Light-Emitting Diode

The light emitting diode (LED) is a diode that emits light when it is for­ward biased and conducting. The LED must not only be forward biased but requires a minimum forward biased voltage drop to draw enough cur­rent to light. The voltage requirement varies based upon the material of the diode. The typical voltage dropped across an on LED ranges from 1.5 V to 5 V. A typical voltage used throughout this book is 2 V. The light bright­ness is proportional to the current, that is, the higher the current through the LED, the brighter the LED. Different materials and construction create different colors of emitted light. LEDs are available with emitted colors throughout the color spec­trum with a variety of light intensities. Typical colors include red, green, yellow/amber, and blue.

Light emitting diodes were developed in the 1960s as an outgrowth of semiconductor technology. The devices emit light when a forward bias voltage is applied to a pn junction in a single crystal of gallium arsenide, gallium arsenide phosphide, or other group III—V compounds. By appro­priate doping and/or the use of crystals containing these III—V materials, it is possible to produce emission of red, green, yellow, and even blue light. It is also possible to produce a variety of colors from a single device.

One of the first reports of visible light emission from a diode ap­peared in 1962. Following this report, the development of light emitting diode displays based on gallium arsenide phosphide pn junctions was carried out at sev­eral laboratories including Bell Laboratories, IBM, RCA Laboratories, Hewlett-Packard, and Monsanto Chemical Company. In 1966, researchers at Bell Laboratories found that the addition of nitrogen to gallium phos­phide substantially improved the performance of green light emitting de­vices. However, the first commercial light emitting diode displays were introduced by Monsanto and Hewlett-Packard in 1968.

The early LED devices were designed to provide a digital readout of information from various test instruments that previously used needle­point gauges. The low operating voltage and relatively high brightness of the light emitting diode displays made them a good choice for this application. In the early 1970s, LEDs began to be used in portable multimeters and eventually in hand-held calculators made by such companies as Hewlett-Packard, Texas Instruments, and Bomar.

 Questionnaire:

• What is light emitting diode? 
• Explain about the history of light emitting diode.