Optical Sources
Optical Sources
Wei-Chih Wang Department of Power Mechanical Engineering National Tsinghua University
w.wang 1 Week 9 • Course Website: http://courses.washington.edu/me557/sensors • Reading Materials: - Week 9 reading materials are from: http://courses.washington.edu/me557/reading/ • HW #3 is assigned and due Week 13 • HW #2 due next Monday • Final Project proposal: Due Monday Week 13 • Set up a time to meet for final project Week 12 • Oral Presentation on 12/23 in class w.wang 2 Light source
- Broadband light source - Light emitting diode - Laser
w.wang 3 What is Light?
All the fifty years of conscious brooding have brought me no closer to the answer to the question, “what are light quanta?” Of course today every rascal thinks he knows the answer, but he is deluding (fooling) himself. - Albert Einstein, 1951
Early days, a light beam was thought to consist of particles. Later, the phenomena of interference and diffraction were demonstrated which could be explained only by assuming a wave model of light. Much later, it was shown that phenomena such as photoelectric effect and Compton effect could be explained on if we assume a particle model of light.
* Ajoy Ghatak, Optics, Macgraw Hill, 2010
* The photoelectric effect is the observation that many metals emit electrons when light shines upon them. Electrons emitted in this manner can be called photoelectrons. The phenomenon is commonly studied in electronic physics, as well as in fields of chemistry, such as quantum chemistry or electrochemistry (Wikipedia). * *Compton scattering is the inelastic scattering of a photon by a charged particle, usually an electron. It results in a decrease in energy (increase in wavelength) of the photon (which may be an X ray or gamma ray photon), called the Compton effect. (Wikipedia) 4 W. Wang Summarize What is Light
- Behave like mechanical particle (reflection, refraction) - Behave like wave (interfere, diffract, partly electric, partly magnetic) - Energy transfer between light and matter (behave like particle with wavelike behavior) Light as particle • A photon is like a particle, but it has very small mass • Think of a photon as a grain of sand. • We see so many photons at the same time it’s like seeing all the sand on a beach; we don’t notice the single grains • When it hit a reflective surface, it reflects like particle and when it travel through different optical medium, it refract to different angle. W. Wang 6 Light as a wave
• But sometimes light acts like a wave • A wave has a wavelength, a speed and a frequency. • We’ll learn more about wave behavior when we talk about polarization, interference and diffraction. • All light travels same speed (in vacuum, index independent of wavelength) • Wavelength gets shorter as frequency goes up (C=f)
W. Wang 7 Light as a photon
• Photon as particle with zero mass • Particles which have wavelike properties • Consisting of a quantum of electromagnetic radiation where energy is concentrated • The energy goes up as frequency goes up (E = h x f
= h x Co/ • Color depends on frequency in non vacuum
medium (E = h x f = h x Co/
W. Wang 8 Wave and Quantum
• When you think of light as a wave of E and B fields, I advice you to not think of them as photons or particles. But just visualize circular waves in water. This is similar except for the fact that it is in three dimensions and that the waves are not through any medium but these 'fields'. So all you need is to disturb a field, and you’ll always get a light wave just like when u disturb water.
2 asin • It’s interesting that field lines are just a mathematical sin ( ) I I o asin convenience or abstract model invented by Faraday, and ( )2 are no more real in the physical sense than isobars on weather maps or contour lines on maps
• On the other hand. When you think of light as a photon, you go quantum. Here everything goes crazy. You no longer use fields. But purely use concept of energy, and discrete orbits (Bohrs model) When electron falls from one discrete orbit to another (and this is not a classical fall, fall, but a quantum fall, where it disappears from one orbit and appears in another). And this leads to generation of photons, or light! w.wang 9 The Bohr Theory of Atomic Structure – Quantum Mechanics
1. The energy of electrons in atoms is QUANTISED,i.e the electron energy cannot vary continuously but is confined to definite fixed energy levels. 2. The electron can only change in energy by moving from one energy level to another. 3. If an electron moves from a lower to a higher level the atom absorbs energy corresponding to the difference in energy between the two levels. 4. If an electron moves from a higher to a lower level the atom emits energy, again corresponding to the difference in energy between the two levels concerned. In the latter Quantum Mechanics has the electron case the energy is emitted as electromagnetic radiation of in a bound state as a probability density function so there is no 'circular frequency given by the equation E = h. orbit'. In these orbits electron do not emit energy in the form of EM radiation w.wang 10 Quantum Theory of Light
Wave-Particle Duality of Light
Quantum theory tells us that both light and matter consists of tiny particles which have wavelike properties associated with them. Light is composed of particles called photons, and matter is composed of particles called electrons, protons, neutrons. It's only when the mass of a particle gets small enough that its wavelike properties show up.
w.wang 11 Particle with Wave-Like Behavior
e - e Photoelectric Effect - At this point you may think that it's pretty obvious that light behaves like a wave. An important feature of this experiment So, why how do we know is that the electron is emitted from the that light is really composed metal with a specific kinetic energy (i.e. a specific speed). (if velocity doesn't of particles called photons? change, kinetic energy doesn’t change) Support for this idea comes from an experiment that is When light intensity increased (brighter light), the kinetic energy of the emitted called the photoelectric electron did not change! The numbers effect. of electrons instead increased! So energy conserved!
w.wang 12 There is a critical frequency for each metal, fo , below which no electrons are emitted. This tells us that the kinetic energy is equal to the frequency of the light times a constant (i.e., the slope of the line). That constant is called Planck's Linear Constant and is given the symbol h= 6.626 x 10-34 J s. equation . 𝑊 =hfo
From the experiment, the kinetic energy of emitted electron shows that it varied with the frequency of the light (f )and this changed the kinetic energy of the
emitted electron (K.Emax)
w.wang 13 EM Theory
The EM radiation emitted by the black body is discrete packets of energy known as quanta..
