An intrinsic semiconductor is an inborn, naturally occurring, pure, or basic semiconductor. The best examples of intrinsic semiconductors are crystals of Pure Silicon and Pure Germanium.
The Atomic Number and Electronic Configuration of Si and Ge Are
Here, we will study these two intrinsic semiconductors.
What are Intrinsic Semiconductors?
We know that Si and Ge have 4 valence electrons and these two elements possess properties like Carbon because they are tetravalent.
All four electrons of Si and Ge crystals are involved in covalent bonding and no electron sets free; this is the property of catenation that we can see in Carbon also. The diagram below shows the catenation property of Si that we can see in Ge also:
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Let’s look at the following points on the effect of temperature on semiconductor:
Working Mechanism of Intrinsic Semiconductors
At 0 K, no free electrons are available because all the electrons are involved in bond formation. All the electrons can’t reach the conduction band and remain in the valence bond.
As there are zero electrons in the conduction band, so no electricity formation (zero conductivity) means the semiconductor behaves as an insulator at 0 K.
Now, what we do is, we increase the temperature of Silicon or Germanium crystals, the thermal energy offered to these crystals may break the bond and release a few electrons, and some free electrons generate electricity. Also, the release of the number of free electrons depends on the temperature.
At room temperature, i.e., 300 K or 27 ℃, only one covalent bond breaks out of 1029 atoms that means very few electrons. So, we couldn’t obtain good conductivity at room temperature.
So, what happens next?
As the temperature rises to 300K, the electron from one of the bond, and vacancy generates in that place. So, the electron that is set free is the thermally free electron.
The place the electron left is the vacancy or the hole, and this electron gains some energy (in electronvolts), crosses the forbidden energy gap and reaches the conduction band; this migration is responsible for electricity generation.
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Here, we focused on one electron, but there are many crystals, and many crystals mean many electrons.
On applying an electric field from the top to the bottom direction, the electrons start flowing in the opposite direction because the electric field or current and electrons flow in an opposite direction.
Concept of Hole and Electron
So far, we understood that on applying an electric field to the semiconductor, the electrons start leaving the conduction band and falls.
Now, electrons start falling and they unite with vacancies or holes, so the process of the meeting of electrons with holes is called recombination.
Now, as the temperature rises, more electrons migrate to the conduction band and thus supersedes the recombination process. At this moment, the semiconductor conductivity increases with the temperature rise.
Points to Remember: Intrinsic Semiconductors
A number of thermally generated electrons equals the number of holes generated. (ne = nh).
The intrinsic concentration is the intrinsic charge carrier density of the semiconductor, and it is symbolized as ni. The ni value for Si and Ge are as follows:
Si = ni = 1.5 x 106 per m3
Ge = ni = 2.4 x 1019 per m3
The equation for the intrinsic concentration of semiconductors shows the direct proportionality of ni with the following quantities:
ni e-Eg/kT
Here,
E = forbidden energy gap
k = Boltzmann constant
T = temperature in Kelvin
So, we understood what is an intrinsic semiconductor. Now, we will differentiate it from the extrinsic semiconductor.
Intrinsic and Extrinsic Semiconductor
FAQs on Intrinsic Semiconductors
1. What is an intrinsic semiconductor, and how does its electrical conductivity change with temperature?
An intrinsic semiconductor is a pure semiconductor material, such as silicon or germanium, that has no intentional doping of impurities. Its electrical conductivity is very low at absolute zero as all valence electrons are involved in covalent bonds. As the temperature increases, some bonds break, releasing free electrons and creating holes, which both contribute to electrical conductivity. Thus, the conductivity of an intrinsic semiconductor increases with temperature.
2. How do the atomic structures of silicon and germanium enable them to act as good intrinsic semiconductors?
Silicon and germanium each have four valence electrons, allowing them to form strong covalent bonds in a crystal lattice. This structure leaves no free electrons at 0 K. However, with rising temperature, thermal energy can break these bonds, freeing electrons and generating holes, which makes these elements highly effective as intrinsic semiconductors in the CBSE Physics syllabus.
