Difference Between Intrinsic and Extrinsic Semiconductors (With Table & Examples)
Intrinsic semiconductors are pure types of semiconducting materials in which the number of free electrons is equal to the number of holes. These materials do not contain any intentional impurities and are often referred to as undoped or I-type semiconductors. Silicon and germanium are classic examples, and understanding their behavior forms the foundation for modern electronics and Physics studies.
What is an Intrinsic Semiconductor?
In an intrinsic semiconductor, atoms are arranged in a crystalline lattice. Each atom, such as silicon or germanium, has four valence electrons. These electrons form strong covalent bonds with four neighboring atoms, leading to a highly stable structure. At low temperatures, all electrons remain in their bonds, resulting in very low conductivity.
As the temperature increases, some electrons acquire enough thermal energy to break free from their bonds. When this happens, a free electron is created in the conduction band, and a corresponding hole (a vacancy with a positive charge) is left behind in the valence band. The number of free electrons is always equal to the number of holes in an intrinsic semiconductor.
Lattice Structure and Charge Carrier Generation
The lattice structure of intrinsic semiconductors closely resembles that of a diamond. Every silicon or germanium atom shares its four outer electrons with its four nearest neighbors, establishing covalent bonds. When thermal energy breaks a bond, two charge carriers are generated: an electron (which moves freely as a conduction electron) and a hole (which acts like a positive charge).
Under an applied electric field, electrons move toward the positive terminal, while holes drift toward the negative terminal. This dual movement contributes to the total current in the material. The total current (I) can be expressed as:
Properties of Intrinsic Semiconductors
- They are composed of pure semiconductor material.
- The density of electrons equals the density of holes.
- At absolute zero temperature (T=0K), they act as perfect insulators.
- Electrical conductivity increases with temperature but is comparatively low.
- Their behavior is highly temperature-dependent.
Fermi Level in Intrinsic Semiconductors
The Fermi level represents the probability of electron occupation in energy states between the valence and conduction bands. In an intrinsic semiconductor, the Fermi level lies exactly in the middle of the band gap. This is because the probability of finding an electron in the conduction band equals the probability of finding a hole in the valence band.
At T=0K, no electrons are excited to the conduction band, so the material behaves like an insulator. As temperature rises, electrons gain enough energy to move to the conduction band, contributing to partial conductivity.
Examples of Intrinsic Semiconductors
- Silicon (Si)
- Germanium (Ge)
- Gray tin (α-Sn)
- Boron arsenide (BAs)
- Gallium arsenide (GaAs)
- Indium antimonide (InSb)
Comparison: Intrinsic vs. Extrinsic Semiconductors
| Parameter | Intrinsic Semiconductor | Extrinsic Semiconductor |
|---|---|---|
| Purity | Pure semiconductor | Impure (doped with controlled impurities) |
| Carrier Density | Electrons = Holes | Electrons ≠ Holes |
| Temperature Dependence | Conductivity depends strictly on temperature | Conductivity depends on both temperature and doping |
| Classification | Not further classified | Divided into n-type and p-type |
Movement and Recombination of Carriers
When an electron breaks free and leaves a hole, both respond to electric fields. Electrons move toward the positive potential, while holes drift toward the negative, just as free positive charges would. The process of electrons filling holes, called recombination, occurs simultaneously. At equilibrium, the generation and recombination rates for electron-hole pairs are balanced.
Key Formulas for Intrinsic Semiconductors
| Quantity | Formula | Description |
|---|---|---|
| Carrier Concentration (ni) | ni = (number of electrons) = (number of holes) | Defines density of charge carriers generated thermally |
| Total Current | I = Ie + Ih | Sum of electron and hole currents |
Stepwise Approach to Example Problem
- Identify if the semiconductor is intrinsic (pure, undoped, electrons = holes).
- Note the temperature: At T=0K, conductivity is zero; as T rises, both electrons and holes are generated equally.
- When asked about current, consider both electron and hole contributions: I = Ie + Ih.
- If comparing to extrinsic, focus on purity, carrier densities, and classification (intrinsic: only one type; extrinsic: n-type or p-type).
Common Uses of Intrinsic Semiconductors
- Manufacturing of transistors.
- Used in the creation of diodes.