w.wang 14 w.wang 15 w.wang 16 i
i i∝𝐼 𝐾. 𝐸𝑚𝑎𝑥 ℎ𝑓 𝑊𝑜
is fixed so more e- more intensity eV Reaching isatuaration
An important feature of this experiment is that the electron is emitted from the metal with a specific kinetic energy (i.e. a specific speed). (if velocity doesn't change, kinetic energy doesn’t change)
When light intensity increased (brighter light), the kinetic energy of the emitted electron did not change! The numbers of electrons instead increased! So energy conserved!
w.wang 17 Atoms and Electrons
• All matter is made up of elements. They are all made of one type of atom, • The three main particles making up an atom are the proton, the neutron and the electron. • Electrons spin around the center, or nucleus, of atoms. The nucleus is made up of neutrons and protons. • Electrons contain a negative charge, nucleus protons a positive charge. Neutrons are neutral HELIUM ATOM Elements are completely pure.
• All Matter is made up of Elements • They are only made of one type of ATOM
w.wang HELIUM ATOM 19 There are many different kinds of atoms, one for each type of element, There are 118 different known elements that make up every thing! w.wang 20 Elements make up all living things. O: Oxygen C: Carbon H: Hydrogen N: Nitrogen
w.wang 21 Atom Smallest particle of an element that still has the characteristics of that element
Planetary Model of Sodium Atom w.wang 22 Actual photograph of atoms of germanium in an ink-blot. w.wang 23 Structure of an atom
Electron cloud or Shell or Level Electron (e-) Surrounded by a cloud of electrons reside at different shell or energy level
• Proton (p+) • Neutron (n)
w.wang 24 Electron energy levels •Are regions where electrons travel around the nucleus . • Each energy level can only hold a certain number of electrons • 1st Shell = 2 electrons • 2nd Shell = 8 electrons • 3rd Shell = 8 electrons
w.wang 25 3 Li
w.wang 26 Groups/Families • The elements in a column are called a group. • Groups are also known as families. • Elements in a group have similar characteristics. • The OUTER SHELL = VALENCE SHELL
w.wang 27 Periods • Each horizontal row is called a period. • A period contains a series of different type of elements from different family. • Row # represent the # of SHELLS (shell #).
w.wang 28 Chemistry 101
When sub-atomic particles were first discovered in 1897, they were thought to orbit the nucleus like planets orbit the sun. Unfortunately, this is the atomic model still taught in many secondary schools. In the mid-20th. century, it was discovered that the structure of the electronic shells was somewhat more complex consisting of: - up to at least seven capital-lettered shells (1=K, 2=L, 3=M, 4=N, 5=O, 6=P, 7=Q) - up to at least 4 small-lettered orbitals (s, p, d, f) - each orbital is further divided into sub-orbitals: s has 1, p has 3, d has 5, f has 7 - each of these sub-orbitals can accommodate two electrons. 29 w.wang w.wang 30 Helium
Helium (He) with its two electrons, has filled its s-orbital and its K-shell. Because its outer shell is filled, Helium (He) does not have any valance electrons. As a result, it tends to be nearly perfectly inert or non-reactive
w.wang 31 What is Light? Matter is composed of atoms, ions or molecules and it is light’s interaction with matter which gives rise to the various phenomena which can help us understand the nature of matter. The atoms, ions or molecules have defined energy levels usually associated with energy levels that electrons in the matter can hold. Light can be generated by the Examples of Light absorption of a matter or a photon of light can photon to raise atom or molecule to interact with the energy levels in higher energy levels and emitting a photon and instead relaxes to the a number of ways. lower energy state w.wang 32 Light Source Electromagnetic waves travel through space at a single constant speed, c=3.00x108 msec-1 sometimes referred to as the 'speed of light' although it is, in fact, the speed of all electromagnetic radiation. The wavelength and frequency of electromagnetic radiation are related to the velocity by the equation: c=
Electromagnetic waves are a form of energy and the energy varies with wavelength and frequency according to the relationship:
E = h = hc / (h = Planck's constant = 6.63 x 10-34 Js) i.e. energy is directly proportional to frequency (high frequency = high energy and vice-versa) but inversely proportional to wavelength (long wavelength = low energy and vice-versa).
The entire range of electromagnetic radiation from the lowest energy (low frequency, long wavelength) to the highest energy (high frequency, short wavelength) is called the electromagnetic spectrum. 33 w.wang The Bohr Theory of Atomic Structure – Quantum Mechanics
1. The energy of electrons in atoms is QUANTISED,i.e the electron energy cannot vary continuously but is confined to definite fixed energy levels. 2. The electron can only change in energy by moving from one energy level to another. 3. If an electron moves from a lower to a higher level the atom absorbs energy corresponding to the difference in energy between the two levels. 4. If an electron moves from a higher to a lower level the atom emits energy, again corresponding to the difference in energy between the two levels concerned. In the latter Quantum Mechanics has the electron case the energy is emitted as electromagnetic radiation of in a bound state as a probability density function so there is no 'circular frequency given by the equation E = h. orbit'. In these orbits electron do not emit energy in the form of EM radiation w.wang 34 w.wang 35 w.wang 36 The Hydrogen Emission Spectrum
The hydrogen emission spectrum is a line spectrum, i.e. only particular frequencies (wavelengths, energies) are observed.
w.wang 37 The Hydrogen Emission Spectrum
The lines in the hydrogen emission spectrum are found to be grouped into line series: Lyman (Ultra Violet), Balmer (Visible), Brackett, Paschen and Pfund (all in the Infra Red region). w.wang 38 WHAT IS GOING ON HERE?
