quiz 3: chapters 22-25, 28 & 29 need to understand the properties of electrons in materials...
TRANSCRIPT
Quiz 3: Chapters 22-25, 28 & 29
• Need to understand the properties of electrons in materials– Colors of materials– Electrical properties (semiconductors, solar
cells)
• Optical properties & optical fibers
Nature of electrons in atoms
• Electron energy levels are quantized• Energy for transition can be thermal or light (electromagnetic), both of which
are quantized resulting in “quantum leap”
Energy E2
Energy E1
Large body (satellite)
Acceleration from E1 to E2 through all intermediate energiesIntuitive!
Electron
Abrupt transition from E1 to E2Intermediate energies not allowedNot intuitive!
Electron energy levels in atoms
• Electrons arranged in shells around the nucleus• Each shell can contain 2n2 electrons, where n is
the number of the shell• Electrons try to achieve 8 electrons in the
outermost shell (octet)• Within each shell there are sub-shells
1st shell: 2 electrons
2nd shell: 8 electrons
3rd shell: 18 electrons
3s
3p
3dSub-shells
Units of energy
• Most common unit is calorie: energy needed to heat 1 gram of water by 1 degree
• 1 food calorie = 1000 calorie
• Appropriate unit for electrons = electron volt (eV)
• 1 calorie = 2.6 x 1019 eV
Typical magnitudes of energy
• Energy difference between electron shells in an atom ranges from a fraction of electron volt (eV) to several eV
• White light consists of VIBGYOR, and comes in small “packets” of energy
• Violet …. Red3.1 eV …. 1.7 eV
Typical magnitudes of energy
2nd shell: 8 electrons
3rd shell: 18 electrons
3s
3p
3d
Sodium (Na) atom has 1 electron in 3rd shell (atomic number 11)
2.1 eV1st shell: 2 electrons
Emitted as visible light
• Violet …. Red3.1 eV …. 1.7 eV
• 2.1 eV corresponds to yellow sodium vapor lamps
Atoms solids
• When atoms come together to form materials, discrete energy levels change to bands of allowed energies Energy
Energy gap
Energy gap
Band diagrams & electron filling
• Electrons filled from low to high energies till we run out of electrons
Empty band
Energy
Partially full band
Metal
Empty band
Full band
Gap ( ~ 1 eV)
Semiconductor
Empty band
Full band
Gap ( > 5 eV)
Insulator
Energy quantization• All forms of energy are “quantized”, meaning they come
in small packets (Planck, Einstein)
• Forms of energy: thermal (heat), light (visible & invisible), electrical
• Even if we dump a lot of energy into a material, remember that they come in numerous identical tiny packets
• Each packet of energy has to be large enough to excite electrons to higher levels
• Example: 1 packet of violet light = 3.1 eV; 1 packet of red light = 1.7 eV
Electron interactions with light: color
• The energy differences between allowed electron levels is of the order of electron volts (eV)
• Packets of visible light are 1.7-3.1 eV
• Ultraviolet light has higher energy, and infrared light has lower energy
• All forms of light are electromagnetic radiation
Electromagnetic spectrum• All electromagnetic radiation are waves• Type of waves is determined by its frequency or wavelength
Decreasing energy
Nature of light
• When light hits a material, it gets reflected and/or transmitted
• Some of the spectral colors may be selectively absorbed or scattered by the material, so that light which is transmitted or reflected into our eyes may be missing some colors (e.g., VisionsWare)
• We should be careful to distinguish between transmitted and emitted light
• Reflection is really light absorbed and re-emitted
Nature of light (contd.)
• Why are some colors absorbed?
• Why are some colors transmitted?
