Final Review Flashcards
What is Elastic Deformation?
a change in material shape that is reversible
What is Plastic Deformation?
a permanent change in material shape when applied force to it
What is stress?
the amount of load (tensile or compressive) on a material that causes deformation
What is the formula for stress?
instantaneous applied force / original surface area = N/m^2 = Pa
What is strain?
relative deformation, % of deformation in material
What is the formula for strain?
delta (change in length) / L (original length) = epsilon
unit-less, expressed as percentage
What is Young’s modulus?
slope of stress strain curve that describes stiffness of a material by stress / strain = psi or Pa
What is yielding strength?
resistance to elastic deformation, when plastic deformation initiates
the stress level at which plastic def begins
initial departure from the linear portion of stress-strain curve
What is tensile strength?
Maximum load-bearing capacity, occurs when necking is observable
What is ductility?
degree of plastic deformation at fracture
What is toughness?
area under the curve, energy absorbing due to plastic deformation
What is the conventional definition of yield strength?
delta y, the stress at a strain offset of 0.2%
How to calculate percent elongation at fracture?
final length - original length / original length
Brittle: < 5%
Ductile: large percentage
What does an elastomer stress-strain curve look?
very shallow slope, almost flat
Difference in Eg for insulators and semiconductors?
Insulators: Eg > 2eV
Semiconductors: Eg < 2 eV
What is the band structure of insulators and semiconductors?
completely filled valence band
empty conduction band
energy gap exists between the two bands
Fermi energy exists in band gap
2 band structures of metals?
partially filled valence band with large band gap (Na 3s1)
filled valence band that overlaps with conduction band (Mg 3s2)
What is the Fermi energy?
Ef = energy corresponding to the highest filled state at 0 K
Insulator band structure
wide band gap > 2eV
heat excitation of electron is difficult
light excitation of electron is easier (hv > Eg)
Uv absorption or blue at 2 eV = 621 nm
Semiconductor band structure
narrow band gap < 2eV
electron excitation easily through heat
optical absorption in IR or visible range
Intrinsic Semiconductor
Behavior dependent on elemental properties
Group IV: Si, Ge
Compounds : II-VI (CdS, ZnTe) and III-V (GaAs, InP)
Larger EN- difference, more ionic bonds, larger band gap
excitation of each electron to conduction band produce one free and hole electron
Extrinsic Semiconductor
Behavior dictated by impurities
Types of charge carriers in semiconductor?
Free electron, e-: in conduction band, e- = -1.6*10^-19C
Holes, h+: in valence band, h+ = 1.6*10-^-19C
Intrinsic property related to temperature?
more electrons are thermally excited under high temperature
What is electron scattering?
contributes to the resistance to the passage of an electric current, related to lift velocity
What is drift velocity?
average electron velocity in direction of field
vd = uE
u = electron mobility (indication of scattering frequency)
What affects mobility?
amount of e scattering
n-type dopant
Group V element replaces a Group IV atom so there is an extra single electron, mostly in conduction band, less in valence band
p-type dopant
Group III element replaces Group IV element so there’s a hole and filled by adjacent electron leaving another hole, more charge holes than free electrons
Extrinsic semiconductor and temperature
Low T: population of the donor band
Intermediate T: donor band fully populated, constant e- concentration
High T: conduction band becomes populated by thermal excitation of electrons in donor band and valence band
Describe the PN junction mechanism
2 pieces of silicon, one of n type and the other p type
One has free e- while the other has holes
e- and h+ diffuse and cancel out, eat each other to create depleted region
What is the depleted region?
filling holes makes negative ion and leaves behind positive ions on the n side
coulomb force prevents migration
What is forward bias?
applied voltage assist e- in overcoming Coulomb barrier of space charge in depletion region
What is reverse bias?
applied voltage impedes e- flow across the junction, creating a larger depletion region
How does Ir compare to If?
Reverse current is much smaller than forward current
What is MosFET?
a type of transistor used for amplifying or switching electronic signals
What is the structure of a MosFET?
2 small islands of p-type Si that are created within a substance of n-type Si
Source and Drain: islands are contacted by metal electrodes to make electrical connection to apply external bias voltage
Gate Oxide: an insulating layer of SiO2 is formed by surface oxidation of Si
Gate Terminal: metal contact on the surface of gate oxide
Describe n-type MosFET
No gate: depletion region between the P and N regions, at least one region is reversed, no current
Positive Gate: induce negative charge on opposite side of the gate oxide, opening up a conductive n-channel between the two n-type islands, this turns it on
p-type: needs a negative gate
What is the eV to wavelength conversion?
