Exam (elaborations)
MATSE FINAL EXTRA GOODS EXAM QUESTIONS AND ANSWERS
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MATSE FINAL EXTRA GOODS EXAM QUESTIONS AND ANSWERS...
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Elastic (young's) Modulus - ANSWERThe conductivity of a pure, defect-free metal is det'd in large by the ?
and ? - ANSWER Electronic structure and bonding of atoms
Metals with the highest conductivity are - ANSWER Ag, Au, Cu (in same
periodic group)
Conductivity σ is changed by the factors that cause scattering of moving
electrons and thereby affect their mobility. Specifically, two factors affect
this change: - ANSWER the temperature effect and impurity/defect
effect.
In temperature effect, ... - ANSWER ...atoms vibrate due to thermal
energy.
As temperature increases, phonon-scattering increases.
In a pure metal, it can express the temperature dependence of resistivity
as ρ=ρ0(1+αRT), where ρ0and αR are constants for a particular metal.
αR= temperature coefficient of resistance.
During impurity/defect effects, in solid solutions... - ANSWER
...impurities will scatter electrons.
We can express resistivity as: ρd=b(1−x)x where:
ρd= an increase in resistivity due to solute atoms,
x = mass fraction of the solute and
b = constant.
Matthiessen's Rule - ANSWER the total resistivity of a crystalline
metallic specimen is the sum of the resistivity due to thermal vibrations
,of atoms of the lattice and the resistivity due to the presence of defects
in the crystal.
This rule is the basis for understanding the resistivity behavior of metals
and alloys at low temperatures.
It is expressed as ρ = ρT + ρd,
where ρT = temperature contribution and
ρd = defect contribution.
Scenario: Adding nickel to copper (alloy). What happens to resistivity
and why? - ANSWER The addition of nickel produces dislocations,
which are a defect, and thus increase resistivity.
Conductivity of Semiconductors? - ANSWER Semiconductors have
moderate and controllable conductivity by nature of an electronic
bandgap.
There are two types of semiconductors:
intrinsic and extrinsic.
In intrinsic semiconductors, the material is pure, and conductivity is
determined by excitation of electrons across a bandgap.
In extrinsic semiconductors, conductivity is determined by the presence
of impurities.
Concept of Energy Levels and Band Diagrams - ANSWER Recall that
electrons in atoms occupy discrete energy levels and that, per the Pauli
exclusion principle, no two electrons can occupy the same state.
These discrete energy levels give way to the concept of energy bands in
a material.
Band Structure in Solids - ANSWER As atoms are brought into proximity
of one another, as in solids, the discrete energy levels split into bands of
closely spaced energy levels.
,The valence band is the highest band that is normally filled with
electrons, in particular, those electrons which are in localized orbitals
and participate in bonding.
Energy states above the valence band are delocalized and comprise the
conduction band.
The separation in energy between the valence and conduction bands is
the bandgap.
Show relative position of the valence and conduction bands for different
types of conductors:
-Metals
-Insulators
-Semiconductors - ANSWER -Metals
Partially filled conduction band or overlapping conduction and valence
bands
-Insulators
Filled valence band, empty conductor band
Eg > 3-4eV
-Semiconductors
0<Eg<3-4 eV
Band gap energy is... - ANSWER ... Eg = Ec - Ev,
where Ec = energy of the bottom edge of the conduction band, and
Ev = energy of the top edge of the valence band.
most prevalent and commercially important semiconductor - ANSWER
crystalline silicon
Silicon has 4 valence electrons that are non-conducting in their normal
valence orbitals. An electron that is excited to the conduction band
, leaves behind an unfilled valence state with a net positive charge (hole).
The excitation requires absorption of energy.
Si atom: 1s22s22p63s23p2. 3s23p2 can be represented by 3sp3, known
as the hybrid valence electrons. The excitation of an electron to the
conductive band can occur by either
thermal energy (kT) or absorption of a photon (hv) if hv>Eg.
Both the electron and the hole can conduct electricity (both effectively
carry charge)
Density of states - ANSWER the fixed number of occupiable states
within the energy bands for any given SEMICONDUCTOR. Whether or
not they are occupied is irrelevant to their existence.
N(E) = "density of states" which is a function of energy
Nv = effective density of states in valence bonds
Nc = effective density of states in conduction band
Example: In silicon: Nv = 1.2x1019/cm3, Nc = 2.8x1019/cm3
The Fermi-Dirac distribution function - ANSWER describes the
probability that there will be an electron in a given energy state E based
on thermal activation (similar to Maxwell-Boltzmann statistics).
The fermi energy Ef, also known as "Fermi Level" - ANSWER
corresponds to the energy level below which all occupiable states would
be filled at absolute zero temperature.
Fermi-Dirac distribution showing the effect of temperature. - ANSWER
describes the probability that an e- will have an energy of some value
greater than Ef .
Equilibrium concentration of electrons and holes of semiconductors -
ANSWER In this concept, the variable n represents the concentration of
free electrons, while p represents the concentration of holes.
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