Chapter 12 Solids and Modern Materials

Which of the following elements, if doped into silicon, would yield an n-type semiconductor: Ga, As, or C?

- An n-type semiconductor means that the dopant atoms must have more calence electrons than the host material, (Silicon). -Look at the periodic table and determien the number of valence electrons associated with Si ...4 Ga ...3 As ...5 C ...4 -The elements with more valance electrons than silicon are the ones that will produce an n-type material upon doping. -As, is the answer.

Compound semiconductors maintain the same _______ valence electron count as ___________ semiconductors- ______ per atom.

-AVERAGE -ELEMENTAL -4

Examples of molecular solids ...

-Ar -H2O -CO2

Silicon and germanium lay directly below ________ in the periodic table, the significance of this is...

-CARBON -Like carbon, each of these elements has four valence electrons, just the right number to satisfy the octet rule by forming single covalent bonds with four neighbors. Hence, silicon and germanium, as well as the gray form of tin, crystallize with the same infinite network of covalent bonds as diamond.

The large difference in physical properties of graphite and diamond - both of which are pure _________ - arise from differences in their ___________________ and _____________.

-CARBON -3-dimentional structure and bonding.

In a semiconductor, the valence band is filled with ___________ and the conduction band is ___________.

-ELECTRONS -EMPTY

Semiconductors can be divided into two classes: - -

-ELEMENTAL semiconductors which contain only one type of atom -COMPOUND semiconductors, which contain two or more elements.

A good example of a polymerization reaction is the formation of polyethylene from __________ __________. In this eraction, the double bond in each ethylene molecule "opens up" and two of the electrons originally in this bond are used to form new C-C single bonds with two other ethylene molecules. This type of polymerization, in which monomers are coupled through their multiple bonds, is called ______ _____________.

-Ethylene molecules -Addition polymerization

The presence of the extra bonds provided by the interstitial component causes the metal lattice to become ___________, ___________, and less_________.

-Harder, stronger, and less DUCTILE.

As we move down the periodic tables, bond distances _________, which __________ orbital overlap.

-INCREASE -DECREASES This decrease in overlap reduces the energy difference between the top of the valence band and the bottom of the conduction band. As a result, the band gap decreases on going from diamond (5.5eV), to silicon (1.11eV) to germanium (0.67 eV) to gray tin (0.08 eV).

The band gap of a compound semiconductor tends to _______ as the difference in group number ______.

-INCREASE -INCREASE

As the difference in electronegativity of the elements _________, the bonding becomes more _______ and the band gap _________.

-INCREASES -POLAR -INCREASE

In the heaviest group 4A element, _______, the band gap collapses altogether. As a result, lead has the structure and properties of a ________.

-LEAD -METAL

Most metals are __________, which means that they can be hamered into thin sheets, and ______, which means that they can be drawn into wires. These properties indicate that the atoms are capable of slipping past one another. ______ and ________ -network solids do not exhibit such behavior; they are typically brittle.

-MALLEABLE -DUCTILE -Ionic and Covalent-Network

As with n-type conductivity, p-type dopant levels of only parts per million can lead to a _________fold increase in conductivity- but in this case ...

-MILLION FOLD -the holes in the valance band are doing the conduction.

Gold is typically alloyed with...

-SILVER -COPPER

In diamond, each carbon atom is bonded to four other carbon atoms, forming a ________ In graphite, sheets are held together by _______ forces.

-Tetrahedron -Dispersion

At the bottom of a plastic container there is number and letter abbreviation below it that indicate the kind of polyer from which the container is made. These symbols make it possible to sort containers by composition. In general, the lower the number, the greater the ease with which ...

-The material can be recycled.

Examples of face-centered cubic metals include...

-aluminium -lead -copper -silver -gold

In pure Si, all of the valence-band molecular orbitals are filled and all of the conduction-band molecular orbitals are empty. Because phosphorus has five valence electrons but silicon has only four, the "extra" electrons that come with the dopant phosphorus atoms are forces to occupy the ___________ _______.

-conduction band

Polyethylene is an important material , although its composition is simple, it is ___________. Today many forms of polyethylene are known.

-difficult to make

The junction of an n-type semiconductor with a p-type semiconductor forms the basis for _____, ________, _________, and other devices.

-diodes -transistors -solar cells

Examples of body-centered cubic metals include...