Incident light(contains various colors) Transmitted light
(some colors missing)
Absorbed light
Reflected light(some colors absorbed & re-emitted)
The color of metals
Empty band
Energy
Partially full band
> 3.1 eV3.1 eV (violet)
1.7 eV (red)2.4 eV (yellow)
Silver
All colors absorbed and immediately re-emitted; this is why silver is white (or silvery)
Empty band
Partially full band
3.1 eV (violet)
1.7 eV (red)2.4 eV (yellow)
Gold
Only colors up to yellow absorbed and immediately re-emitted; blue end of spectrum goes through, and gets “lost”
The color of pure insulators & semiconductors
• Insulators: Diamond & glass (SiO2, band gap ~ 9 eV) are transparent as all visible light goes through
• Semiconductors: Silicon is opaque and silvery as all colors absorbed and re-emitted
Empty band
Energy
Full band
~ 5.5 eV3.1 eV (violet)
1.7 eV (red)2.4 eV (yellow)
Diamond
Full band
~ 1.1 eV
3.1 eV (violet)
1.7 eV (red)2.4 eV (yellow)
Silicon
Insulators with impurities
• Impurities generally result in energy levels (“defect states”) in the band gap
• Two types of impurities:– Donor impurities have more electrons than the
atom they replace– Acceptor impurities have fewer electrons than
the atom they replace
Impurities in diamond• Nitrogen has 5 valence electrons (one
more than carbon)• Thus, nitrogen in diamond produces a
donor level with an electron available for excitation to the empty band
• As the violet end of the spectrum is absorbed during electron excitation, the transmitted spectrum looks yellowish
Empty band
Full band
~ 5.5 eV
3.1 eV (absorptionof violet)
Donor level
• Boron has 3 valence electrons (one less than carbon)
• Thus, boron in diamond produces an acceptor level to which an electron from the full band be transferred
• As the red end of the spectrum is absorbed during electron excitation, the transmitted spectrum looks bluish
Empty band
Full band
1.7 eV (absorption of red)
Acceptor level
In both cases, color is due to the transmitted light
Luminescence: fluorescence
• Light is emitted from a material in an interesting manner• Ultraviolet (UV) light has higher energy than visible light,
and can cause electrons to get excited across large band gaps
• Electrons can then return via impurity states, resulting in emission of visible light
Empty band
Full band
4.5 eV
Absorption of UV
Emission of yellow light2.1 eV
Fluorescent lights
• Tubes that contain mercury and are coated inside with a fluorescent material (e.g., cadmium phosphate, zinc silicate, magnesium tungstate, etc.)
• Electricity acts on mercury vapor in the tube causing it to emit the yellow/green light we usually associate with mercury vapor lamps, but it also emits a lot of UV light
• The UV light hits the fluorescent coating, and we can get light in a variety of colors depending on the coating and impurities
Phosphorescence• Occurs when the impurity levels are “sticky”, that is, if the electron
tends to stay in the impurity level for a little time before jumping to its original state
• Visible light emission continues for maybe a few seconds or minutes after the UV source has been turned off
• Results in an after glow, for instance, in TV screens– Screen coated with material for the three basic colors– Electron beam scans 525 horizontal lines on a screen at a rate of 60
times per second (new picture is formed every 0.017 seconds)– Emission needs to last long enough to bridge the time gap between
successive images– Choosing the right set of phosphorescent (or phosphor) material that
gives the right color, with the right intensity and for the right length of time is very important
Semiconductor microelectronic devices
• In a computer, information is represented in binary code (as 0s and 1s)– Example: 0 000
1 0012 0105 101
• We thus need devices to represent 0s and 1s, and the operations between them
• Till about 60-70 years ago, these were done using vacuum tubes which were huge; a complex contraption as large as a classroom was used to perform the operations of today’s calculators!
• With the discovery and understanding of semiconductor materials, computing “chips” have become much smaller progressively
Moore’s Law
• Number of transistors (or semiconductor devices) per unit area has doubled every 18 months over the last 40 years!
• Cost has also gone down exponentially as the entire chip (containing millions of little devices) is fabricated using an integrated process, resulting in an integrated circuit (IC)
Semiconductor devices
• Today, most of semiconductor devices are based on silicon (Si), and some on gallium arsenide (GaAs)
• These devices help represent 0s and 1s and also perform operations with 0s and 1s
• Basic device is what is called a semiconductor transistor, which is made of a rectifier or diode
• A rectifier allows current to flow along one direction but not along the opposite direction: some sort of a “valve”
Pure Si
• Band gap of Si small enough (1.1 eV) for visible light (1.7-3.1 eV) to excite electrons
• Thus visible light will make Si a conductor! So Si is not exposed to light in devices; it is packaged
Full band
~ 1.1 eV
3.1 eV (violet)
1.7 eV (red)2.4 eV (yellow)
Impurities in Si• Impurities are added to Si in a
controlled manner (by a process called “doping”) to create donor and acceptor levels [What does an impurity do to the band diagram?]
