Light-emitting diodes (LEDs)—small colored lights available in any electronics store—are ubiquitous in modern society. They are the indicator lights on our stereos, automobile dashboards, and microwave ovens. Numeric displays on clock radios, digital watches, and calculators are composed of bars of LEDs. LEDs also find applications in telecommunications for short range optical signal transmission such as TV remote controls. They have even found their way into jewelry and clothing—witness sun visors with a series of blinking colored lights adorning the brim. The inventors of the LED had no idea of the revolutionary item they were creating. They were trying to make lasers, but on the way they discovered a substitute for the light bulb.
Light bulbs are really just wires attached to a source of energy. They emit light because the wire heats up and gives off some of its heat energy in the form of light. An LED, on the other hand, emits light by electronic excitation rather than heat generation. Diodes are electrical valves that allow electrical current to flow in only one direction, just as a one-way valve might in a water pipe. When the valve is "on," electrons move from a region of high electronic density to a region of low electronic density. This movement of electrons is accompanied by the emission of light. The more electrons that get passed across the boundary between layers, known as a junction, the brighter the light. This phenomenon, known as electroluminescence, was observed as early as 1907. Before working LEDs could be made, however, cleaner and more efficient materials had to be developed.
LEDs were developed during the post-World War II era; during the war there was a potent interest in materials for light and microwave detectors. A variety of semiconductor materials were developed during this research effort, and their light interaction properties were investigated in some detail. During the 1950s, it became clear that the same materials that were used to detect light could also be used to generate light. Researchers at AT&T Bell Laboratories were the first to exploit the light-generating properties of these new materials in the 1960s. The LED was a forerunner, and a fortuitous byproduct, of the laser development effort. The tiny colored lights held some interest for industry, because they had advantages over light bulbs of a similar size: LEDs use less power, have longer lifetimes, produce little heat, and emit colored light.
The first LEDs were not as reliable or as useful as those sold today. Frequently, they could only operate at the temperature of liquid nitrogen (-104 degrees Fahrenheit or -77 degrees Celsius) or below, and would burn out in only a few hours. They gobbled power because they were very inefficient, and they produced very little light. All of these problems can be attributed to a lack of reliable techniques for producing the appropriate materials in the 1950s and 1960s, and as a result the devices made from them were poor. When materials were improved, other advances in the technology followed: methods for connecting the devices electronically, enlarging the diodes, making them brighter, and generating more colors.
The advantages of the LED over the light bulb for applications requiring a small light source encouraged manufacturers like Texas Instruments
Diodes, in general, are made of very thin layers of semiconductor material; one layer will have an excess of electrons, while the next will have a deficit of electrons. This difference causes electrons to move from one layer to another, thereby generating light. Manufacturers can now make these layers as thin as .5 micron or less (1 micron = 1 ten-thousandth of an inch).
Impurities within the semiconductor are used to create the required electron density. A semiconductor is a crystalline material that conducts electricity only when there is a high density of impurities in it. The slice, or wafer, of semiconductor is a single uniform crystal, and the impurities are introduced later during the manufacturing process. Think of the wafer as a cake that is mixed and baked in a prescribed manner, and impurities as nuts suspended in the cake. The particular semiconductors used for LED manufacture are gallium arsenide (GaAs), gallium phosphide (GaP), or gallium arsenide phosphide (GaAsP). The different semiconductor materials (called substrates) and different impurities result in different colors of light from the LED.
Impurities, the nuts in the cake, are introduced later in the manufacturing process; unlike imperfections, they are introduced deliberately to make the LED function correctly. This process is called doping. The impurities commonly added are zinc or nitrogen, but silicon, germanium, and tellurium have also been used. As mentioned previously, they will cause the semiconductor to conduct electricity and will make the LED function as an electronic device. It is through the impurities that a layer with an excess or a deficit of electrons can be created.
To complete the device, it is necessary to bring electricity to it and from it. Thus, wires must be attached onto the substrate. These wires must stick well to the semiconductor and be strong enough to withstand subsequent
LEDs are encased in transparent plastic, rather like the lucite paperweights that have objects suspended in them. The plastic can be any of a number of varieties, and its exact optical properties will determine what the output of the LED looks like. Some plastics are diffusive, which means the light will scatter in many directions. Some are transparent, and can be shaped into lenses that will direct the light straight out from the LED in a narrow beam. The plastics can be tinted, which will change the color of the LED by allowing more or less of light of a particular color to pass through.
Several features of the LED need to be considered in its design, since it is both an electronic and an optic device. Desirable optical properties such as color, brightness, and efficiency must be optimized without an unreasonable electrical or physical design. These properties are affected by the size of the diode, the exact semiconductor materials used to make it, the thickness of the diode layers, and the type and amount of impurities used to "dope" the semiconductor.
LPE creates an exceptionally uniform layer of material, which makes it a preferred growth and doping technique. The layers formed are several microns thick.
Quality in semiconductor manufacturing takes two forms. The first concern is with the final produced product, and the second with the manufacturing facility. Every LED is checked when it is wire bonded for operation characteristics. Specific levels of current should produce specific brightness. Exact light color is tested for each batch of wafers, and some LEDs will be pulled for stress testing, including lifetime tests, heat and power breakdown, and mechanical damage.
In order to produce products consistently, the manufacturing line has to operate reliably and safely. Many of the processing steps above can be automated, but not all are. The general cleanliness of the facility and incoming blank wafers is monitored closely. Special facilities ("clean rooms") are built that keep the air pure up to one part in 10,000 for particular processing steps (particularly numbers 1-5 above). All of these checks arise from a desire to improve the yield, or the number of successful LEDs per wafer.
Optoelectronics is blossoming with the advent of better and better processing techniques. It is now possible to make wafers with a purity and uniformity unheard of 5 years ago. This will effect how bright and how efficient LEDs can be made, and how long they will last. As they get better, they are appropriate for increasingly demanding applications, such as communications. The average lifetime of a small light bulb is 5-10 years, but the average modern LED should last 100 years before failure. This makes them suitable for applications where it is difficult or impossible to replace parts, such as undersea or outerspace electronics. Although LEDs are inappropriate for long-range optical fiber transmission, they are often useful for short range optical transmission such as remote controls, chip to chip communication, or excitation of optical amplifiers.
Other materials are being developed that will allow fabrication of blue and white light LEDs. In addition to making possible a wider variety of indicators and toys with more colors, blue light is preferable for some applications such as optical storage and visual displays. Blue and white light are easier on the eyes. Additional colors would certainly open up new applications.
Finally, as process technology advances and it becomes possible to incorporate more devices on a single chip, LED displays will become more "intelligent." A single microchip will hold all the electronics to create an alphanumeric display, and will make instrumentation smaller and more sophisticated.
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— Leslie G. Melcer