Difference Between Intrinsic and Extrinsic Semiconductors with Examples
Extrinsic semiconductors are a key concept in Physics, describing materials whose electrical conductivity is significantly improved by intentionally introducing small amounts of specific impurities. This process is known as doping. The two primary examples of base semiconductors are silicon (Si) and germanium (Ge), which by themselves are called intrinsic semiconductors. When these pure materials are doped with suitable impurity atoms, their conductive properties change, making them suitable for modern electronic devices like diodes and transistors.
Difference Between Intrinsic and Extrinsic Semiconductors
| Intrinsic Semiconductor | Extrinsic Semiconductor |
|---|---|
| Pure semiconductor, no impurities | Doped with impurity atoms to improve conductivity |
| Low electrical conductivity at room temperature | Higher conductivity due to extra charge carriers |
| Only Si or Ge | Si or Ge doped with group-13 or group-15 elements |
| Equal number of electrons and holes | Majority carriers are electrons (n-type) or holes (p-type) |
The process of doping enables us to create two types of extrinsic semiconductors: n-type and p-type. These are named based on the charge of their majority carriers. Understanding this difference is crucial for distinguishing between pure and impurity-driven (extrinsic) semiconductors in Physics.
Types of Extrinsic Semiconductors: n-type and p-type
Doping silicon or germanium with small amounts of certain elements changes the balance between electrons and holes, creating two main categories:
-
n-type Semiconductor:
Created by adding pentavalent atoms (like phosphorus, arsenic, antimony—group-15) to Si or Ge.
Electrons become the majority charge carriers, allowing current to flow more easily. -
p-type Semiconductor:
Created by doping with trivalent atoms (aluminium, boron, gallium—group-13).
This creates 'holes' that act as positive charge carriers.
Holes become the majority carriers, playing a key role in electrical conduction.
Comparison of n-type and p-type Extrinsic Semiconductors
| Parameter | n-type | p-type |
|---|---|---|
| Dopant | Group-15 elements (P, As, Sb) | Group-13 elements (B, Al, Ga) |
| Majority carriers | Electrons | Holes |
| Minority carriers | Holes | Electrons |
| Band diagram feature | Donor level near conduction band | Acceptor level near valence band |
| Example dopant | Phosphorus in Si | Boron in Si |
| Sign of majority carriers | Negative | Positive |
Doping Effect and Band Diagram Concepts
In n-type materials, donor impurities add an energy level just below the conduction band. This makes it easy for electrons to enter the conduction band.
For p-type materials, acceptor impurities create an energy level just above the valence band, so electrons from the valence band fill these levels, leaving behind holes.
These differences are important for understanding real-world applications and electronic circuits.
Key Formulas in Extrinsic Semiconductors
| Formula | Meaning | Application |
|---|---|---|
| n ≈ ND (for n-type) | Electron concentration equals donor atoms | Find majority electrons in n-type |
| p ≈ NA (for p-type) | Hole concentration equals acceptor atoms | Find majority holes in p-type |
| σ = n·e·μe + p·e·μh | Total conductivity | Calculate overall conduction |
| n·p = ni2 | Mass-action law | Relates electrons and holes in any semiconductor |
Step-by-Step Approach for Numerical Problems
| Step | Explanation |
|---|---|
| 1. Identify type | Check if n-type or p-type based on dopant element |
| 2. Relate carrier concentrations | Use n ≈ ND or p ≈ NA accordingly |
| 3. Apply mass-action law | Use n·p = ni2 if both carriers’ values are needed |
| 4. Substitute values | Insert given values for calculations |
| 5. Calculate conductivity | Use the conductivity formula if required |
Example Problems
Q1: A silicon sample is doped with phosphorus. How does this affect electrical conduction?
Solution:
Phosphorus (group-15) donates an extra electron, so electrons become the majority charge carriers. This increases the electrical conductivity, making it an n-type extrinsic semiconductor.
Q2: If antimony (group-15) is added to germanium at a concentration of 1 ppm, what happens to carrier populations?
Solution:
Antimony increases the number of free electrons (majority carriers). The number of holes (minority carriers) is significantly reduced compared to electrons. This enhances the conductivity of the sample.
Core Applications of Extrinsic Semiconductors
-
Used in the construction of fundamental electronic components like diodes and transistors.
- Serve as the base for p-n junctions and integrated circuits, which are present in almost every electronic device.
