Light-emitting Diodes | previous | next | feedback |
(Acronym: LED)
Definition: semiconductor diodes that emit light via electroluminescence
Figure 1: Two light-emitting diodes, emitting white and red light, respectively
A light-emitting diode (LED) is an optoelectronic device which generates light via electroluminescence. It contains a p-n junction, through which an electric current is sent. In the heterojunction, the current generates electrons and holes, which release their energy portions as photons when they recombine. While the fundamental process of light generation is the same as in laser diodes, light-emitting diodes do not exhibit laser action, i.e., they do not exploit stimulated emission.
Materials and Emission Wavelengths
The center wavelength and thus the emission color of an LED is largely determined by the bandgap energy of the used semiconductor material. Essentially the whole visible wavelength region can be covered with LEDs, although the device efficiency is not equally high for all wavelengths.
For red emission, aluminum gallium arsenide (AlGaAs) can be used, which is otherwise common for near-infrared laser diodes. However, the best efficiencies are achieved with the ternary material aluminum indium gallium phosphide (AlInGaP). The internal quantum efficiency can be close to 100% for emission wavelengths around 650 nm, whereas high efficiencies are much more difficult to achieve at shorter wavelengths of e.g. 620 nm, where the human eye is more sensitive. Another challenge is that the temperature increase, which is hardly avoidable in high-power LEDs, decreases the quantum efficiency and increases the emission wavelength. Effective cooling methods are thus essential.
Indium gallium nitride (InGaN) is very suitable for blue and violet LEDs. Despite high defect densities in these materials [1], internal quantum efficiencies of 70% and higher are achieved. Longer wavelengths (green and yellow) are obtained by increasing the indium (In) content, but the efficiency sharply drops as the wavelength is increased.
The technologically most difficult spectral region (in the visible range) is that of green-yellow-orange light. Intense research is pursued to fill this gap. Zinc selenide (ZnSe) and zinc selenide telluride (ZnSeTe) have been developed for green emission, but both device lifetimes and efficiencies are not satisfactory for lighting purposes. Various others materials, including both modified II-VI alloys and oxychalcogenides such as LaCuOS, are therefore under consideration.
White light can be generated either by mixing the outputs e.g. of red, green and blue LEDs, or by using a single blue LED and some phosphor, which converts part of the blue light into light with longer wavelengths. That conversion is normally done either with another semiconductor (e.g., ZnS), or with a scintillator crystal containing rare-earth ions such as Eu2+ (europium) or Ce3+ (cerium). For example, Ce3+:YAG can be used for converting blue light around 440-460 nm into yellow light around 520-640 nm. Modified hosts, e.g. with some of the yttrium in YAG being substituted with gadolinium (Gd3+), lead to somewhat shifted emission ranges of the cerium ions. The color rendering index of such white LEDs is usually not very high, but it can be improved e.g. by combining yellow and red phosphors.
There are also near-infrared LEDs, based e.g. on AlGaAs, and ultraviolet LEDs, using materials like gallium nitride (GaN).
A more recent development is that of LEDs based on organic semiconductors, called OLEDs (organic light-emitting diodes). Such materials have a potential for cheap mass fabrication of large and mechanically flexible devices. They therefore appear to be very promising for future lighting applications. However, further research and development is required for improving the efficiency and particularly the device lifetime. This article focuses mainly on inorganic LEDs.
Emission Properties
Light emitted by LEDs has a low spatial coherence. It is originally emitted in all directions. Even though many LED devices emit light preferentially in one direction (often via built-in reflecting structures), the focusability (beam quality) is very low, comparing e.g. with that of laser diodes.
The emission bandwidth is typically a few tens of nanometers (e.g. 20 nm), i.e., much broader than for laser diodes, and somewhat smaller than that of most superluminescent diodes. This means that the temporal coherence is much lower than that of a laser, although it is much higher than e.g. for an incandescent bulb.
Efficiency
The internal process of generating light in an LED, as described above, can have a very high quantum efficiency and power efficiency, at least in the blue-violet and in the red spectral region. Nevertheless, the device efficiency of early light-emitting diodes was relatively poor. The reason is that it was not possible to efficiently extract the internally generated light; most of the generated light was absorbed inside the device. A key challenge is total internal reflection at the surface of the semiconductor material: due to the high refractive index, light can escape only for relatively small angles of incidence, and even then there is substantial Fresnel reflection.
In the 1990s, advanced LED designs have been developed which allow fairly efficient light extraction and thus reach much higher device efficiencies. The luminous efficacy can be well over 100 lm/W, i.e., even better than that of fluorescent lamps. Also, advanced cooling arrangements have made it possible to generate much higher optical powers.
Device Lifetime
Light-emitting diodes based on inorganic semiconductors can have very long lifetimes, which can exceed 100'000 hours. LEDs are thus belonging to the most long-lived illumination devices.
On the other hand, LEDs are relatively sensitive against excessive reverse voltages, and can be destroyed by electrostatic discharge when improperly handled. Also, the lifetime is severely reduced for operation with a too high current and/or at too high ambient temperatures.
Electrical Characteristics
As any other semiconductor diode, a current can flow through an LED only from the p-doped part to the n-doped part (conventional current direction). A reverse voltage of a more than a few volts can destroy an LED.
