8
2 Basic Principles of LED
material combustion, and easy to produce smoke. In 1879, Edison invented the incandescent lamp with carbon filament. From then on, mankind entered the era of electrical lighting. Incandescent lamps emit light by the thermal effect resulted from
electric current through the filament. Any object heated above 525°C emits visible
light. However, luminous efficiency of incandescent lamps is still very low, because
most of the electric energy is wasted by converting it into heat. Unlike incandescent lamp, fluorescent lamp is based on gas discharge. The tube is filled with argon
and a small amount of mercury. The inner wall of the lamp is coated with fluorescent powder which converts ultraviolet light into visible light. In the luminous
process of fluorescent lamp, the heat generation is small. Furthermore, the emitted
light is cold light which makes the luminous efficiency of fluorescent lamp greatly
improved. However, fluorescent lamp has drawbacks such as electromagnetic pollution, fragility, and mercury pollution in the waste. In the meantime, people have also
developed high-intensity gas discharge lamps which have been used in industrial
and street lighting. The lamp is filled with mercury vapor, high-pressure sodium or
mixed gas vapors. When current passes through the high-pressure gas vapor, the gas
vapor can be ionized, where the collision between electrons, atoms and ions in the
discharge tube takes place and emits light. Unfortunately, this kind of light source
has shortcomings such as high cost, difficult maintenance, high power consumption,
electromagnetic radiation hazards and poor color rendering.
While people are devoting themselves to the research and development of new
light sources, the development of semiconductor technology has led people into the
information age. At the same time, people are paying more and more attention to the
research and application of semiconductor materials in different fields. Under this
background, LED emerges, but over time.
The development of LED is closely related to the progress of materials. From the
invention of red LED in 1962 to the commercialization of blue and green LED in
1994, the progress of materials has greatly accelerated the development of LED [3].
Although the materials used to fabricate LED are different, the basic structure of
LED has not changed, all of them are p-n junctions [4]. For instance, semiconductor
materials can be indirect bandgap and direct bandgap. The spectrum can cover red,
green and blue.
2.1.2 p–n Junction and the Principle of LED Luminescence
The principle of LED is different from that of conventional light sources. Output from
a LED is based on the light radiation (ultraviolet, visible or infrared) produced by
the electron excitation transition in the device or material. It excludes any radiation
(incandescence) caused by pure material temperature.
The basic structure of the LED chip is p–n junction, as illustrated in Fig. 2.1. The
so-called p–n junction is to incorporate p-type (or n-type) impurities into n-type (or
p-type) semiconductor single crystals by appropriate technological methods (such
as alloy, diffusion, growth, ion implantation, etc.). The different regions of the single
2 Basic Principles of LED
material combustion, and easy to produce smoke. In 1879, Edison invented the incandescent lamp with carbon filament. From then on, mankind entered the era of electrical lighting. Incandescent lamps emit light by the thermal effect resulted from
electric current through the filament. Any object heated above 525°C emits visible
light. However, luminous efficiency of incandescent lamps is still very low, because
most of the electric energy is wasted by converting it into heat. Unlike incandescent lamp, fluorescent lamp is based on gas discharge. The tube is filled with argon
and a small amount of mercury. The inner wall of the lamp is coated with fluorescent powder which converts ultraviolet light into visible light. In the luminous
process of fluorescent lamp, the heat generation is small. Furthermore, the emitted
light is cold light which makes the luminous efficiency of fluorescent lamp greatly
improved. However, fluorescent lamp has drawbacks such as electromagnetic pollution, fragility, and mercury pollution in the waste. In the meantime, people have also
developed high-intensity gas discharge lamps which have been used in industrial
and street lighting. The lamp is filled with mercury vapor, high-pressure sodium or
mixed gas vapors. When current passes through the high-pressure gas vapor, the gas
vapor can be ionized, where the collision between electrons, atoms and ions in the
discharge tube takes place and emits light. Unfortunately, this kind of light source
has shortcomings such as high cost, difficult maintenance, high power consumption,
electromagnetic radiation hazards and poor color rendering.
While people are devoting themselves to the research and development of new
light sources, the development of semiconductor technology has led people into the
information age. At the same time, people are paying more and more attention to the
research and application of semiconductor materials in different fields. Under this
background, LED emerges, but over time.
The development of LED is closely related to the progress of materials. From the
invention of red LED in 1962 to the commercialization of blue and green LED in
1994, the progress of materials has greatly accelerated the development of LED [3].
Although the materials used to fabricate LED are different, the basic structure of
LED has not changed, all of them are p-n junctions [4]. For instance, semiconductor
materials can be indirect bandgap and direct bandgap. The spectrum can cover red,
green and blue.
2.1.2 p–n Junction and the Principle of LED Luminescence
The principle of LED is different from that of conventional light sources. Output from
a LED is based on the light radiation (ultraviolet, visible or infrared) produced by
the electron excitation transition in the device or material. It excludes any radiation
(incandescence) caused by pure material temperature.
The basic structure of the LED chip is p–n junction, as illustrated in Fig. 2.1. The
so-called p–n junction is to incorporate p-type (or n-type) impurities into n-type (or
p-type) semiconductor single crystals by appropriate technological methods (such
as alloy, diffusion, growth, ion implantation, etc.). The different regions of the single