Avalanche Breakdown (Reverse biased PN junction)

Transcription

1 Deviation from the Ideal Diode Ideal diode I The mechanism for breakdown 1. Avalanching 2. Zener Process -V BR -Io generation of carriers in the depletion region -I measured Real diode V A Ideal diode equation qva I = IO exp 1 kt Avalanche Breakdown (Reverse biased PN junction) Depletion Region P (-) E-field(+) N (+) (-) (+) (-) (-) N P Diffusion of electrons (-) (-) Diffusion of holes P N - Avalanche breakdown occurs when electrons and/or holes acquire sufficient energy from the electric field to create electron-hole pairs by colliding with atomic electrons within the depletion region - The predominant breakdown mechanism for most PN junction - Reverse bias of PN Junction > critical value High electric field strength acceleration of electron impact ionization Acceleration of electron impact ionization of other atom

2 Zener Breakdown (electron tunneling) - Heavily doped on both sides (the gap between VB and CB in the depletion region becomes narrow when reverse biased.) - Si diode with V BR < 4.5 V tunneling can occur For tunneling 1. Thin potential barrier 2. Large number of electrons available to tunnel on one side of the barrier and a large number of empty states, at the same energy level, into which to tunnel on the other side of the barrier. Remember heavy doping on both side P N kt N N Ecp Evp filled state tunneling empty state Ecn V bi D A ln 2 q ni V bi +V a at reverse bias Evp > Ecn Reverse biased PN junction Evn Zener Diode A conventional solid-state diode will not let current flow if reverse-biased (up to a breakdown voltage). By exceeding the breakdown voltage a conventional diode is destroyed in the breakdown due to excess current and overheating. In case of forward-bias (in the direction of the arrow) the diode exhibits a voltage drop of roughly 0.7 volt. The voltage drop depends on the type of the diode. A Zener diode exhibits almost the same properties, except the device is especially designed so as to have a greatly reduced breakdown voltage, the socalled Zener voltage. A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. A reverse-biased Zener diode will exhibit a controlled breakdown and let the current flow to keep the voltage across the Zener diode at the Zener voltage. For example, a 6.2 volt Zener diode will exhibit a voltage drop of 6.2 volt if reverse biased. However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage. The breakdown voltage can be controlled quite accurately in the doping process. Tolerances up to 0.05% are available though the most widely used tolerances are 5% and 10%. The effect was discovered by the American physicist Clarence Melvin Zener V P= VI = I R= R

3 Zener Diode (Application) Vs 2 2 V P= VI = I R= All diodes, if sufficiently reverse biased, will break down and begin to conduct quite well. This can be a disaster if the current gets high enough to overheat the diode. However, it turns out that one can design diodes that break down at particular and well define voltages in the range V, and these are useful for voltage stabilization. They are called Zener diodes hing/phy107a/pdf/the_zener_diod e.pdf R Zener Diode (

4 We studied -The fundamental of Semiconductor -N and P type semiconductor -the formation of PN junction -I-V characteristics in PN Junction (PN diode) -Rectifying process that convert from dc to ac -Avalanche and zener process for breakdown -The principle of regulating voltage using zener diode What else can be made from PN junction? Solar cell, Photodetector, LED, Laser and so on. Solar Cell (Photo Diode) using PN Junction hυ hυ P I emf N - Conduction band Valence band Light of sufficient energy Electron transfer from VB to CB The electron readily rolls down to N region due to the high electric field in the depletion region The hole readily rolls up to P region Some electrons(or holes) recombine (do not contribute to the current) Some electrons(or holes) within the diffusion length contribute the current The contribution of photocurrent became small as increasing the length from depletion region To reduce the electron consumption by recombination, the defect in semiconductor should be minimized. (e.g. [defect] may diffuse up to 200 µm [defect] diffusion length reduces to 10µm) From Hummel, Electronic Properties of Materials

5 Solar Cell : Schematics Convert Light (Photon) Into Electricity (Voltage) Photovoltaic effect Solar Cell Movie

6 Solar Cell : The Needs The Need for Solar Cells -the need for low maintenance, long lasting sources of electricity suitable for places remote from both the main electricity grid and from people; eg satellites, remote site water pumping, outback telecommunications stations and lighthouses; -the need for cost effective power supplies for people remote from the main electricity grid; eg Aboriginal settlements, outback sheep and cattle stations, and some home sites in grid connected areas. -the need for non polluting and silent sources of electricity; eg tourist sites, caravans and campers -the need for a convenient and flexible source of small amounts of power; eg calculators, watches, light meters and cameras; -the need for renewable and sustainable power, as a means of reducing global warming. Solar Cell: Amorphous or Crystalline Amorphous Si -Deposition on glass from reactive gas such as SiH 4-13% in Market -Efficiency: 6-9% Polycrystalline Si - Casting process (molten silicon is poured into a mold.) - 86% in market - Efficiency: 11-16% Easy to fabricate in a large area There has been progress in fabrication.