1 eV = 1.6*10^-19 J
Light absorption in semiconductors?
Eg < 1.8 eV - all absorbed, appears black (Si)
Eg > 3.1 eV - no absorption, colorless like diamond
1.8 eV < Eg < 3.1 eV - partial light absorption, colored material
What is (Photo-) Luminescence?
material absorbs light at one frequency and emits it at another lower frequency
emitted photons are generated by electron transitions
visible light is emitted when e- falls back to lower energy state is 1.8 eV < hv < 3.1 eV
Effect of delay time
delta time < 1 second - Fluorescence
delta time > 1 second - Luminescence
What does conductivity of semiconductor depend on?
number of free electrons in the conduction band and the number of holes in the valence band
What is photoconductivity?
generation of additional charge carriers from the electron transition as a consequence of the absorption of photons
When the photoconductive material is illuminated, the conductivity _____?
increases
How do LEDs work?
Injection of e- into p-side
Annihilation of electron and emitted photon
Forward-biased potential attracts electrons on the n-side towards the junction and pass into the p-side
Electrons recombine with the holes in the region near the junction
What is electroluminescence?
conversion of electrical energy into light energy
What is the spectrum range of visible light?
400-700 nm
What makes of the EM spectrum
Radio, Micro, IR, UV, X, gamma
What is the intensity of incident light?
energy being transmitted per unit of time across a unit area that’s perpendicular to the direction of propagation
What is Total Extinction?
R + A, the light not transmitted, reflected and absorbed
PETE Commodity Polymer
Polyester
Rigid aromatic rings
high strength, stiff
PE Commodity Polymer
Polyethylene
LDPE - high branching, high ductility, low tensile strength
HDPE - low branching, high tensile strength, chemically stable
PP Commodity Polymer
Polypropylene
tough and flexible (Ziploc containers)
resistance to fatigue
PS Commodity Polymer
Polystyrene
Solid or foamed
Clear, hard, and brittle
foamed is good heat insulator
Polymers with F and Cl
PVC - polar, high strength stiffness and hardness
PTFE - strong C-F bonds, high density, non-stick coating
Thermoplastic
Heated - liquifies
Cooled - hardens
PE, PP, PMMA, Nylon etc
Thermoset
covalent crosslinks
permanently hard
harder and stronger than thermoplastics
Xa and Xb in %IC
Xa is EN- of anion
Xb is EN- of cation
What determines crystal structures of ceramics?
maintenance of charge neutrality - all cation positive charges must be balanced by equal negative
relative size of ions - maximize # of neighbors, stable structures form when anions surrounding a cation are all in contact with that cation, want minimum ration of space
rc/ra for cubic
> .732
rc/ra for tetrahedral
< .414
tetrahedral rc/ra
.225 - .414
octahedral rc/ra
.414 - .732
CN for tetrahedral
4
CN for octahedral
6
CN for cubic
8
What is a nanomaterial
one dimension between 1 and 1000 nanometers, 1-100 nm
comparable to size of light wavelengths
Nanomaterial Classifications by dimension
0D: Clusters - quantum dot
1D: Nanotubes, fibers, rods - nanowire
2D: films and coats - single layer graphene
3D: polycrystals - PS beads superlattice
Unique structure based on size effect
push size down to quantum region increase E level separation, as size decreases, band gap increases
Unique structure based on shape effect
Localized Surface Plasmon Resonance: confinement of a surface plasmon in a nanoparticle of size comparable to or smaller than wavelength of light used to excite the plasmon
shape defines the resonance frequency
Unique structure based on surface effect
surface to volume ratio
Higher surface percentage leads to higher energy (more unsatisfied atoms)
melting point reduces as material gets smaller
cohesive energy - stabilization energy gained by arranging atoms in a crystalline state
melting point is proportional to cohesive energy
Top down nanoscale synthesis methods
Ball milling - grinding to fine material, low cost, suitable for batch and continuous operation
photolithography - uses light to transfer pattern to a photoresist on substrate, fast and low cost, but resolution limit, fix with Critical dimension
Bottom Up
Chemical Vapor Deposition (CVD) - exposing substrate to volatile precursors to react/decompose on substrate surface for desired deposit, for thin film, use Vapor-Liquid-Solid (VLS) mechanism for Si nanowire synthesis (depends on catalyst size, easy morphology to control, high quality, but low yield and high cost
Hot Injection Synthesis of QDs, for quantum dot, low temp, low cost, large yield, easy morphology control, but lower quality and difficult to pattern
What is Modulus of Elasticity and its formula?
The linear part of stress strain curve during elastic deformation
E = stress/straine
SAME AS YOUNG’S MODULUS