In forward direction, the current stays very small for low voltages and then rises very quickly (exponentially) with increasing voltage. Therefore, LEDs can normally not be operated with a constant voltage; the current needs to be stabilized, e.g. by operation with a current source, or by using a simple series resistance for connecting to a constant voltage supply. The optical power is quite precisely proportional to the operation current.
Main Attractions
The main attractions of light-emitting diodes are:
- The device efficiency (see above) can be very high, leading to a small electric power consumption and low heat generation. The effective efficiency of a lamp is often further improved by the more directed emission of LEDs, reducing the amount of light that is lost in the lamp housing.
- The device lifetime (see above) is very long. It normally ends with a gradual loss of brightness, rather than with abrupt failure.
- Larger LED lights contain multiple LEDs, and can remain largely functional when single LEDs fail. This is particularly important where safety is critical (e.g. for traffic lights).
- Light with specific colors (e.g. for traffic lights) can be directly generated. This is more efficient than e.g. using an incandescent lamp with a color filter, which has to absorb much of the generated optical power.
- LEDs can be used in very compact and lightweight packages.
- They are mechanically very robust and can tolerate even severe mechanical shocks.
- The output power of a light-emitting diode can be very quickly modulated. Modulation frequencies of many megahertz are possible, since the carrier lifetime is only a few nanoseconds.
- When white light is generated by using separate red, green and blue LEDs, the color tone can be adjusted by adjusting the relative operation currents.
- Dimming is possible by reducing the current or by quickly switching it on and off with a variable duty cycle. In any case, the power efficiency is preserved when dimming an LED, and the color tone remains quite precisely constant. Both would not be the case for a dimmed incandescent lamp.
- LEDs do contain poisonous substances such as gallium arsenide, but only rather small quantities of them.
Limitations
- The cost per watt of output power of an LED for illumination is quite high. However, fast progress is made to reduce that cost, mainly by increasing the output power of the LED chips. Also, the higher cost can be offset by reduced electricity consumption.
- Although an LED produces less heat than an incandescent lamp with the same optical power, adequate heat sinking is necessary to prevent overheating, which would degrade the lifetime.
- The color rendering index of some white LEDs is low, making them unsuitable for certain applications.
- As the available electric supplies usually provide an (approximately) constant voltage, the operation current needs to be stabilized with some electronics. Quite simple electronics may used, but then often spoil the overall power efficiency.
Applications of Light-emitting Diodes
LEDs are very widely used as small signal lights. Operated with a current of e.g. 5-20 mA, such devices produce enough light to be seen at normal ambient light conditions, and different colors can be used e.g. to signal different states of a device.
As LEDs can be quickly modulated, they are suitable for optical fiber communications over short distances. While the poor directionality of their emission requires the use of multimode fibers and thus restricts the transmission distances, the cost is significantly lower than for a system with single-mode fibers and laser diode transmitters. Fast power modulation is also useful e.g. for application in light barriers, as the modulated LED light is easily distinguished from the ambient light.
The enormous progress on high-power LEDs has recently made it possible to use LEDs for larger signals and for lighting purposes. As the cost per watt is still relatively high, and the output power is limited, such applications started in areas such as traffic lights (where the long lifetime is particularly important) and stop lights for cars. Further improved device make it possible to soon use high-power white LEDs for the main lights of cars, and for residential lighting. In some areas such as airplanes, the compact package size and the low electricity consumption are particularly important.
Special Types of Light-emitting Diodes
Resonant-cavity LEDs (RC-LEDs) have the light-emitting semiconductor junction embedded between two distributed Bragg reflectors (Bragg mirrors), i.e. in an optical resonator. Due to the moderate Q factor, lasing does not occur, but a larger directionality is obtained, comparing with conventional LEDs. This makes it easier to achieve efficient light extraction from the device, and increases the directionality and brightness of the output, from which e.g. applications in fiber-optic communications can profit. The external quantum efficiency can easily exceed 20%. The emission bandwidth is somewhat smaller than for other LEDs.
Bibliography
| [1] | S. Nakamura, "The role of structural imperfections in InGaN-based blue light-emitting diodes and laser diodes", Science 281, 5379 (1998) |
| [2] | J. Han and A. V. Nurmikko, "Advances in AlGaInN blue and ultraviolet light emitters", IEEE Sel. Top. Quantum Electron. 8 (2), 289 (2002) |
| [3] | K. Streubel et al., "High brightness AlGaInP light-emitting diodes", IEEE Sel. Top. Quantum Electron. 8 (2), 321 (2002) |
| [4] | N. K. Patel et al., "High-efficiency organic light-emitting diodes", IEEE Sel. Top. Quantum Electron. 8 (2), 346 (2002) |
| [5] | J. M. Phillips et al., "Research challenges to ultra-efficient inorganic solid-state lighting", Laser & Photon. Rev. 1 (4), 307 (2007) |
| [6] | N. Zheludev, "The life and times of the LED – a 100-year history", Nat. Photonics 1 (4), 189 (2007) |
| [7] | A. Khan et al., "Ultraviolet light-emitting diodes based on group three nitrides", Nat. Photonics 2, 77 (2008) |
| [8] | S. Qingjiang et al., "Bright, multicoulored light-emitting diodes based on quantum dots", Nat. Photonics 2, 717 (2008) |
See also: luminescence, superluminescent diodes, laser diodes