-iron -chromium -Sodium -Tungsten

The doped material is called an ___ _____ semiconductor, __ signifying that the number of negativity charged electrons in the conduction band has increased.

-n-type -n

Typically, the interstitial element is a _______ that makes covalent bonds to the neighboring _________ atomsl

-non-metal -metal

Polymerization results in a micture of macromolecules with varying values of n (________________) and, hence, varying molecular weights.

-numbers of monomer molecules

Some polymers, such as ______ and __________, both of which are ________________ plastics, can be formed into fiber that, like hair, are very long relative to their cross-sectional area.

-nylon and polyesters -thermosetting

Recycling polymeric materials in the U.S. (abbreviations and matched polymer) -PET or PETE -HDPE -PVC -LDPE -PP -PS -none

-polyethylene terephthalate -High-density polyethylene -Polyvinyl chloride -Low-density polyethylene -Polypropylene -Polystyrene -other

Natural polymer consist of thing like...

-wool, leather, silk, and natural rubber

There are 7 possible shapes for a 3D unit cell:

1. Cubic 2. Tetragonal 3. Orthorhombic 4. Rhombohedral 5. Hexagonal 6. Monoclinic 7. Triclinic

Name the five basic lattice shapes:

1. Oblique lattice (most general) 2. Square lattice y=90 3. Rectangular lattice y=90 4. Hexagonal lattice y=120 5. Rhombic lattice

Alloys can be divided into 4 categories:

1. Substitutional alloys 2. interstitial alloys (both of the above are considered are both homogeneous mixtures in which components are dispersed randomly and uniformly.) 3. heterogeneous alloys, 4. intermetallic compounds.

Substitutional alloys are formed when the two metallic components have similar atomic radii and chemical-bonding characteristics. For example, silver and gold form such an alloy over the entire range of possible compositions. When two metals differ in radii by more than about ____%, solubility is greatly more limited.

15%

In the semiconductor community, energies are given in electron volts (eV) 1eV = ...

1eV = 1.602 x 10^-19 J

The elemental semiconductors all come from group ______.

4A

The semiconductor industry uses "nine-nines" silicon to make integrated circuits what this means it that Si must be _____% pure to be technologically useful.

99.999999999999%

An __________ is a material that contains more than one element and has the characteristic properties of a metal.

ALLOY.

___________ solids (from the greek word without form) lack the order found in crystalline solids. These solids are similar to strutures of liquids but do not have the freedom of motion. Examples are rubber, glass, obsidian (volcanic glass).

Amorphous solids

Which of these statements is false? a. as you go down column 4A in the periodic table, the elemental solids become more electrically conducting b. As you go down column 4A in the periodic table, the band gaps of the elemental solids decrease. c. The valence electron count for a compound semiconductor averages out to four per atom. d. Band gap energies of semiconductors range from ~0.1-3.5 eV. e. In general, the more polar the bonds are in compound semiconductors, the smaller the band gap.

B.

Examples of alloys:

Bronze = copper + tin Brass = Copper + zinc

Synthetic polymers have a backbone of ________-________ bonds. Becuase these atoms have an exceptional ability to form strong stable bonds with one another.

CARBON-CARBON

When you place an additional lattice point in a specific location in the unit cell, you create

CENTERED lattices

The band that forms the antibonding orbitals is called the ...

CONDUCTION BAND.

___________ consist of atoms held together in large networks by covalent bonds. Because covalent bonds are much stronger than intermolecular forces, these solids are much harder and have higher melting points than molecular solids.

COVALENT-NETWORK SOLIDS

Polymers may possess short-range order in some regions of the solid, with chains lined up in regular arrays. The extent of such ordering is indicated by the degree of _____________ of the polymer.

CRYSTALLINITY

Compound semiconductors can be doped to make n-type and p-type materials, but the scientist has to make sure that the proper atoms are substituted for example, if Ge were doped into GaAs, Ge could substitute for Ga, making an n-type semiconductor; but if Ge substituted for As, the material would be p-type. Suggest a way to dope CdSe to create a p-type material.

