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Chapter 10: Problem 65

Explain how doping silicon with either phosphorus or gallium increases the electrical conductivity over that of pure silicon

Short Answer

Expert verified
Doping silicon with phosphorus (n-type) introduces an extra free electron per phosphorus atom by utilizing the fifth valence electron, increasing the number of negatively charged carriers. Doping silicon with gallium (p-type) creates positively charged "holes" by utilizing only three valence electrons, making it easier to accept electrons from neighboring atoms. Both n-type and p-type doping increase the number of free charge carriers available in the silicon lattice, ultimately resulting in increased electrical conductivity compared to pure silicon.

Step by step solution

01

1. Understand the concept of doping

Doping is the process of adding impurity atoms to a semiconductor material like silicon to improve its electrical properties. The impurity atoms can either be an n-type dopant (like phosphorus) which contributes extra free electrons or a p-type dopant (like gallium) which creates holes (absence of electrons) in the material. The purpose of doping is to increase the number of free charge carriers in the semiconductor, which in turn increases its electrical conductivity.
02

2. Understand the properties of pure silicon

Pure silicon is a semiconductor with a covalent bond structure. Each silicon atom in the lattice shares its four valence electrons with neighboring silicon atoms, forming a stable and full outer electron shell. This bonding structure leaves no free electrons to conduct electricity, making pure silicon a poor conductor of electricity.
03

3. Analyze the effect of n-type doping (phosphorus)

Phosphorus has five valence electrons, compared to four in a silicon atom. When phosphorus is introduced into the silicon lattice, it forms four covalent bonds with neighboring silicon atoms, leaving one free electron. This free electron is now available to participate in electrical conduction, thereby increasing the conductivity of the doped material. This is called n-type doping because the primary charge carriers are negatively charged electrons.
04

4. Analyze the effect of p-type doping (gallium)

Gallium has three valence electrons, compared to four in a silicon atom. When gallium is introduced into the silicon lattice, it forms only three covalent bonds with neighboring silicon atoms, leaving an empty space called a "hole" in the lattice structure where an electron would typically be found. This hole can easily accept an electron from a neighboring silicon atom, allowing the hole to move through the lattice. A moving hole acts as a positively charged particle, making the doped material more conductive. This is called p-type doping, as the primary charge carriers are positively charged holes.
05

5. Understand the increased electrical conductivity

When silicon is doped with either n-type (phosphorus) or p-type (gallium) impurities, the number of free charge carriers (i.e., electrons for n-type doping and holes for p-type doping) increases. These free carriers can move more easily through the doped material when an electric field is applied, thereby increasing the electrical conductivity compared to pure silicon. In this way, doping silicon with either phosphorus or gallium increases its electrical conductivity, allowing it to be used more effectively in electronic devices.

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Most popular questions from this chapter

You and a friend each synthesize a compound with the formula \(\mathrm{XeCl}_{2} \mathrm{F}_{2} .\) Your compound is a liquid and your friend's compound is a gas (at the same conditions of temperature and pressure). Explain how the two compounds with the same formulas can exist in different phases at the same conditions of pressure and temperature.

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