How to describe the basic characteristics of LEDs?
As an electroluminescent P-N junction device, the characteristics of LED can be described by the electrical parameters of the P-N junction and the optical parameters as a light-emitting device.
The volt-ampere characteristic is an important parameter to describe a P-N junction device. It is an important indicator of the P-N junction performance and the pros and cons of the P-N junction fabrication process. The so-called volt-ampere characteristic is the characteristic that the current flowing through the P-N junction changes with the voltage, which can be very vividly displayed on the oscilloscope. A complete volt-ampere curve includes forward and reverse characteristics. Typically, the reverse characteristic curve changes steeply, and when the voltage exceeds a certain threshold, the current will increase exponentially. Generally, three parameters of reverse breakdown voltage, reverse current and forward voltage can be used to describe the volt-ampere characteristic curve.
The forward voltage VF refers to the voltage drop across the device under the rated forward current, which is not only related to the forbidden band width of the material, but also identifies the bulk resistance and ohmic contact resistance of the P-N junction. The size of VF reflects the advantages and disadvantages of electrode fabrication to a certain extent. Relative to a forward current of 20 mA, the VF value of red and yellow LEDs is about 2 volts, while the VF value of GaN-based green LED devices is usually greater than 3 volts. The reverse leakage current IR refers to the reverse current value flowing through the device under a given reverse voltage, and the value of this value is very sensitive to the quality of the device. Usually at a reverse voltage of 5 volts, the reverse leakage current should not be greater than 10 microamps, and an excessively large IR indicates poor junction characteristics. The reverse breakdown voltage means that when the reverse voltage is greater than a certain value, the reverse leakage current will increase sharply, reflecting the characteristics of the reverse withstand voltage of the device. For a specific device, the standard of leakage current is different. In a more stringent case, the reverse leakage current is required to be no more than 10 microamps under the specified voltage.
In addition to electrical characteristics, a series of optical parameters are also required to describe the performance of LED devices, among which the more important parameters are the peak wavelength and light intensity of the device. Visible light belongs to the category of electromagnetic waves, and wavelengths can usually be used to express what the human eye can perceive. The radiant energy of visible light, generally the wavelength range of visible light is between 380nm and 760nm. The longer the wavelength, the lower the corresponding photon energy, and the redder the color of the light. When the wavelength of the photon becomes shorter, the light will gradually change from Red turns yellow, then green turns blue, until it turns purple. For an LED device, the light emitted by it will expand at the peak λP, and its wavelength half-width is usually 10-30nm. The smaller the half-width, the purer the material of the LED device, the more uniform the performance, and the more complete the crystal. Sex is also better. Light intensity is another important parameter to measure the quality of LED performance, usually represented by the letter Iv. The definition of light intensity is that the light in a given direction emits 1 lumen of light in a unit solid angle as 1 candela, and its unit is expressed in candela (cd). Its relationship can be represented by formula (6-1):
Iv=dφ/dΩ (6-1)
In the formula, the unit of φ is lumens, the unit of Iv is cd, and dΩ is the unit solid angle, and the unit is degree. The normal light intensity of an ultra-bright LED chip is generally between 30-120mcd. After being packaged into a device, its normal light intensity is usually greater than 1cd.
Luminous flux is a more objective parameter for judging the luminous efficiency of LEDs. It represents the size of the light energy emitted by the electroluminescent body per unit time, and the unit is lumens (lm). Generally, the luminous efficacy of incandescent lamps and fluorescent lamps are 15lm/w and 60lm/w respectively. The greater the power of the bulb, the greater the luminous flux. For a high-performance LED device, the luminous efficiency is 20lm/w, and the laboratory level also reaches 100lm/w. In order to make LED devices used for lighting faster, the luminous efficiency of LED devices must be further improved. It is estimated that after 10 years, the luminous efficiency of LEDs can reach 200lm/w. At that time, mankind will usher in a new era in which solid-state light sources completely replace traditional light sources.