B C N
Al Si P
Ga Ge As3 valence electrons
4 valence electrons
5 valence electrons
Empty band
Full band
1.1 eV
Donor level
Empty band
Full band
Acceptor level
Phosphorous impurity Aluminum impurity
Both impurities result in levels that are about 0.03 eV from the main band; thus room temperature thermal energy is sufficient to excite electrons to and from these levels
Impurities in Si: physical picture
• A “hole” is a missing electron, just like a vacancy is a missing atom in an atomic lattice
• A hole has the properties of an electron but has an effective positive charge !
no applied electric field
5+
4+ 4+ 4+ 4+
4+
4+4+4+4+
4+ 4+
Phosphorus atom
no applied electric field
Aluminum atom
valence electron
Si atom
3+
4+ 4+ 4+ 4+
4+
4+4+4+4+
4+ 4+
Free electron
“Hole”
Impurities in Si: band picture
Empty band
Full band
1.1 eV
Donor level
Empty band
Full band
Acceptor level
Phosphorous impurity Aluminum impurity
Hole
n-type semiconductor(charge carriers are negatively charged)
p-type semiconductor(charge carriers are positively charged)
Response to electric field• Say we have two pieces of Si, one is doped with phosphorous (n-
type Si), and the other doped with aluminum (p-type Si)• At room temperature, the first Si piece has a lot of free electrons,
and the second one has free holes• When an electric field is applied, the two types of charge carriers
move in opposite directions, as they are oppositely charged
Free electrons (negative charge) Free holes (positive charge)
Bound electrons (negative charge)
n-type Si p-type Si
The p-n junction rectifier
• When a p-type and a n-type Si are joined together, we have a p-n junction
• A p-n junction has high electron conductivity along one direction, but almost no conductivity along the other! Why?
• Electrons can cross the p-n junction from the n-type Si side easily as it can jump into the holes
• However, along the other direction, electrons have to surmount a ~ 1.1 eV barrier (which is impossible at room temperature in the dark)
p-n junction operation
• This results in a 1-way traffic of electrons, and is a miniature diode that can be used to represent 0s and 1s
Empty band
Full band
1.1 eV
Donor level
Empty band
Full band
Acceptor level
n-type Si p-type Si
Hole
easy hard
The rectifier
• Rectification is the process of converting alternating current (AC) to direct current (DC)
• Used in portable electronic equipment that need to be powered using a wall outlet
n-typeSi
p-typeSi
ACDC
Case Study: Solar cells• Uses the principle of the photoelectric effect
(Einstein: Nobel prize, 1919): light hitting on a material creates current
current
Solar cell
Sun light
current
Silicon based Solar cells• Band gap of Si small enough (1.1 eV) for visible light (1.7-3.1 eV) to
excite electrons
Full band
~ 1.1 eV
3.1 eV (violet)
1.7 eV (red)2.4 eV (yellow)
Full band
Exposure to light
Electron-hole pair
• In solar cells, Si is exposed to light to create electron hole pairs • However, electron-hole pairs created will annihilate themselves, as electron will fall back into the hole
re-emitting light again• So, a p-n junction is used which will prevent the re-emission process, and will result in a net current
p-n junction solar cellp-type Sin-type Si
neutral neutral
Full band
Positively charged
Negatively charged
Full band Exposure to light creates electron-hole pairs
Electric current generated !!
Some holes neutralized by electrons
Basic solar cell
• Anti-reflective coating prevents reflection at top surface to increase efficiency
• Top and bottom contacts help collect the electron and hole currents generating electricity in an external circuit
Prospects of solar cells• Today, only 0.1% of all energy produced come from
solar energy; maximum demonstrated efficiency is 30 %• We want large pieces of crystalline Si to make solar cells
counter to the trend of miniaturization, and difficult to produce large crystalline Si
• Although large, high efficiency amorphous Si solar cells have been demonstrated, production of these is slow
• Lack of sunshine in some parts of the world, and unpredictability in others
• Solar cells produce DC, but AC current required for transmission to large distances
• At present, the most promising applications are in rural and remote areas
• However, this is a very “clean” source of energy, and research is continuing …
Sources of Energy (US)
• Oil 38.8 %• Natural gas 23.2 %• Coal 22.9 %• Nuclear 7.6 %• Hydroelectric 3.8 %• Biomass 3.2 %• Geothermal 0.3 %• Solar 0.07 %• Wind 0.04 %• FUEL CELLS ???