H2 2 H (energy) Molecule Atoms
H H* Atom in 'excited state', i.e. with increased Energy (Hydrogen can be excited by heat or electromagnetic energy)
w.wang 39 H* H + h
The excited atom returns to the 'normal' state (ground state)by releasing the excess energy as electromagnetic radiation of energy (E= h) corresponding to the energy difference between the ground state and the excited state. The fact that the hydrogen emission spectrum is a line spectrum – only radiation of very specific frequencies being emitted – means that only excited states of very specific energies are being formed.
w.wang 40 By inspection an empirical mathematical formula – the Rydberg Equation - was found which predicted the position of all the lines in the spectrum:
When n1 =1andn2 =2,3,4……∞ the positions of the lines in the Lyman (UV) series is predicted. The Balmer (visible) series is predicted by n1 =2,n2 = 3, 4, 5 etc. The Brackett, Paschen and Pfund series (IR) are predicted by n1 =3,n1 =4andn1 = 5 respectively.
w.wang 41 Current • Electrons can be made to move from one atom to another. When those electrons move between the atoms, a current of electricity is created. The electrons move from one atom to another in a "flow." One electron is attached and another electron is lost. It is a situation that's very similar to electricity passing along a wire and a circuit. The charge is passed from atom to atom when electricity is "passed." When electrons move among the atoms of matter, a current of electricity is created. This is what happens in a piece of wire. The electrons are passed from atom to atom, creating an electrical current from one end W. Wang 42 to other, just like in the picture. z x B = H = (1+) H = o(M+H) y
(current density, amp/m2)
W. Wang 43 Displacement current is electric field generated by B field that was radiating in air
Induced current is from dispersive medium like dielectric material
D H J t
use 1 c r o ro D= E W. Wang 44 Electromagnetic Wave
is
id
W. Wang 45 Light Sources
Broadband light sources: incoherent, intensity distribution no uniform across all spectrum (white light source) Hologen:250-1100nm Krpton:350-1700nm Zenon (Xenon) :180- 2200nm Deuterium: 190-500nm Mercury+Argon: 253.65 –1013.98nm Deuterium + halogen:190-900nm LED: range from 400 to 1800nm Tungsten+ Hologen: 350-2000nm
Narrow band light source: Laser- 200 to 1800nm w.wang 46 Noble Gas
Noble Gases • Non-reactive • Full outer shell
w.wang 47 Broadband light source
w.wang 48 Broadband light source
• Incandescent filament Lamps • Gas Discharge light • LED
w.wang 49 Incandescent Filament Lamps In conventional filament lamps or light bulbs, electric current is passed through a coiled tungsten filament, contained in a glass envelope that is filled with an inert gas. When heated by an electrical current, the filament emits electromagnetic radiation. At lower temperatures, radiation is mainly emitted in the infrared part of the spectrum as heat. At Tungsten higher temperatures, the proportion of radiation filament lamp at wavelengths ranging from 380 to 780 nm increases and visible light is produced. In a conventional lamp, the filament temperatures are limited to about 2700 Kelvin. The tungsten filament starts to evaporate and as a result it leads to the blackening of the inside of the lamp envelope. The Great Lightbulb Conspiracy 50 https://spectrum.ieee.org/tech-history/dawn-of-electronics/the-great- w.wang lightbulb-conspiracy Light Bulb Conspiracy (planned Obsolescence) • Intentionally design to last 1000 hours (1924, disbanded 1939) by an organization which name was Phoebus Cartel – memebrships such as Osram, Philips, and General Electric. operated under the premise that their only wish was to standardize the whole light bulb industry • One of the oldest light bulbs that is still in use today is called the “Centennial Light,” and it has been in use for around 108 years. • German Watchmaker Dieter Binninger made light filament light bulb to last 150,000hours (17 years) • Bernard London, who famously proposed ending the Great Depression by mandating planned obsolescence, and Brook Stevens, whose post-war ideas became the gospel of the 1950’s and helped shape the throwaway The Centennial Light, consumer society of today. burning since 1901 • LED bulb designed to last 25 years. Halogen
Halogen lamps are also filament lamps. However, halogen is added to the fill gas to prevent evaporated tungsten from condensing on the inside of the lamp envelope. This feature is used to exploit higher filament temperatures of 3000K and beyond and allows the size of tungsten halogen, quartz- the lamp envelope to be halogen or quartz iodine significantly reduced lamp
w.wang 52 Halogen group
Halogens • Highly Reactive with alkali metals •Missing 1 electron in outer shell
w.wang 53 Halogen Spectrum
W. Wang w.wang 54 Tungsten Halogen Lamp
Fiber light or spectrometer light w.wang 55 Gas discharged lamp
In a gas discharge lamp, once a sufficient voltage is applied, electrons are emitted from a heated electrode, creating a plasma or a gas capable of conducting electricity. In the plasma mobile electrons collide with atoms (predominantly mercury), transfer energy to the atoms and elevate them to an excited state. When these atoms fall back to their original status they emit photons (packages of energy). In many low pressure gas discharge lamps the wavelength of the emitted photon is not in the range of visible light. Mercury, for example, has its major emission in the ultraviolet at 254 nm. w.wang 56 Noble Gas
Noble Gases • Non-reactive • Full outer shell
w.wang 57 high intensity discharge lamp (e.g. metal halide lamp) 6,000 to 15,000 hours High pressure gas discharge lamps emit radiation directly as visible light. In this type of lamp the combination of different element atoms in the hot gas plasma, each emitting at specific wavelengths, determines the color characteristics of the lamp as a whole, as well as the quality of color rendition properties. A metal-halide lamp is an electrical lamp that produces light by an electric arc through a gaseous mixture of vaporized mercury Most gas discharge lamps need at least one free due to excited electrons (mixed with argon electron combined with a high pulse to start the or xenon gas) and metal halides (e.g. lamp operation and to produce light. Usually minute sodium iodide and scandium iodide- compounds of metals with bromine or quantities of materials like tritium or krypton-85 iodine). It is a type of high-intensity are applied either in the lamps themselves or in discharge (HID) gas discharge lamp. starter devices as a source for electrons. High luminous white light -sport stadium, retail sign, head light (xenon) w.wang 58 Sodium Lamp
An amalgam is an alloy of mercury with another metal, which may be a liquid, a soft paste or a solid, depending upon the proportion of mercury
The lamp is operating with liquid amalgam in the tube. Standard: North American / European • Street light Lamp watts: 70-1000w Base type: E27, 40/MOG Bulb shape: TD/ED/TD/E • horticulture LUM. output: 6500-130000lm Color temp.: 2000k (yellow) Rated life: 18000-24000hr w.wang 59 Mercury Lamp
• Gas charging lamp • 10 and 100 times brighter than incandescent lamps (such as the tungsten-halogen) and can provide intense illumination over selected wavelength bands throughout the visible spectral region when combined with the appropriate filters (e.g.. Photolithography)
w.wang 60 low-pressure mercury gas discharge fluorescent lamp
Phosphor coating Ultraviolet photons have the capability to excite fluorescent powders, which are coated on the inside of the tube, with a high degree of efficiency. As a result, these powders emit visible radiation in a range of colors. Lamps based on these principles and operating at low internal gas pressure are called “fluorescent lamps”.
