Semiconductors can create and detect electromagnetic radiation. This radiation carries energy, so it can join conduction currents as part of our electronic circuits. The eye is sensitive to electromagnetic radiation with wavelengths between 400nm and 700nm, and such radiation is called light. I’ll simply call all radiation light in what follows, although it may include radiation to which the eye is not sensitive. Light has only 5 letters, while electromagnetic radiation has 26, including the space.
The frequency of light is connected to its wavelength by λf = c, where c is the phase velocity of light, 2.9979 x 108 m/s (now an arbitrary constant defining the metre), f the frequency in Hz, and λ the wavelength in metres. The frequency is extremely high, so it is more convenient to use wavelegth, usually in nanometres. As we have said, visible light is in the range 400nm (blue) to 700nm (red). The sensitivity of the eye peaks at 550nm (green), falling off smoothly at both limits. We will be concerned with the visible, and with radation of wavelength greater than 700nm, called infrared. At the other end is the ultraviolet, with shorter wavelengths. The deep blue at the short-wavelength end of the visible spectrum was called violet for some reason; indigo would be better–violet it is not. Then we would have had the ultraindigo.
When light interacts with atomic systems, energy is exchanged only in discrete amounts depending on the frequency of the light. The fundamental amount, or quantum, is E = hf, where h is Planck’s constant, 6.6261 x 10-34 Joule-seconds. A friendlier energy unit is the electron-volt, eV, the energy given to an electron by a potential difference of one volt, equal to 1.602 x 10-19 J. In terms of wavelength, the quantum energy is E = 1240/λ eV. For the visual range, this is from 3.1 eV to 1.8 eV. The quantum energy in the ultraviolet is large enough to start smashing molecules, so ultraviolet is bad for you. There is no such problem in the infrared.
A semiconductor is characterized by an energy gap between the electrons that have crowded into all the available places (a band)and so act as if they cannot move because there is no place to go, and the wide-open spaces where the electrons can freely go where they will. N-type semiconductors have extra electrons added to the wide-open spaces by impurities that can easily supply them, and p-type semiconductors have holes that can move around created by impurities that can capture some of the electrons from this filled band. In silicon, the energy or band gap is about 1.1 V, so light can liberate an electron from the filled band, creating a free electron in the upper or conduction band, and a free hole in the lower or valence band. Vibrations in the crystal–heat–can also do this, so there are some free electrons in silicon even in the dark. As a practical matter, though, this intrinsic conduction is not seen. Impurities always dominate, making the material either n-type or p-type.
The diagram at the right shows the conditions in a semiconductor crystal at 0K. All the states in the valence band are full of electrons, absolutely all, so the electrons are packed so tightly they cannot move. All the states in the conduction band, at higher electron energy, are empty. The Fermi level is the energy at which a state is, on the average, occupied half the time. Above the Fermi level, the states are unoccupied, and below it, occupied, at T = 0K. The band gap energy Eg is the energy difference between the valence and conduction bands. Also shown is a donor site, perhaps As in Si, which has an extra electron easily kicked free, since it is not needed for the crystal binding. There is also an acceptor site, Al for example, that could use an extra electron to improve the crystal binding. At T = 0K, the electron is still in the donor, and the acceptor is still empty. The semiconductor is not a conductor at all, since there are no mobile charge carriers. The portion of the crystal with the donors will turn out to be n-type material, with movable electrons, and the part with the acceptors will turn out to be p-type material, with movable holes, when we heat the crystal up.
With the addition of a little heat, the donors have ionized, and the donated electrons are now roaming around the conduction band. The acceptors have also ionized, grabbing electrons from the valence band, leaving holes that can roam about. The donors, fixed in the crystal, are now positively charged, while the acceptors are negatively charged. Nevertheless, the electrons and holes are exactly enough to balance the charges and things are still neutral. However, electrons may well bounce around to the left, and holes to the right, uncovering the fixed charges of the donors and acceptors. Since positive charges are uncovered on the right, and negative on the left, an electric field arises that forces the electrons and holes back where they belong. In the diagram, the Fermi level remains flat, but the bands are bent so that the Fermi level passes between the conduction band and the donor levels in the n-type material, and passes between the valence band and the acceptor levels in the p-type material. The bending of the bands corresponds exactly to the effect of the electric field on the electrons and holes. Some charge has left the region where the bands are sloping to expose the fixed positive and negative charges, and this has required a current to flow briefly, which charges this “capacitor.” The region devoid of electrons and holes is called the depletion layer. In in, an electric field exists that sweeps out any mobile charges that may venture in.
In the energy diagram, electrons roll downhill but holes roll uphill where the bands slope. If light gives an electron in the valence band an energy hf greater than Eg, a free electron and a free hole are produced in this area, and are instantly swept out, creating a photocurrent.
