Problem 4 Why does the conductivity of a s... [FREE SOLUTION] (2024)

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Chapter 13: Problem 4

Why does the conductivity of a semiconductor change with impurity content?Compare this with the behaviour of metallic conductor.

Short Answer

Expert verified

Semiconductor conductivity increases with doping due to more charge carriers, while metallic conductivity decreases with impurities due to electron scattering.

Step by step solution


- Understand semiconductor conductivity

Semiconductors have a conductivity between that of insulators and conductors. Their conductivity is influenced by the presence of impurities or doping.


- Explain the effect of impurities on semiconductors

When impurities, such as phosphorus (donor) or boron (acceptor), are added to a semiconductor like silicon, they introduce extra charge carriers (electrons or holes). This process is known as doping. Increased doping leads to higher conductivity due to a greater number of charge carriers.


- Understand metallic conductors

In metallic conductors, the conductivity is primarily due to the presence of free electrons that move easily within the metal's lattice structure. Adding impurities usually scatters these free electrons, which decreases the metal's conductivity.


- Compare semiconductor and metallic conductor behavior

Unlike semiconductors, where doping increases conductivity, adding impurities to a metallic conductor usually results in increased electron scattering and thus lower conductivity. This contrasting behavior is due to the different ways charge carriers are affected in each material.


- Summarize the comparison

The conductivity of semiconductors increases with impurity content due to additional charge carriers from doping, whereas the conductivity of metallic conductors decreases with impurity content due to increased electron scattering.

Key Concepts

These are the key concepts you need to understand to accurately answer the question.


Doping is a crucial technique used to alter the electrical properties of semiconductors. This process involves adding a small amount of impurity atoms to an intrinsic (pure) semiconductor. These impurities can either donate free electrons to the semiconductor (known as donors) or create holes (known as acceptors) by accepting electrons. Common donor atoms include phosphorus, while boron is a typical acceptor atom.

The purpose of doping is to increase the number of charge carriers, thus enhancing the material’s conductivity. In a pure semiconductor, the number of electrons and holes are very limited. However, doping introduces more free electrons or holes, significantly improving the semiconductor's ability to conduct electricity.

Charge Carriers

Charge carriers are particles that carry electric charge through a material, either electrons (negative charge) or holes (positive charge). In semiconductors, the conductivity is highly dependent on the availability and movement of these charge carriers.

When a semiconductor is doped with donor impurities, additional free electrons become available. Similarly, acceptor impurities create holes by trapping electrons, resulting in positive charge carriers. The movement of these electrons and holes under an electric field facilitates the flow of current through the semiconductor.

In a highly doped semiconductor, there are more charge carriers available, leading to higher conductivity. This is the reason why increasing impurity levels, through doping, directly impacts and increases the conductivity of semiconductors.

Impurities in Materials

Impurities in materials, particularly in semiconductors and metallic conductors, significantly affect their electrical properties. In semiconductors, impurities, or dopants, are intentionally introduced to control their conductivity. The type and amount of impurities can be precisely controlled to tailor the semiconductor for specific applications.

In contrast, impurities in metallic conductors often have a detrimental effect. Metals rely on free electrons for conductivity, and any foreign atoms present can disrupt this electron flow. The presence of these impurity atoms hinders the movement of electrons, causing increased resistance and decreased conductivity.

Therefore, while impurities are beneficial for altering and improving the properties of semiconductors, they generally have a negative impact on metallic conductors.

Metallic Conductors

Metallic conductors work on a different principle compared to semiconductors. They have a lattice structure where free electrons can move easily, allowing them to conduct electricity efficiently. These free electrons come from the outer shells of metal atoms.

Unlike semiconductors, where conductivity is increased by introducing impurities, adding impurities in metallic conductors generally hampers their performance. Impurities in metals cause electron scattering, which obstructs the flow of electrons, resulting in increased resistance and decreased conductivity.

This fundamental difference in how impurities affect semiconductors and metals is essential for understanding material behavior in different applications.

Electron Scattering

Electron scattering is a phenomenon where moving electrons are deflected by impurities, lattice vibrations, or other disturbances within a material. In metallic conductors, this is particularly significant. The presence of impurities disrupts the orderly movement of free electrons, causing them to scatter and lose energy, thus increasing resistance.

In contrast, in semiconductors, the addition of impurities through doping creates more charge carriers (electrons or holes), which increases conductivity. Although scattering can also occur, the overall impact on conductivity is positive due to the larger number of charge carriers.

Understanding electron scattering is key to manipulating and predicting the conductivity of different materials. This knowledge is applied in designing and optimizing electronic devices for improved performance.

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Problem 4 Why does the conductivity of a s... [FREE SOLUTION] (3)

Most popular questions from this chapter

What is intrinsic semiconductor? Obtain an expression for the intrinsiccarrier concentration in an 1 intrinsic semiconductor. Under what conditionwill the Fermi level be in the middle of the forbidden gap?What are intrinsic and extrinsic semiconductors? Discuss the location of theFermi levels under suitable limiting conditions and give the necessary theory.Calculate the current produced in a small germanium plate of area \(1\mathrm{~cm}^{2}\) and of thickness \(03 \mathrm{~mm}\), when a potential difference of 2 volts is apphed across its faces. The intrinsiccarrier concentration of geranium is \(2 \times 10^{19} / \mathrm{m}^{3}\) andthe mobilities of electron and holes are \(0.36\) and \(0.17 \mathrm{~m}^{2}\mathrm{~V}^{-1} \mathrm{~s}^{-1}\), respectively.If a sample of silicon is doped with \(3 \times 10^{23}\) arsenic atoms and \(5\times 10^{23}\) atoms of boron, determine electron and hole concentrations ifthe intrinsic charge carriers of silicon are \(2 \times 10^{16} /\mathrm{m}^{3}\)
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Problem 4 Why does the conductivity of a s... [FREE SOLUTION] (2024)


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