w.wang 61 •Regular •Halogen Incandescent •Nernst •Parabolic aluminized reflector (PAR)
•Fluorescent • Fluorescent lamp (compact) • Fluorescent induction •Photoluminescent • laser lamp Luminescent •Solid-state • LED bulb •Cathodoluminescent • Electron-stimulated •Electroluminescent • field-induced polymer
•Acetylene/Carbide Methods of generation •Argand •Candle •Diya •Flare •Gas •Kerosene Combustion •Lantern •Limelight •Oil •Rushlight •Safety •Tilley •Torch
•Carbon arc Electric arc •Klieg light •Yablochkov candle
•Deuterium arc •Neon •Plasma Gas discharge •Sulfur •Xenon arc •Xenon flash
•Hydrargyrum medium-arc iodide (HMI) •Hydrargyrum quartz iodide (HQI) High-intensity •Mercury-vapor discharge (HID) •Metal-halide • ceramic w.wang •Sodium vapor wikipedia 62 Application
•Floodlight •Footlight •Gobo •Theatrical •Scoop •Cinematic •Spotlight •ellipsoidal reflector •Stage lighting instrument
•Aircraft warning •Balanced-arm lamp •Chandelier •Emergency light •Gas lighting •Gooseneck lamp •Intelligent street lighting •Light tube Stationary •Nightlight •Neon lighting •Pendant light •Recessed light •Sconce •Street light •Torchère •Track lighting •Troffer
•Flashlight •tactical •Glow stick •Headlamp (outdoor) Mobile •Lantern •Laser pointer •Navigation light •Searchlight •Solar lamp
•Germicidal •Grow light •Industrial •Infrared lamp •Scientific •Stroboscope •Tanning
•Aroma lamp •Black light •Bubble light •Christmas lights •Crackle tube •Display •DJ lighting •Decorative •Electroluminescent wire •Lava lamp •Marquee •Plasma globe •Strobe light
•Bioluminescence •Chemiluminescence •Electroluminescence Related topics •Laser w.wang •Photoluminescence 63 •Radioluminescence wikipedia Ideas for final projects- using different Types of lights for unseen application
w.wang 64 Light Emitting Diode
w.wang 65 Light Emitting Diode (LED) The electroluminescent process of LED is to covert input electrical energy into output optical radiation in the visible or infrared (heat) portion of the spectrum, depending on the semiconductor material.
LEDs and laser diodes are very similar devices. In fact, when operating below their threshold current, all laser diodes act as LEDs.
w.wang 66 Light Emitting Diodes
– The Light-Emitting Diode (LED) is a semiconductor pn junction diode that emits visible light or near- infrared radiation when forward biased. – Visible LEDs emit relatively narrow bands of green, yellow, orange, or + - red light (tens of nm). Infrared LEDs emit in one of several bands just beyond red light. w.wang 67 Light Source
Electromagnetic waves travel through space at a single constant speed, c = 3.00 x 108 msec-1 sometimes referred to as the 'speed of light' although it is, in fact, the speed of all electromagnetic radiation. The wavelength and frequency of electromagnetic radiation are related to the velocity by the equation:
c =
Electromagnetic waves are a form of energy and the energy varies with wavelength and frequency according to the relationship:
E = h = hc / (h = Planck's constant = 6.63 x 10-34 Js) i.e. energy is directly proportional to frequency (high frequency = high energy and vice-versa) but inversely proportional to wavelength (long wavelength = low energy and vice-versa).
The entire range of electromagnetic radiation from the lowest energy (low frequency, long wavelength) to the highest energy (high frequency, short wavelength) is called the electromagneticw.wang spectrum. 68 The energy conversion takes place in two stages: first, the energy of carriers in the semiconductor is raised above their equilibrium value by electrical input energy, and second, most of these carriers, after having lived a mean lifetime in the higher energy state, give up their energy as spontaneous emission of photons with energy nearly equal to the bandgap Eg of the semiconductor:
Ego = h hc where h is plank constant and is frequency of emitting light.
