9 Band Theory

Question 1

Metals vs Insulators

Answer 1

Metal: High conductivity (σ)/ low resistivity (ρ)

Insulator opposite

Conductor in between

Question 2

conductivity

Answer 2

large CN in metals give rise directional bonding

• bcc =8, fcc=12

Question 3

Bonding metals

Answer 3

atoms increase the distance b/w E levels w/in bonding/anti-bonding orbitals

Question 4

As number of atoms ______, the distance
between energy levels within bonding and
antibonding regions __________.

Answer 4

Increases, decreases

Question 5

Good conductor must have

Answer 5

1) high [ ] mobile charge carriers
2) delocalized bad where e can move
3) a partially filled band (10-90%)

Question 6

Is Ag or Cu more conductive?
Na or Cu

Answer 6

• Ag cuz 4d e less localized
• Na has fewer e so less conductive

Question 7

optical properties

Answer 7

what’s abs gets reflected (opp molecules)

Question 8

Steel
Lead
Mg

Answer 8

• has impurities
• more directional bonding like C
• liquid so get more scattering of charge carriers

Question 9

can e be exited at RT?

Answer 9

yes, into empty orbitals and are free to move

Question 10

When does conductivity increase? decrease

Answer 10

• increase: # free e increase
• decrease: degree covalent bonding increases

Question 11

Are metals strong absorbers/ reflectors?

• why can electronic transitions be induced

Answer 11

• yes
• there closely spaced empty E levels

Question 12

Electrons in the metal readily _____
electromagnetic radiation and oscillate at the
_____ ___________ as the incoming light.

Answer 12

Absorb, same frequency

Question 13

Bands

Answer 13

closely spaced E levels, not discrete

Question 14

semiconductors and insulators characterized with band gaps (Eg)

what are the 2 bands

Answer 14

CB: empty conduction band
VB: filled valence band

Question 15

ev for insulators, semiconductors

Answer 15

Semi: 0<Eg<= 4ev
- narrow gap: <0.5

insulators: Eg>4ev

Eg~0 semimetal

Question 16

5 types semiconductors

Answer 16

• Elemental semiconductors: same atom (Si, Ge, etc.).
  – Compound semiconductors: two + elements (GaAs, CdSe, etc.)
  – Intrinsic semiconductors: No dopants added.
  – Extrinsic semiconductors: Doped
  – Direct and indirect bandgap semiconductors.

Question 17

when are all the e in VB

Answer 17

0K

Question 18

what is a hole h+

Answer 18

absence e in VB after e promoted

Question 19

Band diagrams

Answer 19

E y-axis, k, x-axis

give info about likely hood of transition

Question 20

Direct vs indirect band gap

Answer 20

• direct: momentum is conserved
• indirect: need photon change momentum not = 0

Question 21

Fermi level

Answer 21

avg E at 0K (no actual e there)

E where probability finding e is 1/2 at 0K

Question 22

can you actually find e in bandgap

Answer 22

• no probability of finding e in bandgap

Question 23

Density of states plot

Answer 23

shows how many E levels for given E (look like half moons)

Question 24

What happen in DOS plots above 0K

Answer 24

some E levels above Fermi level fill

Question 25

how do e excite to CB

Answer 25

Thermal or use photon

Question 26

What is recombonation

Answer 26

e falls CB to VB

Question 27

What is doping

Answer 27

add impurity to improve conductivity

add small amount to not change other properties

Question 28

n- dopping

Answer 28

Add atom with more e get filled E level close to the CB
- e major charge carriers

Question 29

p-doping

Answer 29

add atoms with less e get empty E level just above VB

• h major charge carriers

Question 30

Doping size

Answer 30

similar to other atoms