It is very satisfying to prove to oneself that the PN junction forms a rectifier. In the conditions shown in the band diagram, there is no current through the junction. Actually, there are small currents both ways, but they cancel on the average. The electrons in the conduction band have kinetic energies as shown by the small graph on the right, that shows the thermal energy kT. This is the familiar exponential Boltzmann distribution. There is a similar situation with the holes, but it is not shown to avoid excess complication. If we make the barrier higher, by making the n-type material positive (since we are dealing with electron energies, it looks backwards, but electrons have a negative charge!). If we make the barrier lower, the upper part of the energy distribution of the electrons (and holes) peeks above the barrier, and charges can diffuse through. A current results, which increases exponentially because the Boltzmann distribution increases exponentially.
In a diode, the p region is called the anode, and the n region the cathode. Conduction occurs when the anode is made positive with respect to the cathode, and the current increases exponentially with the applied forward bias, as we well know. If we apply a large reverse bias, the strong electric field accelerates random charges that appear thermally, and if it is strong enough, gives the charges enough energy to knock out further charges. Breakdown then results, and the PN junction ceases to control the current. The heat developed usually destroys the device quickly when breakdown occurs.
With a reverse bias, there is only a small current produced by electron-hole pairs created in the depletion layer by thermal action. The strong electric field sweeps them out. When light falls on the depletion layer, pairs are copiously produced, and a considerable photocurrent results. For this to happen, light must both penetrate to the depletion layer, and also be absorbed there. These are antagonistic effects, so the actual photocurrent is a compromise. In silicon, the band gap of 1.1 eV corresponds to a wavelength of 1130 nm, and here absorption is greatest. As the wavelength decreases, the absorption also decreases, so more light can penetrate through bulk silicon to the depletion layer. However, less is proportionately absorbed, so the amount of light absorbed in the depletion layer decreases, passing on to be uselessly absorbed in the semiconductor beyond. In most practical silicon structures, the maximum sensitivity seems to occur around 850 nm. Other semiconductors also can be used, but silicon is so well known that it is employed for most photojunctions.
An electron and a hole can also recombine, the electron falling into the hole, as it were, releasing its energy, which will be greater than the energy gap. This energy can be released as light, but usually it is not, either because this is not favored by the band conditions (as in silicon) or because there are more efficient competing processes. In some semiconductors, such as gallium arsenide, GaAs, radiative recombination of electron-hole pairs is favored. When current passes through a depletion layer under forward bias, both electrons and holes can be made to take part, and when they encounter one another, light is produced. Since the energies of the electrons and holes are not far from the band limits at normal temperatures, the wavelength of the light corresponds to the energy gap. The wavelength of maximum emission is somewhat shorter than that corresponding to the gap, for some of the same reasons that displace the wavelength of maximum absorption. The light has to get out of the junction.
The band gap of GaAs is 1.4 V, so the emission limit is 885 nm. The peak of the emitted radiation is actually around 940 nm, with a width of about 50 nm (that is, most of the radiation is between 915 and 965 nm). This is a pretty good match with the peak sensitivity of silicon, and many emitters are made from GaAs. The band gap can be varied by adding Al to the mix, and emitters that match silicon closely at about 850 nm are available, but usually are not worth the bother. There is a wide variety of III-V semiconductors like GaAs that allows the tailoring of the band gap to any tastes. When the band gap is larger than about 1.8 V, the peak is in the visible. GaP, for example, with a band gap of 2.25 V, makes a green light-emitting diode (LED). LED’s are now available in all colors from red to blue, with even some white LED’s that mix colors. The working forward voltage of the diode increases as the wavelength decreases, an effect that is easily observed. LED’s are made for greatest efficiency, watts of radiation per milliampere, not for high reverse breakdown voltage, so they should always be guarded against the application of a reverse voltage.
Bart’s Book on Semiconductors – A self help ,complete book on principles of semiconductor devices (includes transistors)
BOIN’s Semiconductor Linkpage – News and Index of links to companies, organizations and universities involved in semiconductor research and development
Compound Semiconductors Online – Resource site pertaining to the extensive needs of the compound semiconductor industry
For The Love of LEDs – Tests on LEDs (Light Emitting Diodes) from infrared to ultraviolet including beam profile images, rated reviews of LED flashlights, and a museum of LED technology from 1960 to present.
ITRS Public Home Page – Provides a roadmap(assessment) of the needs and challenges facing the semiconductor industry over the next 15 years.
LED FAQ – Frequently asked questions and answers on LED’s (Light Emitting Diodes)
NTE Electronics, Inc. – NTE is a supplier of active, passive and electromechanical electronic components. Cross reference for resistors, relays, and semiconductors.