Compound semiconductors have an average of four electron count per atom which constitute to a total of eight valence electrons. If the totaly count is more than eight than the compound is n-type semiconductor and if less than eigth then it is p-type semiconductor. Cd has two valence electrons and Se has six valence electrons whic makes the total count eight. If CdSe is doped with Ag (silver) which has one calence electron in place of two valence electrons, then total electron count along with six electrons of Se is seven which makes is p-type material. Similarly if CdSe is doped with N (nitrogren) in place of Se, then again the total electron count with two valence electrons of Cd and five valence electrons of N, is seven which makes a p-type material.

A second general reaction used to synthesize commercially important polymers is ______________ ______________. In this reaction two molecules are joined to form a larger molecule by elimination of a small molecule, such as H2O. For example, an amine (a compound containing - NH2) reacts with carboxylic acid (a compound containing -COOH) to form a bond between N and C plus an H2O molecule.

Condensation Polymerization

Polymers formed from two different monomers are called ________________. In the formation of nylons, a diamine , a coupond with the -NH2 group at each end, is reacted with a diacid, a compound with the -COOH group at each end.

Copolymers

Responsible for the unique properties of semiconductors as well as materials like diamond.

Covalent-network solids

______________ solids are held together by an extended network of covalent bonds. Resulting in materials are that extremly hard.

Covalent-network solids

Consider what happens when a few phosphorus atoms (known as ______) replace silicon atoms in a silicon crystal.

DOPANTS

The electrical conductivity of a semiconductor is influences by the presence of small numbers of impurity atoms. The process of adding controlled amounts of impurity atoms to a material is known as _______.

DOPING

____________ and ___________ , two allotropes of carbon, are two of the most familiar covalent-network solids. Other examples are silicon, germanium, quartz (SiO2), silicon carbide (SiC), and baron nitride (BN). In all cases, the bonding between atoms is either completely covalent or more covalent than ionic.

Diamond and Graphite

Another type of plastic is the _______________, which is a material that exhibits rubbery or elastic behavior. When subjected to stretching or bending, an elastomer regains its original shape upon removal of the distorting force, if it has not been distorted beyond some elsatic limit. _________ is the most familiar example of this.

ELASTOMER -Rubber

The crystaline structure of graphene illustrates two important characteristics of crystals....

First, we see that no atoms lie on the lattice points. Thus, to build up a structure you must know the location and orientation of the atoms in the motif with respect to the lattice points. Second, we will see that bonds can be formed between atoms in neighboring unit cells and the bonds between atoms need not be parallel to the lattice vectors.

Will GaP have a larger or smaller band gap than ZaN?

For both GaP and GaN the more electropositive element is gallium. SO we need only compare the positions of the more electronegative elements, P and N. Nitrogen is located above phosphorous in group 5A. Therefore, we would expect GaN to have a larger band gap than GaP.

It is also possible to dope semiconductors with atoms that have fewer valence electrons than the host material. Consider what happens when a few aluminum atoms replace silicon atoms in a silicon crystal. Aluminum has only 3 valence electrons compared to silicon's four. Thus, there are electron vacancies, known as ________, in the valence band when silicon is doped with aluminum.

HOLES since the negaitvely charged electron is not there, the hole can be though of as having a positive charge. Any adjacent electron that jumps into the hole leve behind a new hole. Thus the positive hole moves about in the lattice like a particle.

Metallic Solids

Held together by "delocalized sea" of collectively shared valence electrons.

The different types of atoms in an __________ compound are ordered rather than randomly distributed. This leads to better structural stability and higher melting points. Negatively, they tend to be more brittle than substitutional alloys.

INTERMETALLIC

When the solute atoms occupy interstitial positions in the "holes" between solvent atoms, we have an _____________ alloy.

INTERSTITIAL

__________ solids are held together by the mutual electrostatic attraction between cations and anions. Differences between ionic and metallic bonding make the eletrical and mechanicals properities of these solids very different from those of metals.

IONIC

Which of these doped semiconductors would yield a p-type material? (written as host atom:dopant atom) a. Ge:P b. Si:Ge c. Si:Al e. Si:N

If the valence electrons of dopant are more than that of host, the material formed is n-type semiconductor. If the valence electrons of dopant are less than that of host, the material formed is p-type semiconductor. a. Ge:P....... 4-5 n-type b. Si:Ge.......4-4 neither, they form pure semiconductor c. Si:Al.......4-3 p-type e. Si:N........4-5 n-type correct answer is C

For example, Eg = 0.67 in Ge, but Eg =1.43 eV in GaAs. If we increase the difference in group number to four, as in ZnSe (groups 2b and 6A), the band gap _________________

Increases to 2.70 eV. This progression is a result of the transition from pure covalent bonding in elemental semiconductors to polar covalent bonding in compound semiconductors.