7 Solar Cell: the comparison with other energy sources Falling water..rivers, ocean tides Wave motion..in ocean Wind motion..of air Solar radiation..from the sun Fossil fuels..coal, oil, natural gas Fission nuclear energy..splitting atoms Fusion nuclear energy..combining atoms Characteristic features - Environmentally Friendly - Proper energy source in Isolated and Sunny place - Climate-dependent (Hybrid with battery) - Should increase the energy conversion efficiency Dye-Sensitized Solar Cell: The flexible solar cell - Overall conversion efficiency : up to 11% - Convenient and easy manufacturing - Flexibility - Low cost fabrication - Polymer foil is used as substrate instead of glass Enables roll to roll production Enables flexible solar cell Can be used in power generation in wearable computer and flexible display

8 Dye-Sensitized Solar Cell: Working Principle Photon Excitation of dye Fast electron injection into C.B. of TiO 2, SnO 2 Injected electrons percolate through the TiO 2 and are fed into external circuit At counter electrode I e - 3I - Iodide(3I - ) reduces the oxidized dye on the surface of TiO 2 by electron injection Dye-Sensitized Solar Cell: Easy Fabrication Process roll-to-roll, high volume manufacturing process produces thin, flexible cells "In 2005 we will go to market with our first product based on dye-sensitized cell chemistry which currently is on par with today's commercially available thin-film." - Daniel Patrick McGahn, Executive Vice President and Chief Marketing Officer at Konarka Konarka Contracted for Hybrid Solar PV Cells Lowell, Massachusetts - February 18, 2004 [SolarAccess.com] Konarka Technologies has been selected by The Defense Advanced Research Projects Agency (DARPA) to receive a contract in excess of US$6 million for basic research in developing new materials for hybrid photovoltaic (PV) cells.

9 Dye-Sensitized Solar Cell: Application to OLED OLED Display / Organic Solar Cells The anticipated advantages of organic and nano solar cells have prompted accelerated research to leverage the link between organic solar cells and organic LED (light emitting diode). Organic solar cells absorb light and convert it to electricity, while organic LEDs (OLED) use the same materials to perform the reverse process they conduct electricity to emit light. There is great scientific and commercial interest in OLEDs because they make color displays cheaper, sharper, thinner, and mechanically more flexible in consumer devices such as cell phones, personal digital assistants, and car dashboards. OLED technology also uses significantly less energy than current electronic displays, enabling longer battery life and increased usefulness of the products. Solar Cell: competition with other mobile energy sources Other mobile energy source 1. SOFC, PEMFC : - High energy conversion efficiency. - Environmentally Friendly. - The problem in fuel handling. (H 2 ) Prototype PEMFC automobile (by Benz and Ballard) Fuel reforming to hydrogen or H 2 storage is required in using HC - Methanol can be promising fuel but should overcome the poisoning effect of carbon using less expensive catalyst. - Movie (SOFC) - Animation (PEM fuel cell) - Movie (PEM fuel cell) 2. Battery - Relatively easy to fabricate. - Requires relatively long charging time. - Use the highly acidic or alkalic electrolyte solution. - Relatively short lifetime (charging/discharging characteristics).

10 Dye-Sensitized Solar Cell: Combination with fuel cell Capturing sunlight to make enough hydrogen fuel to power cars and buildings has been brought a step closer by a British research company. 1 Ultraviolet sunlight passes through glass skin of cell 2 Light is captured in glass coated with nano-crystalline film 3 Nano-coating properties enable the glass to conduct electricity, which is used to separate the water into oxygen and hydrogen 4 Hydrogen gas is stored for later use as a power source Why H 2 should be prepared by solar energy? If you use a battery, then chances are that the battery was charged with electricity produced by burning fossil fuels, so that the hydrogen you produce isn't produced cleanly. If you use a solar cell, however, then the hydrogen will be produced cleanly, except for any pollutants that were emitted when the cell was made (we say that the solar cell has no "point-of-use" emissions). Burning fossil fuels on the other hand, always results in carbon monoxide (CO) and/or carbon dioxide (CO 2 ), which is produced when the carbon atoms combine with oxygen. These compounds are now considered pollutants because they are greenhouse gases - that is, they help trap heat near the Earth's surface, causing the Earth's surface temperature to rise, i.e. global warming.

11 Two obstacles to a hydrogen-economy There are two obstacles to a hydrogen-economy. It takes a lot of volume (or energy) to store hydrogen - usually five times or so the volume, at reasonable pressures, needed to store an equivalent amount of energy with gasoline. (It triggered the researches on hydrogen storage materials.) There is no hydrogen infrastructure: Making the transition to a hydrogen economy might mean having to scrap the fossil fuel infrastructure that we have already developed. (The in-situ reforming becomes big issue.)