I5 Flashcards
(38 cards)
What is the equation for conductivity?
‘Sigma’= n * e * ‘mu’
n=no. Charge carriers
e= charge
Mu= mobility of carriers
What are the main differences between insolators, conductors (metals) and semiconductors?
Conductivity- metals 10^-1 to 10^5, insulators < 10^-10, semiconductors 10^-5 to 10^2
n is large for metals and independent of temp, where semiconductors and insulators increase exponentially with temp
How do you approximate the band structure of solids?
LCAO for a chain of n atoms so n MO’s in phase so lower energy, vice versa for out of phase. In between N-2nother combinations (between homo and lumo)
As n increases no. Of levels increases and average separation decreases allowing them to be seen as a ‘band’
S and P combine into SP band etc
What does the band structure of Al look like?
1S, 2s, 2p low overlap. 3s and 3P large overlap, electrons are distributed over lower levels of S-P band and Al is CCP this forms the valence band in S-P overlap
What is the fermi level?
Highest occupied level at T=0 K
What is essential for good electronic conductivity?
A partly full valence band for the ease of promoting electrons
What is a phonon?
A set of vibrations where all ionic cores all vibrate together. Which can impede electron passing through the lattice.
What is the key property that lets an insulator act as one?
Completely full valence band and empty conduction band separated by a large energy gap (Eg)
What are the 2 types of charge carrier in semiconductors?
Electrons pro,tied to conduction band
Holes left in valence band- +ve holes ‘move’ when an electron moves into them
What is an intrinsic semiconductor?
Pure materials, electron no. In conduction band controlled by size of energy gap and temp
E.g. silicon sigma band is full and sigma* is empty
What is an extrinsic semiconductor?
Conductivity controlled by addition of dopants of difference valency
Si can become extrinsic via addition of Group 13(Ga) or Group 15 (As) elements
What is P type semiconductivity?
In Si-Ga example Ga replaces some Si making it deficient by 1 electron, this forms a discrete energy level just above valence band- ACCEPTOR LEVEL
Allows easy electron promotion. Conduction occurs via positive (P) holes in valence band
What is n-type semiconductivity?
Si doped with As gains an extra electron which goes into discrete energy level below the conduction band. Electrons cannot move through solid but it acts as a donor level with easy promotion
Conduction occurs via negative(n) electron provided in conduction band
What happens if d-orbitals overlap?
A PARTIALLY FILLED BAND CAN FORM, LEADING TO HIGH CONDUCTIVITY
What is required for good overlapped d-band formation?
1)Formal charge on cations is small
2)Early TM cations
3)Cation in 4dnor 5d transition series
4)An ion is relatively electropositive
What are the 4 most common types of extended defects?
Dislocations, grain boundaries, stacking faults and twinning
What are the 2 most common types of point defect?
Schottky defects- Stoichiometric defects in ionic crystals- alkalihalides
Frenekel defects
What is the schottky defect equation?
ns= N exp(-delta(Hs)/2RT)
N= no. of possible cation sites per unit volume
delta(Hs)= molar enthalpy of formation of schottky defect
What are frenkel defects?
Displacement of atom/ion from its lattice site to an interstitial site creating a vacancy, silver halides commonly exhibit. Normally cation that moves, although anion can occur in fluorite structure
What is the equation for frenkel defects?
nF= (N*Ni)^(1/2) exp(-delta(Hf)/2RT
nF= conc of frenkel defects
N= conc particular lattice sites
Ni= conc interstitial sites available
How do you tell the difference between Frenkel + Schottky defects?
Calculated crystallographic densities- X-ray diffraction, Schottky gives lower density than calculated
How are defects created?
Substitutions will be different charge but similar size.
E.g. NaCln doped with CaCl2 fills 2Na creating a vacancy
What are F and H centres? How do they arise?
F centre- electron trapped in an ion vacancy- leads to adsorption in visible. Created in Alkylhalides via heating AX in A-vapour and then irradiating AX with X-rays
H-Centre - interstitial X binds to an X (Cl) in a normal lattice site. In alkylhalides introduced via heating AX in X2 gas (Cl2) it then forms and occupies single anion site
What are the 2 conductivity mechanisms?
Vacancy mechanism- ‘hops’ to neighbouring vacant site
Interstitial mechanism- ‘hops’ to adjacent equivalent site