The choice of LED materials requires the wavelength light emission to be within visible light or infrared light region. This means that the bandgap of the semiconductor has to be roughly around 2 eV. The most frequency used binary compounds for LED applications are III-V compounds such as GaAs and GaP. In your case, the red diode could be made of a homojunction GaAsP (650nm) diode and the yellow diode made of a homojunction GaAsP:N (585nm) diode, where N representing doping level.
w.wang 69 The typical spectral output of a LED might looks like:
Spectra of different color LEDs
w.wang 70 Why Semiconducting Materials? Rectification and control electrical current with charges and also longer life time • On/off switching (e.g. passing current more easily in one direction than the other) (diode) • Current or voltage driven amplification (diode) • Energy converter (solar cell, photo detector etc.) • Nonlinear IV characteristic (diode) • Easily doped or change layer structure to change electromagnetic properties (mu, epsilon etc.) to create variable resistance, capacitance and its sensitivity to light or heat (e.g. different wavelengths)
w.wang 71 Linear and Nonlinear electronics
current voltage voltage voltage Vaccum tube Thermistor Diode (i.e. type 2A3) (i.e. PN diode, LED, (large negative laser diode, phtodiode) temperature v D nV T coefficient of i D I S e 1 resistivity)
current Normal resistor Ohm’s Law: i=v/R w.wang voltage 72 Introduction to Diodes D1 + ANODE CATHODE - DIODE
• A diode can be considered to be an electrical one-way valve. • They are made from a large variety of materials including silicon, germanium, gallium arsenide, silicon carbide …
W. Wang 73 Introduction to Diodes
• In effect, diodes act like a flapper valve – Note: this is the simplest possible model of a diode
W. Wang 74 Introduction to Diodes
•Fortheflapper valve,asmall positive pressure is required to open. • Likewise, for a diode, a small positive voltage is required to turn it on. This voltage is like the voltage required to power some electrical device. It is used up turning the device on so the voltages at the two ends of the diode will differ. – The voltage required to turn on a diode is typically around 0.6 - 0.8 volt for a standard silicon diode and a few volts for a light emitting diode (LED)
W. Wang 75 Introduction to Diodes
10V
5V
0V
-5V
-10V 0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms V(D1:1) • 10 volt sinusoidal voltage Time source
D1
D1N4002 VAMPL = 10V V1 R1 FREQ = 1k 1k
0 • Connect to a resistive load through a diode
W. Wang 76 Introduction to Diodes D1
V D1N4002 V VAMPL = 10V V1 Only positive R1 FREQ = 1k 1k current flows
0 10V
5V
0V
-5V
-10V 0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms V(D1:1) V(D1:2) Time w.wang 77 What is Semiconductor material?
• The ability to permit the flow of electrons through a substance is its conductivity. The conductivity of conductors is highest, while it is lowest for an insulator, as the electrons flowing through it are negligible. However, as the name suggests, the conductivity of a semiconductor is moderate. • Another interesting thing about a semiconductor is that the current is carried not only by electrons, but by the vacancies they leave behind, which are known as holes. The holes left behind in the valence band can be occupied by electrons from lower states and contribute to the flow of current, thereby leaving a hole in these deeper states as well, which will be occupied by electrons beneath, and so on. • In a typical conductor, like copper, electrons carry the current and act as the charge carrier. In semiconductors, both electrons and holes — the absence of an electron — act as charge carriers. By controlling the doping of the semiconductor, the conductivity and the charge carrier tailored to be either electron or hole based.
w.wang 78 Semiconductor
Variable conductivity A pure semiconductor is a poor electrical conductor as a consequence of having just the right number of electrons to completely fill its valence bonds. Through various techniques (e.g., doping or gating), the semiconductor can be modified to have excess of electrons (becoming an n-type semiconductor) or a deficiency of electrons (becoming a p- type semiconductor). In both cases, the semiconductor becomes much more conductive (the conductivity can be increased by a factor of one million, or even more). Semiconductor devices exploit this effect to shape electrical current. Junctions When doped semiconductors are joined to metals, to different semiconductors, and to the same semiconductor with different doping, the resulting junction often strips the electron excess or deficiency out from the semiconductor near the junction. This depletion region is rectifying (only allowing current to flow in one direction), and used to further shape electrical currents in semiconductor devices. Energetic electrons travel far Electrons can be excited across the energy band gap of a semiconductor by various means. These electrons can carry their excess energy over distance scales of microns before dissipating their energy into heat, significantly longer than is possible in metals. This effect is essential to the operation of bipolar junction transistors. Light energy conversion Electrons in a semiconductor can absorb light, and subsequently retain the energy from the light for a long enough time to be useful for producing electrical work instead of heat. This principle is used in the photovoltaic cell (e.g. solar cell). Conversely, in certain semiconductors, electrically excited electrons can relax by emitting light instead of producing heat. This is used in the light emitting diode. Thermal energy conversion Semiconductors are good materials for thermoelectric coolers and thermoelectric generators, which convert temperature differences into electrical power and vice versa. Peltier coolers use semiconductors for this reason.
w.wang 79 Semiconductor
Variable conductivity A pure semiconductor is a poor electrical conductor as a consequence of having just the right number of electrons to completely fill its valence bonds. Through various techniques (e.g., doping or gating), the semiconductor can be modified to have excess of electrons (becoming an n-type semiconductor) or a deficiency of electrons (becoming a p- type semiconductor). In both cases, the semiconductor becomes much more conductive (the conductivity can be increased by a factor of one million, or even more). Semiconductor devices exploit this effect to shape electrical current. Junctions When doped semiconductors are joined to metals, to different semiconductors, and to the same semiconductor with different doping, the resulting junction often strips the electron excess or deficiency out from the semiconductor near the junction. This depletion region is rectifying (only allowing current to flow in one direction), and used to further shape electrical currents in semiconductor devices. Energetic electrons travel far Electrons can be excited across the energy band gap of a semiconductor by various means. These electrons can carry their excess energy over distance scales of microns before dissipating their energy into heat, significantly longer than is possible in metals. This effect is essential to the operation of bipolar junction transistors. Light energy conversion Electrons in a semiconductor can absorb light, and subsequently retain the energy from the light for a long enough time to be useful for producing electrical work instead of heat. This principle is used in the photovoltaic cell (e.g. solar cell). Conversely, in certain semiconductors, electrically excited electrons can relax by emitting light instead of producing heat. This is used in the light emitting diode. Thermal energy conversion Semiconductors are good materials for thermoelectric coolers and thermoelectric generators, which convert temperature differences into electrical power and vice versa. Peltier coolers use semiconductors for this reason.