Camera photocells & night vision goggles
• Photocells work due to the fact that Si is an insulator in darkness, but is a conductor when exposed to light
• Night vision goggles are of 2 types: active and passive– Passive: uses the low intensity light in dark situations,
and will not work in total darkness• This uses the reverse of the solar cell principle: light creates
electrons, electrons hit other electrons, and create more electrons, which are all accelerated towards a phosphor screen
– Active: uses infrared radiation
Light in materials
• When light enters a transparent medium, it loses some energy by moving electrons
• As a result, light slows down!• And so, light bends! Why?
Incident light Reflected light
Refracted light
Transmitted light
AIR
AIR
GLASS
Bending of light• You are on land, and your friend is in trouble in water• You can run faster than you can swim• What is the path you would take to get to your friend as
quickly as possible?
land
water
Bending of light• The difference in the speed of light in different materials
causes it to bend
Refractive index of material =speed of light in material
speed of light in vacuum
Total internal reflection• Consider light that goes from glass to air• Critical angle is the angle at which the refracted light
goes along the surface• A light ray with greater angles will get totally reflected
back into the glass• This is the principle used in optical fibers
GLASS
AIR
Critical angles
• Critical angle for water/air is 48 degrees, for diamond/air is 24.5 degrees, and in optical fibers is 75 degrees
Optical fiber
Protective cladding
Critical angles & Mirages
Rainbows
Optical fibers
Optical fibers for internet & telephone communication
• Information is “digitized” (converted to 0s and 1s); 0 = no light pulse, 1 = light pulse
• Advantages:– Clarity of signal: copper wires & electricity can
lead to “cross-talk”, as electric current in one wire results in magnetic field which causes a small current in a neighboring wire; “cross-talk” does not occur in optical fibers
– High information density: Two optical fibers can transmit the equivalent of 30,000 telephone calls simultaneously (in 1956, the 1st transatlantic cable could handle only 52 simultaneous conversations)
– Low weight & volume: It requires 30,000 kg of Cu wire to transmit the same amount of information as 0.1 kg of optical fibers
– Transmission at light speed (instead of at drift velocity in the case of Cu wires)
– Long transmission distance: very low intensity attenuation in fibers
Properties of optical fibers
• Fiber has to have two important properties:– Total internal reflection, so that light is contained
within fiber– Low attenuation, so that light can be carried over long
distances with minimal loss• Structure
– Inner core glass: high refractive index (contains light)– Cladding glass: lower refractive index– Outer polymer coating: adds strength & protects fiber
Properties of optical fibers
• Light ray must enter the fiber within a certain acceptance angle. If not, light will get refracted out as condition for total internal reflection will be violated; this becomes important when a fiber bends
• The way to avoid losing light is to make fibers with small diameters; thinner fibers also better from a flexibility and weight point of view
Manufacture of optical fibers• The core glass needs
– To be super-pure (to ensure extremely small absorption)– A smooth defect-free surface– Small diameter (~ 10 microns)
• Base material for both core glass and cladding glass is SiO2, made using chemical vapor deposition (CVD):– SiCl4 + O2 === SiO2 + 2Cl2
• Since core glass need to have a higher refractive index (i.e., it has to be denser) Germanium (Ge) is added. Ge has 4 valence electrons like Si, but is much heavier. This is another example of “doping” as Ge is an intentional impurity, which substitutes for some of the Si atoms.
Manufacture of optical fibers (contd.)
• The next step of the process is to increase the temperature of the furnace so that the glass softens and the tube collapses to form a solid rod
• The rod is then placed in a high temperature furnace and drawn to form a thin fiber
• Finally, a thin protective plastic layer is placed on the surface to complete the manufacturing process
Purity of optical fibers
• This technology is made possible by breakthroughs in the glass manufacturing process
• In 1970, only 1% of light entering a 1 km long fiber made it to the other end; but today almost 100% of it is transmitted
• 2 Co atoms for every billion Si atoms can cause only 1% transmission; so can 20 iron atoms or 50 copper atoms!
Quiz 3 (April 19)Chapters 22-25, 28 & 29
• Need to understand the properties of electrons in materials– Colors of materials– Electrical properties (semiconductors, solar
cells)
• Optical properties & optical fibers
• GOOD LUCK!