__________________ forces between the polymer chains hold the chains together in the ordered crystalline regions, making the polymer denser, harder, less soluable, and more resistant to heat.

Intermolecular

_______ compounds tend to be brittle because when stress is applied, there is a shift so that the alignmnet becomes cation-cation, anion-anion. The resulting repulsive interaction causes the planes to split away. (carving of gemstones).

Ionic solids/o

In 1827, ___________________ coined the word polymer (from the greek polys, "many", and meros, "parts").

Jons Jakob Berzelius this word was to denote molecular substances of high molecular weight formed by the polymerization (joining together) of monomers, moleculars with low molecule weight.

_______________are held together by the intermolecular forces we studied in ch. 11. Because these forces are relatively weak, they tend to be soft and have low melting points.

MOLECULAR SOLIDS

In most crystals, the atoms are not exactly coincident with the lattice points. Instead, a group of atoms, called a _______, is associated with each lattice point.

MOTIF

_________ __________ or pulling to align the chains as the molten polymer is drawn through small holes can frequently enhance the crystallinity of a polymer.

Mechanical stretching.

Are solids in which the dimensions of individual crystals have been reduced to the order of 1-100nm,

NANOMATERIALS

In a heterogeneous alloy, the components are ...

NOT dispersed uniformly. In general, the properties rely on both the composition and the manner in which the solid is formed from the molten mixture.

A material like this is called a _____-type semiconductor.

P-TYPE P signifying that the number of positive holes in the material has increased.

____________ are polymeric solids that can be formed into various shapes, usually by the application of heat and pressure. There are several types.

PLASTICS

Contained long chains of atoms (usually carbon) where the atoms within a given chain are connected by covalent bonds and adjacent chains are held to one another largely by weaker intermolecular forces.

POLYMERS

Two classes of solids that do not fall into the other categories:

POLYMERS and NANOMATERIALS

When you place a lattice point at each corner of a unit cell, you get a _________________ lattice

PRIMITIVE

Intermetalic compounds have definite...

PROPERTIES and their composition cannot be varied.

Are normally stronger and have higher melting points than molecular solids, more flexible then metallic, ionic, or covalent-network solids.

Polymers

Structures of metallic solids: 1. 2. 3.

Primative cubic structure Body-centered cubic structure Face-centered cubic structure

The centered rectangular lattice is another name for the ______

RHOMBIC lattice

Metals conduct electricity extremely well. Many solids, however, conduct electricty somewhat, but nowhere near as well as metals, which is why such materials are called _____________.

SEMICONDUCTORS

When atoms of the solute in a solid solution occupy positions normally occupied by a solvent atom, we have a ___________________ alloy.

SUBSTITUTIONAL

Two example of semiconductors are _______ and _______.

Silicon and germanium

The simple structural formulas given for polyethylene and other polymers are deceptive. Becuase four bond surround each carbon atom in polyethylene, the atoms are arranged in a __________ fashion, so that the chain is not straight.

TETRAHEDRAL the flexibility in these chains causes any material made of this polymer to be very flexible.

There are _____ atoms in the primitive hexagonal unit cell, one from each layer.

TWO

Responsible for the fact that most metals are relatively strong without being brittle.

The "delocalized sea" that holds together metallic solids.

This form of bonding allows metals to conduct electricity...

The "delocalized sea" that holds together metallic solids.

Diamond is strong because ...

The carbon atoms are sp3-hybridized and held together by strong carbon-carbon single covalent bonds. The strength and directionality of these bonds make diamond the hardest known material.

What is the coordination number?

The number of atoms immediately surrounding a given atom in a crystal structure.

Will GaP have a larger or smaller band gap than ZnS?

The size of the band gap depends on the vertical and horizontal positions of the elements in the periodic table. Thus, we would expect the electronegativity difference to be larger for ZnS, which should result in ZnS having a larger band gap than GaP.

_______________ can be reshaped. For example, plastic milk containers are made from ___________ polymer polyethylene. These containers can be melted down and the polymer recycled for some other use.