w.wang 80 Doping and the control of current through a device
To contemplate its utility, one must understand that the current flowing through a semiconductor, unlike a conductor, is not an uncontrollable surge of electrons, but rather a delicate combination of charges and their steady flow. Innovative engineering put forward the idea of contaminating a silicon or germanium atom on purpose to induce new energy levels. The amount of contamination or doping provides the means of controlling current
w.wang 81 Doping
Silicon atoms are arranged in a definite symmetrical pattern making them a crystalline solid structure. A crystal of pure silica (silicon dioxide or glass) is generally said to be an intrinsic crystal (it has no impurities) and therefore has no free electrons. But simply connecting a silicon crystal to a battery supply is not enough to extract an electric current from it. To do that we need to create a “positive” and a “negative” pole within the silicon allowing electrons and therefore electric current to flow out of the silicon. These poles are created by doping the silicon with certain impurities. w.wang 82 P type and N type Materials The semiconductor can be modified to have excess of electrons (becoming an n-type semiconductor) negatively charge ion or a deficiency of electrons (becoming a p-type semiconductor) positively charged ion.
In both cases, the semiconductor becomes much more conductive (the conductivity can be increased by a factor of one million, or even more). Semiconductor devices exploit this effect to shape electrical current. w.wang 83 Donor ( n type semiconductors)
In semiconductor physics, a donor is a dopant atom that, when added to a semiconductor, can form a n-type region. For example, when silicon (Si), having four valence electrons, Extra e- needs to be doped as an n-type semiconductor, elements from group V like phosphorus (P) or arsenic (As) can be used because they have five valence electrons. A dopant with five valence electrons is also called a pentavalent impurity. Other Group pentavalent dopants are antimony (Sb) and bismuth (Bi). V
When substituting a Si atom in the crystal lattice, four of the valence electrons of phosphorus form covalent bonds with the neighbouring Si atoms but the fifth one remains weakly bonded. At room temperature, all the fifth electrons are liberated, can move around the Si crystal and can carry a current and thus act as charge carriers. The initially neutral donor becomes positively charged (ionised).
Diffusion, implant etc. w.wang 84 Acceptor (P type semiconductors)
Extra hole In semiconductor physics, an acceptor is a dopant atom that when added to a semiconductor can form a p‐type region. For example, when silicon (Si), having four valence electrons, needs to be doped as a p‐type semiconductor, elements from group III like boron(B) or aluminium (Al), having three valence electrons, can be used. The latter elements are also called trivalent impurities. Other trivalent dopants include indium (In) and gallium (Ga). Group When substituting a Si atom in the crystal lattice, the three III valence electrons of boron form covalent bonds with three of the Si neighbors but the bond with the fourth neighbor remains unsatisfied. The unsatisfied bond attracts electrons from the neighbouring bonds. At room temperature, an electron from the neighboring bond will jump to repair the unsatisfied bond thus leaving a hole (a place where an electron is deficient). The hole will again attract an electron from the neighbouring bond to repair this unsatisfied bond. This chain‐like process results in the hole moving around the crystal and able to carry a current thus acting as a charge carrier. The initially electroneutral acceptor becomes negatively charged (ionised). w.wang Diffusion, implant etc. 85 Semiconductor
Variable conductivity A pure semiconductor is a poor electrical conductor as a consequence of having just the right number of electrons to completely fill its valence bonds. Through various techniques (e.g., doping or gating), the semiconductor can be modified to have excess of electrons (becoming an n-type semiconductor) or a deficiency of electrons (becoming a p- type semiconductor). In both cases, the semiconductor becomes much more conductive (the conductivity can be increased by a factor of one million, or even more). Semiconductor devices exploit this effect to shape electrical current. Junctions When doped semiconductors are joined to metals, to different semiconductors, and to the same semiconductor with different doping, the resulting junction often strips the electron excess or deficiency out from the semiconductor near the junction. This depletion region is rectifying (only allowing current to flow in one direction), and used to further shape electrical currents in semiconductor devices. Energetic electrons travel far Electrons can be excited across the energy band gap of a semiconductor by various means. These electrons can carry their excess energy over distance scales of microns before dissipating their energy into heat, significantly longer than is possible in metals. This effect is essential to the operation of bipolar junction transistors. Light energy conversion Electrons in a semiconductor can absorb light, and subsequently retain the energy from the light for a long enough time to be useful for producing electrical work instead of heat. This principle is used in the photovoltaic cell (e.g. solar cell). Conversely, in certain semiconductors, electrically excited electrons can relax by emitting light instead of producing heat. This is used in the light emitting diode. Thermal energy conversion Semiconductors are good materials for thermoelectric coolers and thermoelectric generators, which convert temperature differences into electrical power and vice versa. Peltier coolers use semiconductors for this reason.
w.wang 86 PN junction In the previous tutorial we saw how to make an N-type semiconductor material by doping a silicon atom with small amounts of Antimony and also how to make a P-type semiconductor material by doping another silicon atom with Boron.
This is all well and good, but these newly doped N-type and P-type semiconductor materials do very little on their own as they are electrically neutral. However, if we join (or fuse) these two semiconductor materials together they behave in a very different way merging together and producing what is generally known as a “PN Junction“.
