Understanding the Principle and Applications of Light Emitting Diodes (LEDs), Schemes and Mind Maps of Chemistry

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Light Emitting Diodes (LEDs)

ELE 432 Assignment # 3 Vijay Kumar Peddinti

Light Emitting Diodes Principle Synopsis: To explain the theory and the underlying principle behind the functioning of an LED

Brief History:

• The first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter H. J. Round.

(material.eng.usm.my/stafhome/zainovia/EBB424e/ LED 1.ppt)

• In the mid 1920s, Russian Oleg Vladimirovich Losev independently created the first LED, although his research was ignored at that time.
• In 1955, Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys.
• Experimenters at Texas Instruments, Bob Biard and Gary Pittman, found in 1961 that gallium arsenide gave off infrared radiation when electric current was applied. Biard & Pittman received the patent for the infrared light-emitting diode.
• In 1962, Nick Holonyak Jr., of the General Electric Company and later with the University of Illinois at Urbana-Champaign, developed the first practical visible- spectrum LED. He is seen as the "father of the light-emitting diode".
• In 1972, M. George Craford, Holonyak's former graduate student, invented the first yellow LED and 10x brighter red and red-orange LEDs.
• Shuji Nakamura of Nichia Corporation of Japan demonstrated the first high- brightness blue LED based on InGaN. The 2006 Millennium Technology Prize was awarded to Nakamura for his invention.

electroluminescence. These photons should be allowed to escape from the device without being reabsorbed.

The recombination can be classified into the following two kinds

• Direct recombination
• Indirect recombination

Direct Recombination: In direct band gap materials, the minimum energy of the conduction band lies directly above the maximum energy of the valence band in momentum space energy (Figure 2 shows the E-k plot (see Appendix 2)^ of a direct band gap material). In this material, free electrons at the bottom of the conduction band can recombine directly with free holes at the top of the valence band, as the momentum of the two particles is the same. This transition from conduction band to valence band involves photon emission (takes care of the principle of energy conservation). This is known as direct recombination. Direct recombination occurs spontaneously. GaAs is an example of a direct band-gap material.

Figure 2: Direct Bandgap and Direct Recombination

Indirect Recombination: In the indirect band gap materials, the minimum energy in the conduction band is shifted by a k-vector relative to the valence band. The k-vector difference represents a difference in momentum. Due to this difference in momentum, the probability of direct electron- hole recombination is less. In these materials, additional dopants(impurities) are added which form very shallow donor states. These donor states capture the free electrons locally; provides the necessary momentum shift for recombination. These donor states serve as the recombination centers. This is called Indirect (non-radiative) Recombination. Figure3 shows the E-k plot of an indirect band gap material and an example of how Nitrogen serves as a recombination center in GaAsP. In this case it creates a donor state, when SiC is doped with Al, it recombination takes place through an acceptor level.

The indirect recombination should satisfy both conservation energy, and momentum. Thus besides a photon emission, phonon (See Appendix 3)^ emission or absorption has to take place. GaP is an example of an indirect band-gap material.

Figure 3: Indirect Bandgap and NonRadiative recombination

The wavelength of the light emitted, and hence the color, depends on the band gap energy of the materials forming the p-n junction. The emitted photon energy is approximately equal to the band gap energy of the semiconductor. The following equation relates the wavelength and the energy band gap. hν = Eg hc/λ = Eg

λ = hc/ Eg Where h is Plank’s constant, c is the speed of the light and E (^) g is the energy band gap Thus, a semiconductor with a 2 eV band-gap emits light at about 620 nm, in the red. A 3 eV band-gap material would emit at 414 nm, in the violet. Appendix 4 shows a list of semiconductor materials and the corresponding colors.

LED Materials: An important class of commercial LEDs that cover the visible spectrum are the III-V(see Appendix 5). ternary alloys based on alloying GaAs and GaP which are denoted by GaAs 1- yP^ y. InGaAlP is an example of a quarternary (four element) III-V alloy with a direct band gap. The LEDs realized using two differently doped semiconductors that are the same material is called a homojunction. When they are realized using different bandgap materials they are called a heterostructure device(see Appendix 7). A heterostructure LED is brighter than a homoJunction LED.

Applications: LED have a lot of applications. Following are few examples.

• Devices, medical applications, clothing, toys
• Remote Controls (TVs, VCRs)
• Lighting
• Indicators and signs
• Optoisolators and optocouplers

Figure 5: Optocoupler schematic showing LED and phototransistor (Wikipedia)

• Swimming pool lighting(see Appendix 9).

