ENGR 220 Final Chapters: 17,18, 20, 21

Optical Properties of Insulators&Semiconductors

-Band structure...with band gaps... • May be transparent or opaque to visible light • Thus, also need consider refraction and transmission

For an Extrinsic semiconductor

-n≠p - Occurs when impurities with different # valence e- than Si host are added (doping)

μr

-relative permeability (unitless) -measure of degree to which material can be magnetized, or ease B-field can be induced in presence of external H-field

Domains in Ferromagnetic & Ferrimagnetic Materials

As the applied field (H) increases the magnetic domains change shape and size by movement of domain boundaries.

Applied Magnetic Field (Field Vector)

Created by current flow through a coil (solenoid)

Ef = Fermi Energy (level)

Energy corresponding to highest filled e- state

BOND RUPTURE

due to heat, radiation or chemical reaction

If Egap < 1.8 eV

full absorption; opaque (Si, GaAs)

Consequences of Scission (3)

- Chain separation - Reduction in MW - Reduction of mechanical properties (brittleness, cracking, discoloration)

For metals, resistivity is increased by (3)

- Deformation - Increasing defects; imperfections, alloying, etc. - Increasing temperature

Polymers

- Degradation... - Dissolve in solvents...absorb & swell - UV radiation & heat can break covalent bonds

For semiconductors, conductivity is increased by (2)

- Increasing temperature - Doping (e.g., adding B to Si (p-type) or P to Si (n-type)

Hard magnetic materials:

- Large coercivities (High resistance to demagnetization) and remanence - Used for permanent magnets

Electrical conductivity and resistivity are (2)

- Material parameters - Geometry independent

Magnetic moment for an electron (2)

- Orbital motion of e- around nucleus - Electron spin

Causes of Scission (4)

- Radiation (e-beams, X-rays, gamma rays, etc.) Ø UV, Oxygen & Ozone - Heat - Weathering

When light (em radiation) shines on a material, it may be:

- Reflected, absorbed, transmitted, Scattered

Ceramics

- Relatively deterioration-resistant - Issue mainly at high temps & v. extreme environments

Soft magnetic materials:

- Small coercivities and remanence - Used for electric motors

Optical classification:

- Transparent, translucent, opaque

Corrosion may be controlled by (5)

- Using metals which form a protective oxide layer - Reducing T - Adding inhibitors - Painting - Using cathodic protection

Magnetic Induction (B) Summary :

- occurs when material subjected to a magnetic field - is a change in magnetic moment from UNPAIRED electrons

e- always has

- orbital magnetic moment - spin magnetic moment

H called (2)

-Externally applied magnetic field -magnetic field strength

3 Types of Magnetism

-Ferromagnetic and Ferrimagnetic (large χm) -Paramagnetic (small χm) -Diamagnetic (very very small χm, can not be magnetized)

n-type Semiconductors (3)

-Loosely bound e- occupies energy state within forbidden band gap --Excitation supplies or "donates" e- to conduction band -Because e- excited from "donor" impurity level, just below the conduction band, no hole left in valence band

p-type Semiconductors (3)

-Weakly bound hole liberated from impurity atom via thermally excited transfer of e- from adjacent bond -Excited holes participate in conduction -Impurity atoms introduce "Acceptor" energy state within band gap, just above valence band...no free e- in impurity level or CB

hc

1240 ev nm

Visible Light:

400-700 nm

Conductors, semiconductors, and insulators

Differ in accessibility of energy states for conduction electrons

Anything that decreases e- mobility (μe)

will increase resistivity

Electron Transitions

Ø Excitation e- from one occupied state to vacant higher energy state Ø Only specific values of ΔE allowed Ø All photon energy absorbed Ø e- cannot remain in excited state forever...decays back to ground state Ø Re-emits photon(s)

Net magnetic moment for an atom:

Ø Sum of magnetic moments from all electrons Ø orbital moments of some e- pairs will cancel each other Ø same true for spin moments

Electronic Polarization

Ø em wave includes fluctuating E-field Ø E-field interacts with e- cloud around each atom Ø Perturbs or shifts e- cloud relative to nucleus: - some radiation energy may be absorbed - light waves retarded in vel. in medium...refraction

Photon energy

ΔE = hν = hc/ λ E= Energy (J) λ= Wavelength (m) v or f= Frequency (Hz) h= Planck's constant (6.62 x 10-34 J.s) c= Speed of light (3 x 108 m/s)

spontaneously

ΔV must be +ve for the reaction to proceed spontaneously in the direction assumed Nernst equation