- When the N-type semiconductor and P-type semiconductor materials are first joined together a very large density gradient exists between both sides of the PN junction. The result is that some of the free electrons from the donor impurity atoms begin to migrate across this newly formed junction to fill up the holes in the P-type material producing negative ions.
w.wang 87 - However, because the electrons have moved across the PN junction from the N-type silicon to the P-type
silicon, they leave behind positively charged donor ions ( ND ) on the negative side and now the holes from the acceptor impurity migrate across the junction in the opposite direction into the region where there are large numbers of free electrons. - As a result, the charge density of the P-type along the junction is filled with negatively charged acceptor ions (
NA ), and the charge density of the N-type along the junction becomes positive. This charge transfer of electrons and holes across the PN junction is known as diffusion. The width of these P and N layers depends on how
heavily each side is doped with acceptor density NA, and donor density ND, respectively. - This process continues back and forth until the number of electrons which have crossed the junction have a large enough electrical charge to repel or prevent any more charge carriers from crossing over the junction. Eventually a state of equilibrium (electricallyneutral situation) will occur producing a “potential barrier” zone around the area of the junction as the donor atoms repel the holes and the acceptor atoms repel the electrons. - Since no free charge carriers can rest in a position where there is a potential barrier, the regions on either sides of the junction now become completely depleted of any more free carriers in comparison to the N and P type materials further away from the junction. This area around the PN Junction is now called the Depletion Layer. Too far away so no combining The total charge on each side of a PN Junction must be equal and opposite to maintain a neutral charge condition around the junction. If the depletion layer region has a distance D, it therefore must therefore
penetrate into the silicon by a distance of Dp for the positive side, and a distance of Dn for the negative side giving a relationship between the two of:
Dp*NA = Dn*ND in order to maintain charge neutrality also called equilibrium.
w.wang 88 Diode Fabrication Process
w.wang 89 By manipulating this non- Pictorial description conductive layer, p–n junctions are commonly used as diodes: circuit elements that allow a flow of electricity in one direction but not in the other (opposite) direction. In a p– n junction, without an external applied voltage, an No equilibrium condition is reached in which a potential difference forms across the charges junction. This potential difference is called built-in in this potential When put two materials together, free region electrons from the N-type material fill holes from the P-type material. This creates an insulating layer in the middle w.wang of the diode called the depletion zone. 90 D1 How Diodes Work ANODE CATHODE DIODE
Electrons can be excited across the energy band gap of a semiconductor by various means. These electrons can carry their excess energy over distance scales of microns before dissipating their energy into heat, significantly longer than is possible in metals. This effect is essential to the operation of bipolar junction transistors.
w.wang narrowed 91 D1 How Diodes Work ANODE CATHODE DIODE
When the positive end of the battery is hooked up to the N-type layer and the negative end is hooked up to the P-type layer, free electrons collect on one end of the diode and holes collect on the other. The depletion zone gets bigger and no current flows. w.wang 92 Introduction to Diodes D1
V D1N4002 V VAMPL = 10V V1 Only positive R1 FREQ = 1k 1k current flows
0 10V
5V
0V
-5V
-10V 0s 0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms V(D1:1) V(D1:2) Time w.wang 93 Forward and Reverse Bias
w.wang 94 Semiconductor
Variable conductivity A pure semiconductor is a poor electrical conductor as a consequence of having just the right number of electrons to completely fill its valence bonds. Through various techniques (e.g., doping or gating), the semiconductor can be modified to have excess of electrons (becoming an n-type semiconductor) or a deficiency of electrons (becoming a p- type semiconductor). In both cases, the semiconductor becomes much more conductive (the conductivity can be increased by a factor of one million, or even more). Semiconductor devices exploit this effect to shape electrical current. Junctions When doped semiconductors are joined to metals, to different semiconductors, and to the same semiconductor with different doping, the resulting junction often strips the electron excess or deficiency out from the semiconductor near the junction. This depletion region is rectifying (only allowing current to flow in one direction), and used to further shape electrical currents in semiconductor devices. Energetic electrons travel far Electrons can be excited across the energy band gap of a semiconductor by various means. These electrons can carry their excess energy over distance scales of microns before dissipating their energy into heat, significantly longer than is possible in metals. This effect is essential to the operation of bipolar junction transistors. Light energy conversion Electrons in a semiconductor can absorb light, and subsequently retain the energy from the light for a long enough time to be useful for producing electrical work instead of heat. This principle is used in the photovoltaic cell (e.g. solar cell). Conversely, in certain semiconductors, electrically excited electrons can relax by emitting light instead of producing heat. This is used in the light emitting diode. Thermal energy conversion Semiconductors are good materials for thermoelectric coolers and thermoelectric generators, which convert temperature differences into electrical power and vice versa. Peltier coolers use semiconductors for this reason.
w.wang 95 Energy levels
Helium (He) with its two electrons, has filled its s‐orbital and its K‐shell. Because its outer shell is filled, Helium (He) does not have any valance electrons. As a result, it tends to be nearly perfectly inert or non‐ reactive w.wang 96 Conductor Semiconductor and Insulator
The first crystal in the diagram has an odd number of electrons in its valenceband and no electrons in the next band, which makes for a single, loose electron inits highest energy level. This will readily flow to the conduction band when given a small kick or connected to a battery, providing a huge current. This crystalisa conductor; examples of conductors are metals, such as Copper and Iron.The electrons of the second crystal are not only very stable and tied to each other, but there is also a pair of electrons in its conduction band too, thus making the flow of electrons to the conduction band nearly impossible. This represents an insulator. Paper, rubber and glass are some common insulators. The third crystal has a loose electron, but not an empty conduction band. However, it does contain half-filled energy levels that can accommodate more electrons. This loose electron can be projected to the conduction band whengivena strong enough kick, rendering a small amount of current. This crystal is a semiconductor; major examples are Silicon and Germanium. w.wang 97 Band theory of Solid
A useful way to visualize the difference between conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. Instead of having discrete energies as in the case of free atoms, the available energy states form bands. Crucial to the conduction process is whether or not there are electrons in the conduction band. In insulators the electrons in the valence band are separated by a large gap from the conduction band, in conductors like metals the valence band overlaps the conduction band, and in semiconductors there is a small enough gap between the valence and conduction bands that thermal or other excitations can bridge the gap. With such a small gap, the presence of a small percentage of a doping material can increase conductivity dramatically.