Physics Encyclopedia: Solid State Physics and Semiconductors – Educational link based resources on solid state physics, semiconductors,diodes & transistors and nanotechnology
Semiconductor Glossary – Explanation of key terms in semiconductor materials, processes, and devices.
Semiconductor Hall of Fame – A link list of the major contributors to the Science and technology of semiconductors, biographies and articles.
Semiconductor Links – Industry oriented semiconductor related resources
The Semiconductor Resource Page – Alphabetically arranged list of semiconductor manufacturers.
Transistor Cross Reference Database – Based on Pacific Semiconductor’s 40,000 transistors database.
Transistorized – Know about transistors chronologically
Transistors – Transistors tutorials, With some very good transistor theory, AC, DC amp, bias, and clipping fundmentals.
The Unusual Diode FAQ – All about the unusual types of semiconductor diodes.
Access Group Inc. – Independent stocking distributor of excess inventory, obsolete and hard to find parts. Specialises in integrated circuits, semiconductors, hard drives and peripherals. Online part search and inventory available for download.
All American Semiconductor Inc. – Distributor of application specific semiconductors, microcontrollers, microprocessors and memory products, flat panel display products and passive, electromechanical, power and discrete components. Online line card, links to manufacturers and purchasing.
Ambassador Components Inc. – Distributor of new and obsolete semiconductors, integrated circuits, passive and discrete parts. Online line card part search.
American Microsemiconductor Inc. – Broadline semiconductor supplier. Includes product line card, online store, FAQ and tutorials.
Amps Abundant – Supplies obsolete, discontinued and hard to find diodes, power rectifiers and thyristors (SCRs) and SCR/diode test equipment. Online technical specifications and company news.
APP Systems Services Private Limited – Distributor of vacuum technology, thin films, microelectronics, optics, and opthalmics in South East Asia.
Asgard Inc – Distributor of impossible to find electronic components and semiconductors.
Attiva SAS – Electronic component distributor. Boardlevel, CPU, memory, semiconductors, I.C., passive components
Azzurri Technology Ltd. – Distributor of proprietary and manufacturers’ semiconductors and electronic components. Links to manufacturer’s product information.
B & D Enterprises International – Factory authorized distributor for NEC, Sanyo, Sanken and Shindengen semiconductors. Distributes Sanyo OSCON and aluminum electrolytic and Poscap capacitors. Online line card, links to manufacturers’ specifications and crossmatch database.
BA-Electronics – Distributor of a wide selection of replacement semiconductors, including: transistors, diodes, rectifiers, voltage regulators, FETs.
Bakis Electronics LTD – An import and export company of semiconductors.
Bowers Semiconductors Ltd. – Sole UK distributor for Powerex and Mitsubishi power semiconductors. Online line card and value added services include manufacture and supply of high-current assemblies using diodes and thyristors.
California Eastern Laboratories – Supplies NEC Corporation’s RF, wireless, optoelectronic and fiber optic semiconductor products. Online line card and PDF data sheets available for download.
Cardinal Tech Corp. – Distributor specialising in locating current and discontinued or obsolete ICs. Online line card, part search and post excess inventory.
Certified Components Group, Inc. – Distributor of memory, logic, microprocessor, discrete and passive components. Online line card, part search and RFQ.
Circuit Solutions Inc. – Distributes allocated and hard to find memory products, discrete semiconductors and ICs. Online part search, line card, manufacturers list and links to industry news.
CounterTrade Products Inc. – Stocking distributor of electronic components and semiconductors. Online part search and inventory available for download.
Die Technology Ltd, – Distributor for un-encapsulated semiconductor die to the microelectronics component industry. Offering assessment, bare die in wafer form or waffle pack.
E & K International Ltd. – Specialised semiconductor distributor representing Gunter Semiconductor GMbH and Lesag HBB. Products include power, high voltage, small signal, schottky diode, SCR and Triac devices and semiconductor wafer.
EBV Elektronik GmbH & Co. KG – European Avnet semiconductor distributor. Online line card, part search and industry links.
eeParts, Inc. – Stocking supplier of electronic components.
Elcom Inc. – Manufacturer’s representative for Intel, Philips and Linear Technology. Online line card, links to manufacturers’ data sheets and sample requests. ( SE USA )
007 Electronics Network, Inc. – Distributor of semiconductors. Online part search, represented manufacturers list and RFQ.
Endrich Bauelemente Vertriebs GmbH – An European electronic distribution company specialized for the distribution of active, passive and electromechanical components.
Essential IC – Independent distributor of semiconductors, ICs, eproms, DRAM, SRAM, capacitors, resistors and obsolete Intel, AMD, Motorola and Texas Instruments parts. Online line card and links to manufacturers.
Eun Hyun Ltd. – Stocking distributor of semiconductors. Online line card, part search, industry links and purchasing. (Korean, English, Chinese, Japanese)
Excelpoint Systems Pte Ltd – Distribution of electronic components.