Advantages of using LEDs

• LEDs produce more light per watt than incandescent bulbs; this is useful in battery powered or energy-saving devices.
• LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.
• The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.
• When used in applications where dimming is required, LEDs do not change their color tint as the current passing through them is lowered, unlike incandescent lamps, which turn yellow.
• LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or High Intensity Discharge (HID) lamps that require a long time before restarting.
• LEDs, being solid state components, are difficult to damage with external shock. Fluorescent and incandescent bulbs are easily broken if dropped on the ground.
• LEDs can have a relatively long useful life. A Philips LUXEON k2 LED has a life time of about 50,000 hours, whereas Fluorescent tubes typically are rated at about 30,000 hours, and incandescent light bulbs at 1,000–2,000 hours.
• LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.
• LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds; Philips Lumileds technical datasheet DS23 for the Luxeon Star states "less than 100ns." LEDs used in communications devices can have even faster response times.
• LEDs can be very small and are easily populated onto printed circuit boards.
• LEDs do not contain mercury, unlike compact fluorescent lamps.

Disadvantages:

• LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than more conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten the future existence of compact fluorescent lamps.
• LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life (See Appendix8 9).
• LEDs must be supplied with the correct current. This can involve series resistors or current-regulated power supplies.
• LEDs do not approximate a "point source" of light, so they cannot be used in applications needing a highly collimated beam. LEDs are not capable of providing divergence below a few degrees. This is contrasted with commercial ruby lasers with divergences of 0.2 degrees or less. However this can be corrected by using lenses and other optical devices.
• There is increasing concern that blue LEDs and white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in the eye safety specifications for example ANSI/IESNA RP-27.1-05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems. LEDs in the future: LEDs have come a long way and currently they are widely used in many applications. In future, I believe research will continue for high intenisty LEDs, eventhough heat dissipation is an issue (see appendix8 9)

References:

• Discussions with Dr Fischer.
• ELE 432 Notes and Solid State Electronic Devices Ben G Streetman, Sanjay K Banerjee.
• Wikipedia.org (http://en.wikipedia.org/wiki/Led)
• http://www.ialb.uni-bremen.de/downloads/Semiconductor%20Device.pdf
• material.eng.usm.my/stafhome/zainovia/EBB424e/ LED 1.ppt
• pn Junction Devices and Light Emitting Diodes by Safa Kasap University of Saskatchewan Canada.
• Solid State Light Emitters, Light Emitting Diodes, Dr. János Schanda ,Colour and Multimedia Laboratory of the University of Veszprém.
• Light Emitting Diode, Bill Wilson Future Reading: LEDs are very interesting and involved. It’s difficult to summarize all the information in one report. If interested, additional information can be obtained from the above references. Acknowledgements: I would like to take this opportunity to thank Dr Fischer and others who have assisted me in editing this report.

Solid State Electronic Devices Ben G Streetman, Sanjay K Banerjee

(material.eng.usm.my/stafhome/zainovia/EBB424e/ LED 1.ppt)

5. Semiconductors in the periodic table: The following table shows the semiconductors in the periodic table. An example of III-V components is GaP or GaAs.

II III IV V VI B C Al Si P S Zn Ga Ge As Se Cd In Sn Sb Te

6. LED dome shapes: The LED domes are constructed such most of the light gets emitted efficiently. Following picture shows the two different kinds of domes.

(pn Junction Devices and Light Emitting Diodes by Safa Kasap)

7. Heterojunction High intensity LEDs: A semiconductor device that has junctions between different bandgap materials is called a heterostructure device. Following picture shows an example

(pn Junction Devices and Light Emitting Diodes by Safa Kasap)

8. Luminous Intensity over the year: The following graph shows the improvement of luminous intensity of LEDs over the years.

(Wikipedia)

Most LEDs were made in the very common 5 mm T1-3/4 and 3 mm T1 packages, but with higher power, it has become increasingly necessary to eliminate the heat, therefore the packages have become more complex and adapted for heat dissipation. Packages for state-of-the-art high power LEDs bear little resemblance to early LEDs. For example, the following picture shows a Philips Lumiled LUXEON K2.

Following pictures shows Color Logic, a Goldline Controls Product (company I work with). Color Logic is used to light up the swimming pool with different colors. The board uses about 25 LEDs (Philips Lumiled LUXEON K2). Currently Heat dissipation is issue. When closely seen, (picture on the right shows an enlarged view) a lot of copper is placed underneath these LEDs for better heat dissipation in to the board and to the heat sink.

Following is a new version of the board. This board uses even smaller, but brighter LEDs. Heat dissipation is even more crucial. It can be seen that there is even more amount of copper and the LEDs are separated a little bit.

9. Temperature effects on LEDs Following picture shows the effect of temperature on LEDs.

(Solid State Light Emitters, Light Emitting Diodes, Dr. János Schanda ,Colour and Multimedia Laboratory of the University of Veszprém)