Thermoplastics

Unlike thermoplastic, a _________________ plastic (also called a _________) is shaped through irreversible chemical processes and, therefore cannot be reshaped readily. Vulcanized rubber and polyurethanes found in commercial products including insultating foams and mattresses are familiar examples of ____________ plastics.

Thermosetting also called a thermoset

The small repeating unit found in crystalline solids

UNIT cell

Unlike metals, in semiconductors an energy gap develops between the filled and empty states, much like the energy gap between bonding and antibonding orbitals. The band that forms from bonding molecular orbitals is called the ....

VALENCE BAND.

Hexagonal close packing ...

When the fourth layer lays directly over the first layer, leading to the ABAB stacking pattern which is called this.

These two bands are separated by the energy _______ __________

_BAND GAP -Eg

High-density polyethylene (HDPE), used to form bottles, drums, and pipes, has an average molecular weight in the range of 10^6amu and has a density of 0.94 g/cm^3 or higher. This form has fewer side chains and thus ....

a higher degree of crystallinity.

These extra electrons can move very easily in the conduction band. Thus, just a few parts per million (ppm) of phosphorus in silicon can increase silicon's intrinsic conductivity by a factor of ___________.

a million!

The structure of a crystalline solid is defined by a.) b.)

a.) the size and shape of the unit cell b.) the locations of atoms within the unit cell

In part because of there distribution in molecular weights, polymers are largely ______________ (noncrystalline) materials,

amorphous

The crystal structures of many metals are simple and can be generated by placing a single ...

atom on each lattice point.

When atomic s and p orbitals overlap, they form...

bonding molecular orbitals and antibonding molecular orbitals. Each pair of s orbitals overlaps to give one bonding and one antibonding molecular orbital, whereas the p orbitals overlap to give three bonding and three antibonding molecular orbitals.

Geometrical pattern of points on which the unit cells are arranged is called a _____

crystal lattice (it is an abstract, not real) Imaginary.

Solids in which atoms are arranged in an orderly repeating pattern are called...

crystalline solids

These solids usually have flat surfaces or faces that make definite angles with one another. Examples are sodium chloride, quartz, and diamond.

crystalline solids

When the spheres in the third layer do not lay directly over the first or second layer. When this sequence repeats itself, you get ABCABCABC results known as ...

cubic close packing.

Molecular solids consist of atoms or neutral molecules held together by: - - -

dipole-dipole forces Dispersion forces Hydrogen bonds

Because the valence electron in ionic compounds are confined to the anions, rather than being delocalized, ionic compounds are typically ______________.

electrical insulators.

The dramatic change in conductivity in response to the addition of a trace amount of a dopant means that ...

extreme care must be taken to control the impurities in semiconductors.

The linear structure of polyethylene is conducive to .... However, the degree of crystallinity in polyethylene strongly depends on the average ____________ _____________.

intermolecular interactions that lead to crystallinity. molecular weight

Both synthetic and natural polymers commonly consists of a collection of____________ (large molecules) of different molecular weights.

macromolecules

A Face-centered cubbic lattice has...

one lattice point at the center of each of the six faces of the unit cell in addition to the lattice points at the eight corners.

A body-centered cubic lattice has ...

one lattice point at the center of the unit cell in addition to the lattice points at the eight corners.

Rather than exhibiting a well-defined crystalline phase with a sharp melting point, polymers soften .....

over a range of temperatures.

Low density polyethylene (LDPE) used in forming films and sheets, has an average molecular weight in the range of 10^4 amu, has a density of less than 0.94 g/cm^3, and has substantial chain branching. That is, there are side chains off the main chain of the polymer. These side chains inhibit the formation of crystalline regions, thus ...

reducing the density of the material.

For example, in gallium arsenide, GaAs, each Ga atom contributes three electrons and each As atom contributes five, which averages out to four per atom- the same number as in silicon or germanium. Hence, GaAs is a ___________.

semiconductor

The most efficient way to pack one layer of equal-sized spheres is to ...

surround each sphere by six neighbors.

Examples of metals with primative cubic structure:

they are rare, radioactive element polonium is an example.

What needs to happen for crystal structure and the lattice points to have identical patterns?

this happens in solids which have atoms that are identical, in other words, ELEMENTS can form structures of this type.

Band gaps greater than ~_______ are so large that the material not a semiconductor; it is an __________ and does not conduct electricity.

~3.5 eV INSULATOR