3. What is the difference between intrinsic and extrinsic semiconductors on a microscopic level?
- Intrinsic semiconductors are made of pure material with equal numbers of free electrons and holes generated only by thermal energy.
- Extrinsic semiconductors are doped with specific impurities to increase either electrons (n-type) or holes (p-type), altering charge carrier concentration and conductivity even at room temperature.
4. Why does an intrinsic semiconductor behave like an insulator at absolute zero temperature?
At absolute zero (0 K), all electrons in an intrinsic semiconductor are tightly bound in covalent bonds, leaving no free electrons or holes. As a result, the material exhibits zero electrical conductivity and acts as an insulator at this temperature.
5. How is the intrinsic carrier concentration (ni) of a semiconductor calculated, and what factors influence its value?
The intrinsic carrier concentration, ni, is given by the formula ni = A·e−Eg/2kT, where Eg is the energy band gap, k is Boltzmann's constant, and T is the temperature in Kelvin.
- A larger band gap (Eg) results in fewer charge carriers.
- Higher temperatures (T) increase ni sharply.
6. What happens to the flow of majority charge carriers when an electric field is applied across an intrinsic semiconductor?
When an electric field is applied to an intrinsic semiconductor, electrons migrate toward the positive terminal while holes move toward the negative terminal. Both types of charge carriers contribute equally to electric current, and their directions are always opposite due to their charge.
7. How does recombination of electrons and holes occur in intrinsic semiconductors, and what role does temperature play?
Recombination in intrinsic semiconductors occurs when a free electron falls into a hole, neutralizing both as they return to a lower energy state. As temperature increases, the rate of generation of electron-hole pairs surpasses the recombination rate, leading to higher conductivity.
8. In CBSE board exam terms, why is it important to understand the distinction between intrinsic and extrinsic semiconductors?
Distinguishing between intrinsic and extrinsic semiconductors is vital for board exams as it forms the base for understanding modern electronics, such as diodes and transistors. Questions on their properties, differences, and role in device functioning are common in the CBSE Physics syllabus.
9. What would happen if a pentavalent impurity is added to a pure intrinsic semiconductor?
Adding a pentavalent impurity (like phosphorus) to an intrinsic semiconductor results in an n-type semiconductor. This process introduces extra electrons as majority carriers and increases the material's conductivity even at lower temperatures.
10. Why is the number of electrons equal to the number of holes in an intrinsic semiconductor, and how does this relate to its overall neutrality?
For intrinsic semiconductors, thermal excitation generates each electron with a corresponding hole, keeping the numbers equal (ne = nh). This ensures charge neutrality of the material at all times and directly affects its conduction mechanism.
11. Who is considered the pioneer in semiconductor technology, and what was their contribution?
Dr. William Shockley is often called the father of semiconductors, credited with inventing the transistor and explaining the theory behind semiconductor action, which is fundamental in the Physics curriculum.
12. Can a material like aluminium be used as an intrinsic semiconductor?
Aluminium is a good conductor due to its metallic nature and does not function as an intrinsic semiconductor. Only materials with four valence electrons, such as silicon and germanium, exhibit true intrinsic semiconductor properties.
13. What misconceptions do students often have about the conduction process in intrinsic semiconductors?
Students often think only electrons are responsible for current; however, in intrinsic semiconductors, both electrons and holes equally contribute to electrical conductivity. Another misconception is that increasing temperature always increases conductivity indefinitely, but extremely high temperatures can damage the lattice structure.
14. How are intrinsic semiconductors relevant to the physics studied for CBSE Class 12 board exams?
Intrinsic semiconductors are fundamental to understanding electronic devices covered in CBSE Class 12 Physics. Mastery of these concepts helps students solve complex problems on diodes, transistors, and integrated circuits, which are frequently asked in board exams as per the 2025–26 syllabus.
