9 Band Theory Flashcards
Metals vs Insulators
Metal: High conductivity (σ)/ low resistivity (ρ)
Insulator opposite
Conductor in between
conductivity
large CN in metals give rise directional bonding
- bcc =8, fcc=12
Bonding metals
atoms increase the distance b/w E levels w/in bonding/anti-bonding orbitals
As number of atoms ______, the distance
between energy levels within bonding and
antibonding regions __________.
Increases, decreases
Good conductor must have
1) high [ ] mobile charge carriers
2) delocalized bad where e can move
3) a partially filled band (10-90%)
Is Ag or Cu more conductive?
Na or Cu
- Ag cuz 4d e less localized
- Na has fewer e so less conductive
optical properties
what’s abs gets reflected (opp molecules)
Steel
Lead
Mg
- has impurities
- more directional bonding like C
- liquid so get more scattering of charge carriers
can e be exited at RT?
yes, into empty orbitals and are free to move
When does conductivity increase? decrease
- increase: # free e increase
- decrease: degree covalent bonding increases
Are metals strong absorbers/ reflectors?
- why can electronic transitions be induced
- yes
- there closely spaced empty E levels
Electrons in the metal readily _____
electromagnetic radiation and oscillate at the
_____ ___________ as the incoming light.
Absorb, same frequency
Bands
closely spaced E levels, not discrete
semiconductors and insulators characterized with band gaps (Eg)
what are the 2 bands
CB: empty conduction band
VB: filled valence band
ev for insulators, semiconductors
Semi: 0<Eg<= 4ev
- narrow gap: <0.5
insulators: Eg>4ev
Eg~0 semimetal
5 types semiconductors
- 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.
when are all the e in VB
0K
what is a hole h+
absence e in VB after e promoted
Band diagrams
E y-axis, k, x-axis
give info about likely hood of transition
Direct vs indirect band gap
- direct: momentum is conserved
- indirect: need photon change momentum not = 0
Fermi level
avg E at 0K (no actual e there)
E where probability finding e is 1/2 at 0K
can you actually find e in bandgap
- no probability of finding e in bandgap
Density of states plot
shows how many E levels for given E (look like half moons)
What happen in DOS plots above 0K
some E levels above Fermi level fill
how do e excite to CB
Thermal or use photon
What is recombonation
e falls CB to VB
What is doping
add impurity to improve conductivity
add small amount to not change other properties
n- dopping
Add atom with more e get filled E level close to the CB
- e major charge carriers
p-doping
add atoms with less e get empty E level just above VB
- h major charge carriers
Doping size
similar to other atoms
