Light Emitting Diodes PrincipleSyno

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/LED1.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 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.
Schematic:
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 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 electronhole
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 NonRa

ผลลัพธ์ (อังกฤษ) 2:

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 in 1907 by
the British experimenter HJ Round.
(Material. .eng.usm.my / Stafhome / Zainovia / EBB424e / LED1.ppt)
Mid • In the 1920s, Russian Oleg Vladimirovich Losev independently Created the
First LED, Research Although his was ignored at that time.
• In 1955, Rubin Braunstein of the. Radio Corporation of America reported on
gallium arsenide Infrared emission from (GaAs) and Other Semiconductor alloys.
• Experimenters at Texas Instruments, Bob Biard and Gary Pittman, the 1,961th Found in
gallium arsenide that Gave off Infrared Radiation Electric current was when
Applied. Biard & Pittman received the Light-emitting diode Patent for the Infrared.
• In 1 962, Nick Holonyak Jr., of the General Electric Company and later with the
University of Illinois at Urbana-Champaign, developed the First Visiblespectrum practical
LED. He is seen as the "Father of the Light-emitting diode".
• In 1 972, 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. First of Japan demonstrated the Highbrightness
based on InGaN blue LED. The Millennium Technology Prize 2 006
to Nakamura was Awarded for his Invention.
Schematic:
Theory:
A Light emitting diode (LED) is essentially a PN Junction diode. When Carriers are
injected Across a Forward-biased Junction, emits incoherent Light. Most of the
Commercial LEDs are realized using a highly doped n and AP 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 is Process Called Minority Carrier
injection. But the Hole injection from the n + P Side to 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). Effect is this 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. Energy of the maximum momentum in Valence Band Space Energy (Figure 2
shows the plot Ek (See Appendix 2) of a Direct Band Gap Material). Material in this, free
at the bottom of the conduction electrons recombine directly with Band Can free holes at
the top of the Valence Band, as the momentum of the particles Two is the Same. This
transition from conduction to Valence Band 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 Band-Gap Direct Material.
Figure 2: Direct bandgap recombination Direct and
Indirect recombination:
In the indirect Band Gap Materials, the minimum Energy conduction in the 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 Electronhole
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 for Shift recombination. These serve as the recombination donor States
Centers. This is Called Indirect (non-radiative) recombination.
Figure3 shows the plot of an indirect Ek 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 NonRa.