ELEC 211 Lecture Notes - Lecture 15: Electric Field, Covalent Bond

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fuchsiacicada94

2 Feb 2020

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Materials can be classified by their ability to conduct electricity. This ability is related to the valence electrons. Copper is an example of an excellent conductor. It has only one electron in its valence band, which can easily escape to the conduction band, leaving behind a positive ion (the core). Like all metals, copper has many free electrons which are loosely held by the attraction of the positive metal ions. Insulators have tightly bound electrons with few electrons available for conduction. Nonmetals, such as glass, air, paper and rubber are excellent insulators and widely used in electronics. Even these materials can break down and conduct electricity if the voltage is high enough. Semiconductors are between conductors and insulators in their ability to conduct electricity. Silicon is an example of a single element semiconductor. It has four electrons in its valence band. Unlike metals, silicon forms strong covalent bonds (shared electrons) with its neighbors.

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The conduction of electricity inmaterials is explained in terms of band theory. The band that isformed from the filled bonding molecular orbitals is called thevalence band, and the band that is formed from the emptyantibonding molecular orbitals is called the conductionband. The energy difference between these bands is known as theband gap energy, or the band gap.

Click on the image below to explore the activity, which will help you to identify the insulator and thetypes of semiconductors based on band gap energy. Blue-coloredbands indicate conduction bands, and red-colored bands indicatevalence bands.

When you click on each box, you will observe the changes in bandgap energy for that material.

Part A

In the interactive activity, the bandshown at the top is the conduction band, and the band shown at thebottom is the valence band. The valence band consists of bondingmolecular orbitals that are filled with electrons, and theconduction band consists of vacant antibonding molecular orbitals.The red color indicates that the orbitals in that band are filledwith electrons, and the blue color indicates that the orbitals inthat band are empty. The space between the two bands shows theenergy difference between them and is known as the band gapenergy, or the band gap.

Which of the following statements are correct with respect tothe band structure and the band gap of different materials?

Check all that apply.

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Part B

When you add a controlled amount ofsubstance known as an impurity to a pure semiconductor, itsconductivity increases. This process is known as doping. Theaddition of an impurity can increase or decrease the number ofavailable electrons. If the number of electrons is increased, theextra electrons conduct electricity. If the number of electrons isdecreased, a vacant hole is created. The adjacent electron entersthe vacant hole and conducts electricity. Thus, it seems like theholes act as charge carriers. They are considered to be positivecharge carriers. Depending upon the charge carriers that conductelectricity, there are two types of semiconductors: p-typesemiconductors and n-type semiconductors.

Which of the following statements are correct with respect tosemiconductors?

Check all that apply.

	The undoped semiconductor has a partiallyfilled conduction band.
	The n-type semiconductor has apartially filled valence band.
	The p-type semiconductor has apartially empty conduction band.
	The p-type semiconductor has apartially empty valence band.
	The band gap of a semiconductor is lessthan that of an insulator.
	The conduction band for insulators isalways vacant.

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Part C

Germanium is a group 4A semiconductor. Theaddition of a dopant atom (group 3A element) that has fewer valenceelectrons than the host atom results in a p-typesemiconductor. The addition of a dopant atom (group 5A elements)that has more valence electrons than the host atom results in ann-type semiconductor.

Which of the following elements when used for doping germaniumwill yield p-type semiconductors, and which elements whenused for doping germanium will yield n-typesemiconductors?

Drag the appropriate elements totheir respective bins.

	The conductivity of semiconductors can beincreased by the doping process.
	The addition of group 3A elements to group4A semiconductors decreases the conductivity of the group 4Asemiconductors.
	As the temperature increases, theconductivity of semiconductors decreases.
	Holes are charge carriers in p-typesemiconductors.
	The addition of group 5A elements to group4A semiconductors increases the conductivity of the group 4Asemiconductors.
	Electrons are charge carriers inn-type semiconductors.

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Part D

Observe the band structure for fourdifferent substances in the following figure. The band shown at thetop is the conduction band, and the band shown at the bottom is thevalence band. The red color indicates that the orbitals in thatband are filled with electrons, and the blue color indicates thatthe orbitals in that band are empty. Identify which band structurecorresponds to which substance.

Drag the appropriate labels totheir respective targets.

ELEC 211 Lecture Notes - Lecture 15: Electric Field, Covalent Bond

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