w.wang 98 Fermi Level
"Fermi level" is the term used to describe the top of the collection of electron energy levels at absolute zero temperature. This concept comes from Fermi-Dirac statistics. Electrons are fermions and by the Pauli exclusion principle cannot exist in identical energy states. So at absolute zero they pack into the lowest available energy states and build up a "Fermi sea" of electron energy states. The Fermi level is the surface of that sea at absolute zero where no electrons will have enough energy to rise above the surface. The concept of the Fermi energy is a crucially important concept for the understanding of the electrical and thermal properties of solids. Both ordinary electrical and thermal processes involve energies of a small fraction of an electron volt. But the Fermi An important parameter in the band theory is energies of metals are on the order of electron volts. This the Fermi level, the top of the available implies that the vast majority of the electrons cannot electron energy levels at low temperatures. The position of the Fermi level with the relation to receive energy from those processes because there are no the conduction band is a crucial factor in available energy states for them to go to within a fraction determining electrical properties. of an electron volt of their present energy. Limited to a tiny depth of energy, these interactions are limited to "ripples on the Fermi sea". w.wang 99 Fermi Level
In metals, the Fermi energy gives us information about the velocities of the electrons which participate in ordinary electrical conduction. The amount of energy which can be given to an electron in such conduction processes is on the order of micro-electron volts (see copper wire example), so only those electrons very close to the Fermi energy can participate. The Fermi velocity of these conduction electrons can be calculated from the Fermi energy.
The Fermi energy also plays an important role in understanding the mystery of why electrons do not contribute significantly to the specific heat of solids at ordinary temperatures, while they are dominant contributors to thermal conductivity and electrical conductivity. Since only a tiny fraction of the electrons in a metal are within the thermal energy kT of the Fermi energy, they are "frozen out" of the heat capacity by the Pauli principle. At very low temperatures, the electron specific heat becomes significant.
w.wang 100 • In a p-type material the fermi-level is closer to the valence band than to the conduction band and the opposite is true for n-type materials. When these are drawn in thermal equilibrium the result is an image like the one above. • The image above doesn't really give energy levels in the sense but rather show potential difference. Given the two different materials they accumulate positive and negative charges respectively. As we know, when we interface these two materials carriers drift into the other material in an attempt to achieve charge neutrality, this manifests itself as a diffusion current and a potential difference. • The actual energy levels of the materials is given by the vacuum-function which is essentially the energy-difference relative to an electron at rest unaffected by the material ( e.g an electron in vacuum ). • The semiconductor above is drawn in thermal equilibrium, as a result of this (the fermi level being constant throughout the material) the p-side appears to have bands at higher energy levels
w.wang 101 Spontaneous Emission
h hc
Whole bunch of interbands not shown
w.wang 102 The energy conversion takes place in two stages: first, the energy of carriers in the semiconductor is raised above their equilibrium value by electrical input energy, and second, most of these carriers, after having lived a mean lifetime in the higher energy state, give up their energy as spontaneous emission of photons with energy nearly equal to the bandgap Eg of the semiconductor:
Ego = h hc where h is plank constant and is frequency of emitting light.
The choice of LED materials requires the wavelength light emission to be within visible light or infrared light region. This means that the bandgap of the semiconductor has to be roughly around 2 eV. The most frequency used binary compounds for LED applications are III-V compounds such as GaAs and GaP. In your case, the red diode could be made of a homojunction GaAsP (650nm) diode and the yellow diode made of a homojunction GaAsP:N (585nm) diode, where N representing doping level.
w.wang 103 What is an i-v characteristic curve? • Recall that the i-v relationship for a resistor is given by Ohm’s Law: i=v/R • If we plot the voltage across the resistor vs. the current through the resistor, we obtain i The slope of the straight line is given by 1/R v
w.wang 104 i-v characteristic of a real diode The current‐voltage relation of a diode is derived based on the Boltzman’s and Maxwell’s equations. The equation of voltage and current is given:
v D nV T i D I S e 1 Ideal Diode
Threshold or bias voltage
• vD is the voltage across the diode and iD is the current through the diode. n and Is are constants. VT is a voltage proportional to the temperature, we use 0.0259V. • Note that for vD less than zero, the exponential term vanishes and the current w.wang iD is roughly equal to minus the saturation current. 105 • For vD greater than zero, the current increases exponentially. Real diode characteristics
• A very large current can flow when the diode is forward biased. For power diodes, currents of a few amps can flow with bias voltages of 0.6 to 1.5V. Note that the textbook generally uses 0.6V as the standard value, but 0.7V is more typical for the devices. • Reverse breakdown voltages can be as low as 50V and as large as 1000V.
• Reverse saturation currents Is are typically 1nA or less. w.wang 106 Facts about LEDs ~ cutoff frequency, – LEDs switch off and on rapidly, are very capacitance value rugged and efficient, have a very long lifetime, and are easy to use (~ns to s). – They are current-dependent sources, and their light output intensity is directly proportional to the forward current through the LED. – Always operate an LED within its ratings to prevent irreversible damage.
– Use a series resistor (Rs) to limit the current through the LED to a safe value. VLED is the LED voltage drop. It ranges from about 1.3V to about 3.6V. Intrinsic resistance of VVin LED LED is small (~ 10ohms) R s ILED –ILED is the specified forward current. (Generally 20mA).
w.wang 107 The current‐voltage relation of a diode is derived based on the Boltzman’s and Maxwell’s equations. The equation of voltage and current is given: