LED’s light-emitting mechanism and luminescent materials

LED's light-emitting mechanism and luminescent materials

LED is the abbreviation of Light emitting diode (light emitting diode), essentially a semiconductor diode, including positive and negative electrodes, P-type semiconductor layer, N-type semiconductor layer, light-emitting layer and base layer (as shown in Figure 1), the actual LED needs to consider factors such as light emission and heat dissipation, and its structure is much more complicated. As mentioned earlier, P-type semiconductor is a semiconductor material doped with acceptor impurities, which will generate a certain number of holes; N-type semiconductor is a semiconductor material doped with donor impurities, which generates a certain number of free electrons. Because the light-emitting layer is very thin, the holes and electrons in the P-type semiconductor layer and the N-type semiconductor recombine at the interface, forming a PN junction in the light-emitting layer. The PN junction has a certain potential barrier, which can prevent the electrons and holes from spreading further, so that the entire PN junction is in equilibrium. The working principle of the LED is shown in Figure 2. When a forward bias voltage is applied to the PN junction, the potential barrier of the PN junction will be lowered. Holes will be injected into the N-type material, thereby breaking the original equilibrium state of carriers in the PN junction. These injected electrons and holes meet and recombine at the PN junction. As long as the configuration voltage applied to the PN junction does not change, the carriers will be continuously injected, and the minority carriers on both sides of the PN junction will be continuously injected. In a steady state, the balance of carriers will be re-established on both sides of the PN junction, that is, the recombination rate of carriers will be equal to the injection rate. In the process of carrier recombination, its excess energy is released in the form of light, and of course some recombined energy is released in the form of heat. The recombination of these injected minority carriers does not occur instantaneously, and must meet the conditions of conservation of momentum and energy. Conservation of energy is relatively easy to satisfy. The radiated photon can take away the remaining energy of the hole-electron pair during recombination, but the photon has almost no contribution to the conservation of momentum, so electrons and holes can only be in the same momentum and opposite directions. Compound. Such conditions are not easily met, which will cause a delay in the recombination of electrons and holes. Therefore, the injected minority carriers will have a certain lifetime when radiative recombination occurs, which can be expressed by τr.

LED's light-emitting mechanism and luminescent materials
Figure 1 LED structure diagram
LED's light-emitting mechanism and luminescent materials
Figure 2 Schematic diagram of the working principle of LED

However, the recombination of injected electrons and holes is not completely radiative recombination. Due to the defects of the crystal inside the LED, such as impurities, dislocations and surfaces, the injected carriers will be trapped. Such a process may or may not emit light, and the conservation of kinetic energy and momentum can be achieved through phonon emission. The minority carriers that also undergo non-radiative recombination also have a certain lifetime, because it takes time for the carriers to diffuse to the recombination point.

The essential structure of LED is semiconductor PN junction. When a forward voltage is applied to the PN junction, minority carriers are injected, and the light-emitting recombination of the minority carriers is the working mechanism of the light-emitting diode. Electricity in LED

By injecting carriers into light energy, how to increase the ratio of radiation recombination to generate high-efficiency light radiation has become the main design focus. That is to design various working conditions so that the rate of radiative recombination is much faster than that of non-radiative recombination. The efficiency of the luminescence process is the probability of injected carriers to generate recombination radiation, also known as internal quantum efficiency.

In order to find high-efficiency LED light-emitting materials, it is necessary to understand the energy band structure of semiconductor materials, and the luminous efficiency is related to the characteristics of the transition band gap. In semiconductors, the lower energy band of the semiconductor is usually called the valence band, and the upper energy band of the semiconductor is usually called the conduction band. The valence band structure of semiconductors is relatively simple, generally a parabolic structure along the crystal <0, 0, 0> direction, usually the hole momentum is very small, near the top of the valence band parabola. The conduction band of semiconductors varies with different materials. The conduction band has multiple energy valleys. The valley with the lowest energy is the most important for the transition, and the electrons are usually near the valley of the lowest energy valley. Semiconductor materials can generally be divided into direct band gap semiconductors and indirect band gap semiconductors. The luminous efficiency is related to whether the material is a direct band gap. Direct band gap materials include GaN-InN-AIN, GaAs, InP, InAs and GaAs, etc. The lowest point of the conduction band and the highest point of the valence band of these materials are under the same momentum coordinate, so The radiation recombination of electrons and holes easily meets the conditions of conservation of momentum, and the radiation lifetime of carriers is very short, for example, the radiation lifetime of GaAs is 1-100ns. However, indirect band gap materials, such as germanium and silicon, have a radiation lifetime on the order of seconds, which is much longer than the non-radiative lifetime, and the luminous efficiency is low. Currently, LEDs use direct band gap materials. The width of the semiconductor band gap can determine the threshold voltage of the LED. The larger the band gap width of the semiconductor material, the higher the barrier of the PN junction and the corresponding threshold voltage. For example, the band gap width of GaN is larger than that of AlGaAs, so the operating voltage of GaN is higher.

The luminescent materials of LEDs are mainly compound semiconductors of Group III-V and Group II-VI. According to the different band gaps and lattice constants of these two types of elements, radiation of different wavelength spectra can be realized. The wavelength range of these luminescent materials covers the red light to ultraviolet light region. Although II-VI group materials can also obtain red light and green light, the stability of this group of materials is relatively poor, and it is difficult to prepare luminescent stable compounds. Therefore, most of the currently used luminescent materials are III-V group. The color of LED light is determined by the material of the semiconductor. There are two major types of LEDs currently used in the lighting field. One type is aluminum phosphide, gallium phosphide and indium phosphide alloys (AlGaInP or AlInGaP) that can be made into red. , Orange and yellow LEDs, the other is the alloy of indium nitride and gallium nitride (InGaN) which can be made into green, blue and white LEDs. Figure 3 shows the wavelength range emitted by different luminescent materials. In the design of LEDs, as long as there are subtle changes in the components of the luminescent material, the wavelength and color of the light will have a relatively large change.

LED's light-emitting mechanism and luminescent materials
Figure 3 The wavelength range of the luminescent material of the LED (unit: nm)

In semiconductors, the excess energy of electrons in the recombination process can be released in the form of radiation (emitting photons). This recombination is called radiation recombination, which is the reverse process of light absorption. During the recombination process, the excess energy of the electrons can also be released in other forms without emitting photons. This recombination is called non-radiative recombination. The purpose of the light-emitting diode electrical device is to make the probability of the radiation recombination process much greater than that of the non-radiation recombination process. Understanding the radiation recombination process and non-radiation recombination process in semiconductors is the basis for understanding the working mechanism of optoelectronic devices and device design.