Optical and thermal properties of LED

LED is a photoelectric device made of pn junction using compound materials . It has the electrical characteristics of pn junction devices: IV characteristics, CV characteristics and optical characteristics: spectral response characteristics, luminous intensity directional characteristics, time characteristics and thermal characteristics.

LED Optical Characteristics

There are two series of light-emitting diodes: infrared (non-visible) and visible light. The former can be measured by radiometry, while the latter can be measured by photometry.

Luminous normal light intensity and its angular distribution Iθ

Luminous intensity (normal light intensity) is an important performance that characterizes the luminous intensity of light-emitting devices . A large number of LED applications require cylindrical and spherical packaging. Due to the effect of convex lenses , they all have strong directivity: the light intensity is the largest in the normal direction, and its intersection angle with the horizontal plane is 90°. When the light intensity deviates from the positive normal by different θ angles, it also changes accordingly. The luminous intensity depends on the angular direction with different package shapes. The angular distribution of luminous intensity Iθ describes the light intensity distribution of LED light in all directions in space. It mainly depends on the packaging process (including the bracket, die head, and whether scattering agent is added to the epoxy resin),

the peak wavelength of light emission and its spectral distribution .

The luminous intensity or light power output of LED varies with the wavelength, and a distribution curve is drawn - the spectral distribution curve. Once this curve is determined, the relevant chromaticity parameters of the device, such as the dominant wavelength and purity, will also be determined accordingly. The spectral distribution of LEDs is related to the type, properties and pn junction structure (epitaxial layer thickness, doping impurities) of the compound semiconductor used

for preparation , but has nothing to do with the geometric shape and packaging method of the device. Regardless of the material the LED is made of, there is a point where the relative light intensity is the strongest (the light output is the largest), and a corresponding wavelength is called the peak wavelength, represented by λp. Only monochromatic light has a λp wavelength. Spectral line width At ±△λ on both sides of the peak of the LED spectrum line, there are two points where the light intensity is equal to half of the peak value (maximum light intensity). These two points correspond to λp-△λ and λp+△λ respectively. The width between them is called the spectral line width, also known as half- power width or half-height width. The half-height width reflects the width of the spectrum line, that is, the parameter of the monochromaticity of the LED. The LED half-width is less than 40 nm. Dominant wavelength Some LEDs emit not only a single color, that is, not only one peak wavelength; there are even multiple peaks, which are not monochromatic light. The dominant wavelength is introduced to describe the chromaticity characteristics of LEDs. The dominant wavelength is the wavelength of the main monochromatic light emitted by the LED that can be observed by the human eye. The better the monochromaticity, the more λp is the dominant wavelength. For example, GaP materials can emit multiple peak wavelengths, but there is only one dominant wavelength. As the LED works for a long time and the junction temperature rises, the dominant wavelength tends to be long-wave. Luminous flux Luminous flux F is the radiation energy that characterizes the total light output of the LED , which indicates the performance of the device. F is the sum of the energy emitted by the LED in all directions, which is directly related to the working current. As the current increases, the luminous flux of the LED increases. The unit of luminous flux of visible light LEDs is lumen (lm). The power radiated outward by the LED - the luminous flux is related to the chip material, the packaging process level and the size of the external constant current source. At present, the maximum luminous flux of a monochromatic LED is about 1 lm, and the F of a white light LED is ≈ 1.5~1.8 lm (small chip). For a white light LED made of a 1mm×1mm power-level chip, its F=18 lm. Luminous efficiency and visual sensitivity ① LED efficiency includes internal efficiency ( the efficiency of converting electrical energy into light energy near the pn junction) and external efficiency (the efficiency of radiating to the outside). The former is only used to analyze and evaluate the quality of the chip. The most important characteristic of LED optoelectronics is the ratio of radiated light energy (luminous amount) to input electrical energy, that is, luminous efficiency. ② Visual sensitivity is the use of some parameters in lighting and photometry. Human visual sensitivity has a maximum value of 680 lm/w at λ = 555nm. If visual sensitivity is recorded as Kλ, the relationship between luminous energy P and visible light flux F is P=∫Pλdλ; F=∫KλPλdλ ③ Luminous efficiency - quantum efficiency η= number of emitted photons /number of pn junction carriers=（e/hcI）∫λPλdλ If the input energy is W=UI, then the luminous energy efficiency ηP=P/W If the photon energy hc=ev, then η≈ηP, then the total light flux F=（F/P）P=KηPW Where K= F/P ④ Lumen efficiency: LED luminous flux F/external power consumption W=KηP It is used to evaluate the characteristics of externally packaged LEDs. The high lumen efficiency of LEDs means that the energy of radiated visible light is larger under the same external current, so it is also called visible light luminous efficiency. High-quality LEDs require large outward radiated light energy and as much light as possible, that is, high external efficiency. In fact, the LED's outward emission is only a part of the internal emission, and the total luminous efficiency should be η=ηiηcηe, where ηi is the minority carrier injection efficiency in the p and n junction regions, ηc is the minority carrier and majority carrier recombination efficiency in the barrier region, and ηe is the external light extraction efficiency. Since the refractive index of LED materials is very high, ηi≈3.6. When the chip emits light at the interface between the crystal material and the air (without epoxy packaging), if it is vertically incident and reflected by the air, the reflectivity is (n1-1)2/(n1+1)2=0.32, and the reflected light accounts for 32%. In view of the fact that the crystal itself absorbs a considerable part of the light, the external light extraction efficiency is greatly reduced. In order to further improve the external light extraction efficiency ηe, the following measures can be taken: ① Cover the chip surface with a transparent material with a higher refractive index (epoxy resin n=1.55 is not ideal); ② Process the chip crystal surface into a hemispherical shape; ③ Use a compound semiconductor with a large Eg as a substrate to reduce the light absorption in the crystal. Someone once used low-melting-point glass [composition As-S(Se)-Br(I)] with n=2.4~2.6 and high thermoplasticity as a cap, which can increase the efficiency of infrared GaAs, GaAsP, and GaAlAs LEDs by 4~6 times. Luminescence Brightness Brightness is another important parameter of LED luminescence performance, which has strong directionality. The brightness in the direction of the positive normal line BO=IO/A, which specifies the brightness of the light source surface in a certain direction, which is equal to the luminous flux radiated by the unit projection area on the surface of the light source within the unit solid angle, and the unit is cd/m2 or Nit.






