Ohm'sLaw

ΔV=I*R (current*resistance)

equation for μr

μr = μ/μ0

Conductivity

σ = 1/ρ (p is resistivity)

For an intrinsic semiconductor

σ = ni|e|(μe +μh) # electrons = # holes (i.e. n = p = ni)

Conduction in metals

σ=n|e|μe σ = 1/ρ

n-type Extrinsic: (n >> p)

σ≈n|e|μe

p-type Extrinsic: (p >> n)

σ≈p|e|μh

Selective Absorption: Semiconductors

• Absorption by electron transition occurs if hν > Egap

Refractive Index, n

• Transmitted light distorts electron clouds • Light is slower in a material vs. in vacuum n = refractive index ≡ c (velocity of light in vacuum) /v (velocity of light in medium)

You cannot have magnetism without

unpaired electrons

Equation for Drift Velocity

vd = μe*E μe=electron mobility

Oxidation

Metals lose or give up valence e- e- generated from oxidized metal atom become part of another chemical species:

B is called

Flux Density or Induction

Magnetization of solid

M

physiochemical SWELLING & DISSOLUTION

Polymers exposed to liquids

Metals (Conductors)

Thermal energy excites many e- into higher, empty, available energy states

Oxidation occurs at the ?

anode

Color determined by

light wavelengths that are transmitted or re-emitted from electron VB to CB transitions

Optical Properties of Metals: Absorption

All frequencies visible light absorbed: Ø Metals opaque to all em radiation at low end of f-spectrum Ø Most absorbed radiation re-emitted from surface, gen. w. same λ...i.e. as reflected light

Drift Velocity, Vd

Average e- vel. in direction of force due to applied E-field

Magnetic Flux Density (Field Vector)

B

Equation for M

B = μ0H + μ0M

Equation for B, and equation for B in a vacuum

B = μH B0 = μ0H

Refraction:

Bending" of light

BOND RUPTURE by Scission

Breakage/rupture of bonds within molecular chain

Corrosion

Chemical reaction with transfer of e- from one material to another

Deterioration Mechanisms Polymers

Degradation

corrosion

ELECTROCHEMICAL

Atomic & Electronic Interactions 2 Key Optical Phenomena

Electronic Polarization Electron Transitions

Overall Corrosion

MUST have at least 1 Oxidation + 1 Reduction reaction

electron mobility

Frequency of scattering events

Electrical resistance is

Geometry and material dependent parameter

Relation for the applied magnetic field, H

H= NI/L -H=Applied magnetic field units = (Ampere-turns/m) -N= # Turns - I = Current (A) - L= Coil Length (m)

SEMICONDUCTORS TWO TYPES

INTRINSIC & EXTRINSIC

B is

Magnitude of internal field strength within material subjected to H-field

Incident light either transmitted, absorbed, reflected or scattered:

Io = IT +IA +IR +IS

J = E*σ

J= current density E= Electric Field σ= conductivity

Metal corrosion

Loss of material via dissolution

Magnitude of M proportional to applied H-field

M=χm*H

degradation

Polymers physiochemical

Resistance (Ω)

R = ρl/a (resistively (constant for material)* length)/ area

Deterioration Mechanisms Ceramics

Relatively deterioration-resistant

Refraction

Speed of transmitted light varies among materials

Units of B

Tesla

Color may be changed by

adding impurities which change the band gap (Eg) magnitude

Reduction occurs at the?

cathode

Hysteresis and Permanent Magnetization

coercivity & remanence saturation

Dissolution

complete solubility

Deterioration Mechanisms Metals

corrosion,oxidation

# of e- and holes in semiconductors

increase with T

In presence of H-field

magnetic moments within material align with field and reinforce it by virtue of their magnetic fields (μ0M)

χm

magnetic susceptibility (also unites) χm = μr - 1

If Egap in between, partial absorption

material has color

Metals with a more negative Standard Electrode Potential

more likely to corrode relative to other metals

Electrical Conductivity of SEMICONDUCTORS

n= # of electrons/m^3 μe= electron mobility p= number of holes/m^3 μh= hole mobility

If Egap > 3.1 eV

no absorption; colorless/transparent (diamond)

Two types of band structures in metals:

o Partially filled o Overlapping

Electron (e-) accelerated in direction

opposite to E-field

Swelling

partial dissolution w. limited solubility

μ0

permeability of vacuum (4π x 10-7 H/m)

μ

permeability...property of medium through which H-field passes and B-field is measured

Galvanic Series

ranks the reactivity of metals in seawater

Increasing T

speeds up oxidation/reduction reactions

Net magnetic moment

sum of magnetic moments of all e- (both orbital and spin), taking cancellation into account