impurity atoms in it, even at room temperature.

p-type semiconductor examples: Aluminum-doped silicon, Boron-doped silicon, Aluminum-doped germanium, etc. are examples of p-type semiconductors.

Difference between n-type and p-type semiconductors

  n-type semiconductor  p-type semiconductor
1. To get an n-type semiconductor we need to dope Pentavalent atoms in an intrinsic Semiconductor.
2. Majority Carriers in n-type semiconductors are negatively charged electrons.
3. Minority carriers in n-type semiconductors are holes.
4. n-types are also called donor type semiconductors.  
1. To get a p-type semiconductor we need to dope Trivalent atoms in an Intrinsic Semiconductor.
2. Majority carriers in p-type semiconductors are positively charged Holes.
3. Minority carriers in p-type semiconductors are electrons.
4. P-types are also called acceptor type semiconductors.
Difference between n-type and p-type semiconductors

Why the Conductivity of semiconductors increases with an increase in temperature?

If we increase the temperature of a semiconductor, free electrons and holes are produced by breaking the covalent bonds. Both the free electrons in the conduction band and holes in the valence band carry current. Thus, the conductivity of semiconductors increases with the increase in temperature.

Are the n-type and p-type semiconductors positively and negatively charged?

No, n-type semiconductors and p-type semiconductors are not positively and negatively charged respectively. It is true that an n-type semiconductor has an electron concentration greater than the hole concentration, but the total number of electrons and protons in the crystal are equal. N-type semiconductors have unpaired, free electrons, but it does not have a greater number of electrons than the number of protons. Similarly, for p-type semiconductors hole concentration is greater than electron concentration, but the total number of electrons and protons are equal in the entire crystal.

“At a high temperature Extrinsic Semiconductors behave like Intrinsic Semiconductors”- Explain it.

In n-type semiconductors, the electron concentration is larger than the hole concentration. Again, in p-type semiconductors, the hole concentration is larger than the electron concentration. Now, if we increase the temperature of the semiconductor, then one electron and one hole appear by breaking a covalent bond. At very high temperatures, a large number of covalent bonds break and the number of free electrons and holes becomes so large that the difference between the electron and hole concentration becomes negligible. Thus, at very high temperatures the concentration of electrons and holes becomes nearly equal. Hence we can call the extrinsic semiconductor an intrinsic one at a high temperature.

This is all from this article on intrinsic and extrinsic semiconductor materials. If you have any doubts on this topic feel free to ask me in the comment section. Read how to increase the conductivity of semiconductors by clicking on the Next Article button below.

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