If the surface of the light source is an ideal diffuse reflector, the brightness BO is a constant regardless of direction. The surface brightness of a clear blue sky and a fluorescent lamp is about 7000Nit (nits), and the surface brightness of the sun as seen from the ground is about 14×108Nit.
The brightness of an LED is related to the external current density. For a general LED, the BO will increase approximately as the JO (current density) increases. In addition, the brightness is also related to the ambient temperature. As the ambient temperature increases, ηc (compound efficiency) decreases, and BO decreases. When the ambient temperature remains unchanged, the current increase is sufficient to cause the pn junction temperature to rise. After the temperature rises, the brightness is saturated.

Lifetime

The brightness of LED light will decay with long-term operation. The degree of device aging is related to the size of the external constant current source, which can be described as Bt=BO et/τ, where Bt is the brightness after t time and BO is the initial brightness.
The time t that it takes for the brightness to drop to Bt=1/2BO is usually called the life of the diode. It takes a long time to measure t, and the life is usually calculated. Measurement Method: Pass a constant current source through the LED, and after ignition for 103 ~104 hours, measure BO, Bt=1000~10000, substitute Bt=BO et/τ to find τ; then substitute Bt=1/2BO to find the life t.
For a long time, it has been believed that the life of LED is 106 hours, which refers to a single LED under IF=20mA. With the development and application of power LEDs, foreign scholars believe that the percentage value of LED light attenuation is used as the basis for life. For example, if the light attenuation of LED is 35% of the original, the life is >6000h.

Thermal properties

The optical parameters of LEDs are closely related to the junction temperature of the pn junction. Generally, when the LED is operated at a low current IF < 10 mA, or when it is continuously lit at 10-20 mA for a long time, the temperature rise is not obvious. If the ambient temperature is high, the main wavelength or λp of the LED will drift to a longer wavelength, and BO will also decrease. In particular, the temperature rise of dot matrix and large display screens will affect the stability of LEDs, and a scattering ventilation device should be specially designed.

The relationship between the main wavelength of LED and temperature can be expressed as λp(T′)=λ0(T0)+△Tg×0.1nm/℃.
It can be seen from the formula that when the junction temperature rises by 10℃, the wavelength drifts 1nm to the long wave, and the uniformity and consistency of the light emission deteriorate. This is for the design of lighting lamps and lanterns that require miniaturization and dense arrangement to increase the light intensity and brightness per unit area. In particular, attention should be paid to the use of lamp housings with good heat dissipation or special general equipment to ensure the long-term operation of LEDs.