Fibra Brandt Semiconductors – Distributor of obsolete and hard to find electronic components, semiconductors, integrated circuits, transistors and capacitors.
Function Technology Corp. – Purchases OEM excess, refurbished, pull and new semiconductors. Online line card and links to represented manufacturers.
Global IC Trading Group – Distributor of DRAM, CPUs, flash, memory modules, DSPs, microcontrollers, microprocessors, analog, logic and telecom/datacom components. Online industry news.
Golledge Electronics Ltd. – Supplier of crystals, oscillators, TCXOs, VCXOs, OCXOs, resonators and filters for frequency control applications. Online line card, technical specifications and image gallery.
Grove Electronics – Independent stocking distributor of electronic components. Asset management and recovery, procurement of hard-to-find components, provides diagnostic and packing services.
Hero Electrical – Distributor of optoelectronic and LSI components.
I.C.T. Power Company Inc. – Provider of semiconductor products and solutions for the power electronics industry, and is the Canadian representative of EUPEC Inc.
Industrial Semiconductor – Distributor of high power industrial scrs, diodes, igbts, transistors, modules, heatsinks, rectifiers.
Ineltron – A distributor of high power semiconductors.
Infinity Components – Distributor of integrated circuits and semiconductors specialising in locating hard to find and obsolete components. Online line card.
K-1 Technologies Inc. – Distributor of electronic components including semiconductors, integrated circuits, connectors, capacitors, diodes and transistors. Specializes in obsolete ICs and hard-to-find parts. customers.
Magnitude Electronics – Distributor of semiconductors and integrated circuits, including excess inventory. Online line card, part search and inventory available for download.
Manistar Electronics Inc. – Distributor and exporter of semiconductors, memory devices, diodes, transistors and connectors. Online line card, industry news and member’s area.
Memec – Global semiconductor distributor. Online line cards, industry news, company newsletter and application and manufacturers’ selection criteria. Offers design services, workshops and seminars.
National Team Components, Inc – Semiconductor distributor. Also buys surplus inventory.
New York Semiconductor – Independent stocking distributor of electronic components and semiconductors. Hard-to-find or obsolete parts, excess inventory solutions.
Nu Horizons Electronic – Franchised distributor of semiconductors. Online product change notifications, development tools and part search.
Olympus Products – Distributor of semiconductors, ICs and passive components. Sources obsolete and hard to find parts. Online part search and RFQ.
Plextron Technology Co. Ltd. – Distributor of surge and circuit protection devices, TVS diodes, chip varistors and polyreset components. Online data sheets and sample request.
Prelco Electronics Inc. – Provides consumer and industrial semiconductors. Specialises in obsolete and hard to find components and offers pre-programming of eeproms. Online line card and links to manufacturers and industry news.
Profusion Plc – Specialist distributor of discrete power semiconductors and linear integrated circuits. Online line card, part search and purchasing.
RFQ Electronics, Inc – Distributor focused on major IC vendors.
Rochester Electronics, Inc. – Distributor and manufacturer of discontinued semiconductors. Online search of wafer/die and tooling inventory.
SASCO Semiconductor – Arrow’s semiconductor distributor and design-in service in Central Europe. Online line card, part search and capability statement.
Semiconductor Logistics Corporation (SLC) – Buys and sells hard to find and discontinued heremtically sealed discrete semiconductors and wafer components. Site lists available inventory.
Semtex Industrial Corp. – Franchised distributor of power semiconductors including rectifier, thyristor, bridge, SCR, diode, Triac, relay, module, transistor, IBGT and hybrid components.
Serenity Electronics – An independent distributor of commercial and military electronic components such as semiconductors, active and passive components.
Smith Semiconductor, Inc – Stocking distributor for new & obsolete electronic components including integrated circuits. Online part search.
Solid State Inc. – An online distributor of semiconductors and tool kits.
Stock’n Source Inc. – Independent stocking distributor of allocated and obsolete integrated circuits and semiconductors. Online line card, inventory search and links to manufacturers.
SurplusSalesCo.com – Discount distributor of semiconductor, electronic devices, diodes, transistors, thyristors, capacitors, igbts, varistors, scr’s.
Techsonic – Supplies semiconductors including microprocessors, ICs and transistors. Includes product listing and pricing.
Transfer Electronik – Distributor of electronic micro-controllers, transistors and other electronic devices.
Transpac Technology, Inc. – Premier global supplier of electronic components and services. Source for actives, passives, and electromechanical parts. Packing and testing services also available.
Universal Semiconductor, Inc. – Authorized Sanyo distributor. Specializing in Japanese components.
U.S. Circuits, Inc., – Distributor of semiconductors and transistors including new, obsolete and discontinued.
The Vital – Distributor of memory, passive components and printed circuit boards. From China.