To explain the theory and the underlying principle behind
the functioning of an LED
The first known report of a light-emitting solid-state
diode was made in 1907 by
the British experimenter H. J. Round. 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 visible spectrum 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 high brightness blue LED
based on In GaN. 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.
To understand the principle, let’s consider an unbiased
pn+ junction band . 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. 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
. 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.
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.
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, phononemission or
absorption has to take place. GaP is an example of an indirect band-gap
material.
The wavelength of the light emitted, and hence the color,
depends on the band gap energy of the materials forming the p-n junction. The
emitted photon energy is approximately equal to the band gap energy of the semiconductor.
The following equation relates the wavelength and the energy band gap.
LED Materials
An important class of commercial LEDs that cover the visible
spectrum are the III-V(see Appendix 5). ternary alloys based on alloying GaAs
and GaP which are denoted by GaAs1- yPy. InGaAlP is an example of a quarternary (four
element) III-V alloy with a direct band gap. The LEDs realized using two
differently doped semiconductors that are the same material is called a homo junction.
When they are realized using different band gap materials they are called a
hetero structure device hetero structure LED is brighter than a homo Junction
LED.
LED Structure
The LED structure plays a crucial role in emitting light
from the LED surface. The LEDs are structured to ensure most of the
recombinations takes place on the surface by the following two ways.
• By increasing the doping concentration of the
substrate, so that additional free minority charge carriers electrons move to
the top, recombine and emit light at the surface.
• By increasing the diffusion length L = √ Dτ, where D is
the diffusion coefficient and τ is the carrier life time. But when increased
beyond a critical length there is a chance of re-absorption of the photons into
the device. The LED has to be structured so that the photons generated from the
device are emitted without being reabsorbed. One solution is to make the p
layer on the top thin, enough to create a depletion layer. Following picture
shows the layered structure.
There are
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