- Vital in the making of solar cells, LEDs, and sensors.
Further Study and Practice
- For foundational knowledge, review intrinsic semiconductor concepts and energy bands.
- Practice with structured MCQs and numerical questions in Semiconductor Devices.
- Strengthen concepts for real-life devices through Semiconductor Diode and NPN vs PNP transistor topics.
This holistic approach ensures clarity in distinguishing between intrinsic and extrinsic semiconductors, provides stepwise methods for calculations, and highlights direct practical applications in electronic circuits.
FAQs on Extrinsic Semiconductors Explained for Class 12, JEE & NEET
1. What are extrinsic semiconductors?
Extrinsic semiconductors are impure semiconductors obtained by adding small amounts of specific impurity atoms (doping) to pure (intrinsic) semiconductors. This process enhances their electrical conductivity. There are two types based on the dopant used: n-type (donor impurities add free electrons) and p-type (acceptor impurities create holes).
2. What is the difference between intrinsic and extrinsic semiconductors?
Intrinsic semiconductors are pure and have equal numbers of electrons and holes, while extrinsic semiconductors are doped with impurities to increase charge carriers. Key differences include:
- Intrinsic: Only silicon or germanium, low conductivity.
- Extrinsic: Doped, higher conductivity; charge carriers dominated by electrons (n-type) or holes (p-type).
3. How are n-type and p-type semiconductors formed?
n-type semiconductors are formed by adding pentavalent impurities (group-15 elements such as phosphorus or arsenic) to silicon or germanium, supplying extra electrons. p-type semiconductors are created by adding trivalent impurities (group-13 elements such as boron or gallium), which create additional holes. The type of majority charge carrier depends on the type of dopant.
4. What are the majority and minority carriers in n-type and p-type semiconductors?
- In n-type semiconductors, electrons are the majority carriers and holes are the minority carriers.
- In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers.
5. What is semiconductor doping?
Doping is the process of intentionally adding small amounts of impurity atoms (dopants) to a pure semiconductor to increase its electrical conductivity. The dopant atoms can be donors (creating n-type) or acceptors (creating p-type), thereby determining the type of majority carrier in the material.
6. What is the energy band diagram of an extrinsic semiconductor?
In n-type extrinsic semiconductors, a donor energy level appears just below the conduction band, making it easier for electrons to jump into the conduction band.
In p-type extrinsic semiconductors, an acceptor level appears just above the valence band, allowing electrons to move out of the valence band, leaving holes. These shallow impurity levels greatly increase conductivity compared to intrinsic semiconductors.
7. Why do we use extrinsic semiconductors in electronic devices?
Extrinsic semiconductors offer much higher conductivity and controlled electrical properties compared to intrinsic semiconductors. They are used as the foundation for electronic devices such as diodes, transistors, integrated circuits, solar cells, and LEDs, enabling modern technology and communication systems.
8. What are common examples of n-type and p-type extrinsic semiconductors?
- n-type examples: Silicon doped with phosphorus (Si + P), germanium doped with arsenic (Ge + As).
- p-type examples: Silicon doped with boron (Si + B), germanium doped with gallium (Ge + Ga).
9. How does the conductivity of an extrinsic semiconductor compare to an intrinsic semiconductor?
Extrinsic semiconductors have considerably higher electrical conductivity than intrinsic semiconductors.
- Doping increases the number of charge carriers.
- Conductivity can be controlled by the amount and type of dopant added.
This enables their use in practical electronic devices.
10. What is the mass action law in semiconductors?
The mass action law states that the product of the electron concentration (n) and hole concentration (p) in a semiconductor is always equal to the square of the intrinsic carrier concentration (ni2):
n·p = ni2
This relation holds for both intrinsic and extrinsic semiconductors and is useful for calculating minority carrier concentration.
11. Can a semiconductor be both n-type and p-type simultaneously?
No, a semiconductor cannot be both n-type and p-type at the same time. However, by creating different regions of n-type and p-type material within a single crystal, devices like p-n junctions are formed, which are the basis for many electronic components.
12. What happens when a small amount of group-15 element is added to silicon?
When a group-15 element (such as phosphorus or arsenic) is added to silicon, it acts as a donor and supplies extra free electrons, resulting in the formation of an n-type extrinsic semiconductor where electrons are the majority carriers.