- Study of semiconductor physics and device basics.
Next Steps and Further Study
- To learn more about advanced applications, visit Semiconductor Materials & Circuits.
- For details on extrinsic semiconductors, see Extrinsic Semiconductors.
- Practice problems and revision notes are available in related links and resources throughout the Physics section.
Understanding intrinsic semiconductors is essential for mastering modern electronics and Physics concepts. These concepts set the stage for more complex devices such as diodes and transistors. Keep practicing with varied examples and tables to strengthen your foundational knowledge.
FAQs on Intrinsic Semiconductors Explained for Physics Exams
1. What is an intrinsic semiconductor?
An intrinsic semiconductor is a pure form of semiconductor material without any significant impurity atoms. Its electrical conductivity depends only on the properties of the material and temperature, not on external dopants. Examples include pure silicon (Si) and germanium (Ge).
2. Give examples of intrinsic semiconductors.
Common examples of intrinsic semiconductors are:• Silicon (Si)• Germanium (Ge)• Gray tin (α-Sn)• Other materials like boron arsenide (BAs), gallium arsenide (GaAs), and indium antimonide (InSb) can also exist in intrinsic (pure) form.
3. What is the difference between intrinsic and extrinsic semiconductors?
Intrinsic semiconductors are pure materials with equal numbers of electrons and holes, while extrinsic semiconductors are doped with impurities to increase conductivity. Key differences:• Intrinsic: Pure, undoped, electrons = holes.• Extrinsic: Contains dopants, either electrons (n-type) or holes (p-type) are the majority.• Extrinsic semiconductors have higher conductivity than intrinsic semiconductors.
4. Why are intrinsic semiconductors not widely used?
Intrinsic semiconductors are rarely used in practical electronic devices because their electrical conductivity is too low at room temperature. In most cases, extrinsic (doped) semiconductors are preferred for better control over conductivity and device performance.
5. What is the band gap of intrinsic semiconductors?
The band gap is the energy difference between the top of the valence band and the bottom of the conduction band in a semiconductor. For intrinsic semiconductors:• Silicon (Si): 1.12 eV at 300K• Germanium (Ge): 0.66 eV at 300K• A suitable band gap allows limited charge carriers at room temperature, giving semiconductors unique properties.
6. What is the formula for intrinsic carrier concentration?
The formula for intrinsic carrier concentration (ni) is:
ni = √(NcNv) × exp(-Eg / 2kT)
Where:
• Nc, Nv = effective density of states in conduction/valence bands
• Eg = band gap energy
• k = Boltzmann constant
• T = absolute temperature (in Kelvin)
7. What happens to the conductivity of intrinsic semiconductors with increase in temperature?
As the temperature increases, more electrons gain enough energy to jump from the valence band to the conduction band in an intrinsic semiconductor. This increases the number of free charge carriers, so conductivity increases with temperature in intrinsic semiconductors.
8. What is the Fermi level in an intrinsic semiconductor?
The Fermi level is the energy level where the probability of finding an electron is 50%. In an intrinsic semiconductor, the Fermi level lies almost exactly halfway between the valence band and conduction band due to equal numbers of electrons and holes.
9. What is the movement of holes in intrinsic semiconductors?
In an intrinsic semiconductor, when an electron leaves the valence band to jump to the conduction band, it leaves behind a vacancy called a hole. Under an electric field:• Holes move toward the negative terminal (as if they are positive charges).• This movement contributes to the total current along with electrons.
10. How is current conducted in intrinsic semiconductors?
In intrinsic semiconductors, current is carried by two types of charge carriers:• Electrons (in the conduction band)• Holes (in the valence band)The total current is the sum of electron and hole contributions, and both move in opposite directions under an electric field.
11. What are the main uses of intrinsic semiconductors?
Though not widely used in actual devices, intrinsic semiconductors are important for:• Understanding the fundamental principles of semiconductors• Basic research and teaching in Physics• Reference in the manufacturing of semiconductor devices before doping
12. Why are electrons and holes equal in intrinsic semiconductors?
In an intrinsic semiconductor, every time an electron is thermally excited from the valence band to the conduction band, it leaves behind a hole. Therefore, the number of electrons equals the number of holes at all times.
