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.
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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 visiblespectrum 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 highbrightness blue LED based on InGaN. The 2006 Millennium Technology Prize was awarded to Nakamura for his invention.
Theory: A Light emitting diode (LED) is essentially a pn junction diode. When carriers are injected across a forward-biased junction, it emits incoherent light. Most of the commercial LEDs are realized using a highly doped n and a p Junction.
Figure 1: p-n+ Junction under Unbiased and biased conditions. (pn Junction Devices and Light Emitting Diodes by Safa Kasap)
To understand the principle, let’s consider an unbiased pn+ junction (Figure1 shows the pn+ energy band diagram). The depletion region extends mainly into the p-side. There is a potential barrier from Ec on the n-side to the Ec on the p-side, called the built-in voltage, V0. This potential barrier prevents the excess free electrons on the n+ side from diffusing into the p side. When a Voltage V is applied across the junction, the built-in potential is reduced from V0 to V0 – V. This allows the electrons from the n+ side to get injected into the p-side. Since electrons are the minority carriers in the p-side, this process is called minority carrier injection. But the hole injection from the p side to n+ side is very less and so the current is primarily due to the flow of electrons into the p-side. These electrons injected into the p-side recombine with the holes. This recombination(see Appendix 1) results in spontaneous emission of photons (light). This effect is called injection
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...
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