Electronic Propeties Of Solids

Question 1

What three charge carriers can a material conduct via?

Answer 1

Electrons, holes or ions

Question 2

What is the equation for ionic conductivity?

Answer 2

n = no. of charge carriers
e=charge
Mew=mobility

Question 3

What are the minimum requirements for a material to be an ionic conductor?

Answer 3

Either:
Some ion sites in the crystal structure are vacant so neighbouring ions can hop into vacant sites
Or
Some interstitial sites are occupied allowing ions to hop into neighbouring interstitial sites

Question 4

What happens to the number of point defects as temperature increases?

Answer 4

The number of stoichiometric point defects increases

Question 5

How do you calculate the number of Schottky defects in an ionic lattice?

Answer 5

Question 6

How do you calculate the number of Frenkel defects in an ionic lattice?

Answer 6

Question 7

When do intrinsic (Schottky) contributions dominate the ionic conductivity?

Answer 7

At high temperatures (T)

Question 8

When do extrinsic (impurity) contributions dominate the ionic conductivity?

Answer 8

At low temperature (T)
The extrinsic contribution will depend only on the concentration of impurities and their mobility

Question 9

What are the observed trends between (temperature) and (log conductivity) for metals,superconductors,semiconductors and insulators?(General graph shapes)

Answer 9

Question 10

Define band gap energy and bandwidth.

Answer 10

Band gap energy: energy between two different bands.
Bandwidth: the energy spread of a band. Increased atomic orbital overlap increases bandwidth.

Question 11

What is the Fermi level?

Answer 11

Below Fermi level= valence band
Above= conduction band

Question 12

How does the FCC crystal structure lead to an insulating material?

Answer 12

Low coordination number (4) means reduced ways in which the 2s and 2p AO’s can overlap. Therefore a small bandwidth.

Question 13

Why is Silicon a semiconductor while diamond is an insulator?

Answer 13

The band structures are similar however silicon has a smaller band gap. At Temp> Eg, electrons can be promoted into the conduction band.

Question 14

Explain the increase in conductivity down group 14

Answer 14

-AO’s involved in band formation become more diffuse( better overlap)
-Electronegativity decreases- weaker bonds formed so top of conduction band and bottom of valance band are closer in energy.

Question 15

What is an intrinsic semiconductor ?

Answer 15

Question 16

What is an extrinsic semiconductor?

Answer 16

Doping a material results in the presence of electrons or holes (mobile charge carriers)

Question 17

What equation does the temperature dependence of the conductivity of an intrinsic semiconductor follow?

Answer 17

Arrhenius type behaviour
Eg= band gap energy
Kb= Boltzmann constant

Question 18

What equation tells us the fraction of states in the conduction band that may be populated at a given T?

Answer 18

Question 19

If you dope silicon with some boron atoms, what type of doping is this?

Answer 19

P-type, boron has one less valance electron than Silicon. Therefore for every boron atom there is one less electron in the band structure (holes used as carriers)

Question 20

If you dope silicon with some phosphorus atoms, what kind of doping is this?

Answer 20

Extrinsic n-type doping. Phosphorous has one more valence electron, so for every doped phosphorous atom there is an extra electron in the band structure (electrons used as carriers)

Question 21

What makes a semiconductor photoconductive?

Answer 21

Their conductivity increases upon irradiation with light. (Electrons can move from the valence to the conduction band)

Question 22

A large electronegativity difference between elements in a solid favours what bonding?

Answer 22

Ionic bonding (large electro negativity difference usually means larger band gap)

Question 23

What is the Hubbard energy (U)

Answer 23

Energy cost associated with having two electrons on the same atom when a conduction electron hops from one atom to the next.
-If bandwidth energy<U electrons remain localised (material is semiconducting or insulating)
-If bandwidth energy>U then electrons can delocalise (material becomes metallic)

Question 24

What are 4 things to look for when determining whether good overlap of d-orbitals is likely in TM compounds?

Answer 24

-charge on cation is small
-cation sits early in transition row
-cation sits in second or third transition row
-anion is reasonably electro positive

Question 25

What two unique properties define superconductors ?

Answer 25

They conduct electrical current with zero resistance (below Tc)

They expel magnetic fields from their interior

Question 26

What are phonons?

Answer 26

Collective lattice vibrations in a solid (increase with temperature)

Question 27

What is the isotope effect?

Answer 27

For metal superconductors, it can be shown that Tc depends on the isotopic mass of the metallic element.
This dependence bears resemblance to that known for reduced mass and bond vibrational frequencies,suggesting superconductivity is somehow related to the vibrational modes of the crystal lattice.

Question 28

What is the mechanism of BCS theory ?

Answer 28

-As a conduction electron passes through the superconductor, it distorts the lattice creating a region of enhanced positive charge
-Next conduction electron is attracted to this enhanced positive charge created by the lattice vibration of the first electron, so can pass through the material easily.
Electrons 1&2 are coupled though this PHONON MEDIATED ATTRACTION and form a COOPER PAIR.

Question 29

What equation would you use to calculate the vacancy conduction of alkali halides?

Answer 29