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Extrinsic semiconductors are semiconductors formed when a small measured amount of chemical impurity is added to intrinsic semiconductors. They are also called doped semiconductors or impurity semiconductors. Semiconductors in general, are amorphous or crystalline solids that possess electrical conductivity between a conductor and an insulator. Adding the impurities (doping) alters the electrical properties of the semiconductor making it suitable for applications in electronic devices like diodes and transistors. This article discusses the types of dopants and applications of extrinsic semiconductors.
| Table of Content |
Key Terms: Semiconductor, Extrinsic Semiconductor, Doping, Dopant, Impurity, Diode, Transistors, Intrinsic, Pentavalent, Trivalent, P-Type, N-Type
What are Extrinsic Semiconductors?
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Doping is a process in an extrinsic semiconductor when a bandgap is controlled by specifically adding minor impurities to the material. Doping the semiconductor with some suitable impurity can also increase its conductivity by several times. These impurities are called dopants.
The selection of dopants is based on the application of the final semiconductor. A good Extrinsic Semiconductor can be made by adding just a small amount of dopant. Although we should always make sure to check and confirm the size of the dopant atom to be equal to that of the original atom. A Dopant’s presence should not disturb the original semiconductor’s shape as it occupies only some positions in the crystal lattice of the semiconductor.
Semiconductor Electronics Class 12 Important Notes PDF
Semiconductor Electronics Class 12 Important Notes
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Types of Dopants in Extrinsic Semiconductors
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Doping causes the conductivity to change in a semiconductor. Elements in the fourth group of the periodic table like Germanium and Silicon are commonly used for making semiconductors. Silicon and Germanium have tetravalent crystals which can be doped with these two kinds of dopants:
- Pentavalent: These Atoms have a valency of 5. Examples: Phosphorous (Pi), Antimony (Sb), Arsenic (As), etc.
- Trivalent: These Atoms have a valency of 3. Examples: Aluminium (Al), Indium (In), Boron (B), etc.
The third and fifth groups of the periodic table consist of pentavalent and trivalent dopants respectively, and they are close to the fourth group. So, the size of the atoms is basically not so different from that of the fourth group of elements’ atoms. Extrinsic semiconductors can be further classified into two kinds:
- P-type semiconductor
- N-type semiconductor
Let us look at each of the two types in detail below.
P-Type Semiconductor
A dopant atom can be made with a tetravalent atom such as Ge or Si by doping them with a trivalent impurity like B, Al, In, etc, then the dopant atom consists of one less electron than the atoms of Ge or Si surrounding it. Thus, a hole or a vacant area is generated in the trivalent atom as the fourth atom of the tetravalent atom is free. Such semiconductors in which the holes are considered as the charge carriers are known as P-Type Semiconductors.
N-Type Semiconductor
A tetravalent atom-like Ge or Si occupies the position of an atom in the crystal lattice of the elements like Si atom when it’s doped with a pentavalent atom. The four silicon atoms that are around the four of the electrons of the pentavalent atom bond together and the fifth electron remains weakly bound to the main atom. Due to this, the electrons become free and move in the semiconductor’s lattice because the ionization energy required to free the fifth electron is very small. These types of semiconductors are known as n-type semiconductors.
Applications of Extrinsic Semiconductors
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Extrinsic semiconductors are the primary components of most electronic devices. Some applications are as follows:
- Diodes are installed in electronic devices as they require a designated unidirectional current flow.
- Both P and N-type semiconductors join together to construct a P-N junction.
- Extrinsic semiconductors are used in bipolar junction transistors and field-effect transistors.
- Most extrinsic semiconductors work as switch devices.
Extrinsic Vs Intrinsic Semiconductors
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Tabulated below are the key differences between extrinsic and intrinsic semiconductors:
| Extrinsic Semiconductors | Intrinsic Semiconductors |
| Pure semiconductors are doped to produce extrinsic semiconductors. | The pure form of semiconductors is used for intrinsic semiconductors. |
| High electrical conductivity. | Low electrical conductivity at room temperature. |
| The number of holes is not equal to the number of electrons. | The number of holes equals the number of electrons. |
| Dependent on temperature and amount of impurity. | Dependent on temperature only. |
| Classified as p and n-type. | No further classification. |
| Example: Ge is doped with Al, P, or As. | Example: Pure Ge |
Things to Remember
- An extrinsic semiconductor is made by doping a pure/intrinsic semiconductor with a particular concentration of impurity.
- Extrinsic semiconductors can be doped using two types of dopants: pentavalent and trivalent.
- An extrinsic semiconductor can be either a P-Type or N-Type.
- Holes are the majority of the charge carriers in P-Type semiconductors.
- Electrons are the majority of the charge carriers in N-Type Semiconductors.
- Doping a semiconductor does not affect the overall neutrality.
- The majority of applications of extrinsic semiconductors are in electronic devices due to their ability of unidirectional current flow.
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Sample Questions
Ques. What are semiconductors? Name the types of semiconductors. (3 marks)
Ans. Semiconductors are crystalline solids that possess an electrical conductivity of a degree falling between that of a conductor and an insulator. The valence band and conduction band in semiconductors have a small energy gap. Semiconductors allow electrons to jump up to the conduction band a little less easily than a conductor.
- P-Type
- N-Type
Ques. What are Extrinsic Semiconductors? (1 mark)
Ans. Extrinsic Semiconductors are created when we add particular impurities to a pure semiconductor. There are two kinds of Semiconductors, P-Type, and N-Type.
Ques. Name the elements that make a good semiconductor. (1 mark)
Ans. Elements such as Silicon and Germanium make good conductors as their energy band gap is small. Also, they both are tetravalent elements so, they consist of four valence electrons.
Ques. What happens when a pentavalent impurity is added to a pure semiconductor? (1 mark)
Ans. Upon adding a pentavalent impurity to an intrinsic semiconductor, an N-Type Extrinsic Semiconductor is produced.
Ques. What is doping and what are dopants? (2 mark)
Ans. A process of increasing carrier concentration by adding particular impurities to an intrinsic semiconductor is known as Doping. The atoms that are used as an impurity are called dopants.
Ques. What are pentavalent and trivalent dopants. Give examples. (2 mark)
Ans. Atoms with five valence electrons are called Pentavalent Dopants. They are used for the doping of semiconductors to produce N-Type semiconductors.Some examples are Antimony (Sb), Phosphorous (P), and Arsenic (As), etc.
Atoms with three valence electrons are called Trivalent Dopants. For example Aluminium (Al), and Boron(B).
Ques. Why are semiconductors doped? (3 marks)
Ans. Extrinsic semiconductors are those semiconductors that are doped with specific impurities. The impurity changes the electrical properties of the semiconductor, making it more suitable for electronic devices like diodes and transistors. The dopant that is added to the material is chosen in such a way that the original lattice of the pure semiconductor is not distorted. A small amount of suitable impurity when added to pure material increases its conductivity by many times. The dopants occupy only a few of the sites in the crystal of the original semiconductor which is necessary that the size of the dopant is nearly equal to the size of the semiconductor atoms.
Ques. List down some applications of semiconductors in day-to-day life? (3 marks)
Ans. Some of the major applications of semiconductors are as follows:
- Used in temperature sensors
- 3-D printing machines
- Self-driven cars
- Microchips
- Solar panels, calculators, computers, and other electronic devices
- Microprocessors used in robots and space vehicles
Ques. List down the important features of semiconductors. (3 marks)
Ans. Some of the key features of semiconductors are:
- Long life span
- Noise-free operation
- Shock-proof
- Lower power consumption
- Portable
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