Analysis of LED optical and thermal characteristics

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 to measure its optical properties.

Normal light intensity and its angular distribution Iθ

Luminous intensity (normal light intensity) is an important performance that characterizes the luminous intensity of a light-emitting device . A large number of LED applications require cylindrical or 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 angle with the horizontal plane is 90°. When the light intensity deviates from the positive normal by different angles θ, it also changes. 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)

Luminescence peak wavelength and its spectral distribution

The luminous intensity or light power of LED output varies with wavelength, and is plotted as a distribution curve - the spectral distribution curve. Once this curve is determined, the relevant colorimetric parameters of the device, such as the main wavelength and purity, are also determined accordingly. The spectral distribution of LED is related to the type and properties of the compound semiconductor

used in the preparation, as well as the pn junction structure (epitaxial layer thickness, doping impurities), but has nothing to do with the device's geometric shape and packaging method.

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 there is a corresponding wavelength, which is called the peak wavelength and is represented by λp. Only monochromatic light has a λp wavelength.

Line width

At ±△λ on both sides of the peak of the LED spectrum line, there are two points where the light intensity is 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 spectrum 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 LED monochromaticity. 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; they even have multiple peaks, and are not monochromatic. For this reason, 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 λp is the dominant wavelength. For example, GaP materials can emit multiple peak wavelengths, but 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 radiant energy that characterizes the total light output of an LED . It indicates the performance of the device. F is the sum of the energy emitted by the LED in all directions, and it is directly related to the operating 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 LEDs - luminous flux - is related to the chip material, packaging process level and the size of the external constant current source. Currently, the maximum luminous flux of a single-color 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 the radiated light energy (luminous energy) to the input electrical energy, that is, the luminous efficiency.

2 Visual sensitivity is a parameter used 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 luminous 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. 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 light energy radiated outwards and as much light as possible, that is, high external efficiency. In fact, LEDs emitting light outwards is only a part of the internal light, 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 output (light extraction efficiency) efficiency.

Since the refractive index of LED materials is very high, ηi≈3.6. When the light emitted by the chip is incident vertically at the interface between the crystal material and the air (without epoxy packaging), it is reflected by the air, and the reflectivity is (n1-1)2/(n1+1)2=0.32, and the reflected light accounts for 32%. Since the crystal itself absorbs a considerable part of the light, the external light output 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 high refractive index (epoxy resin n=1.55 is not ideal);

2. Process the chip crystal surface into a hemispherical shape;

3 Use compound semiconductors with large Eg as substrates to reduce light absorption in the crystal. Some people have used low-melting-point glass [composition As-S(Se)-Br(I)] with n=2.4~2.6 and high thermoplasticity as caps to increase the efficiency of infrared GaAs, GaAsP, and GaAlAs LEDs by 4~6 times.

Luminescence Brightness

Brightness is another important parameter of LED luminous performance, which has strong directionality. The brightness in the normal direction is BO=IO/A, which specifies that the brightness of the light source surface in a certain direction is equal to the luminous flux radiated by the unit projection area on the light source surface within the unit solid angle, and the unit is cd/m2 or Nit.

If the light source surface 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 LEDs is related to the applied current density. For general LEDs, the increase in JO (current density) will increase the BO. In addition, the brightness is also related to the ambient temperature. As the ambient temperature rises, ηc (compound efficiency) decreases, and BO decreases. When the ambient temperature remains unchanged, the increase in current is sufficient to cause the pn junction temperature to rise. After the temperature rises, the brightness is saturated.

life

The brightness of LED will decrease with long-term operation. The aging degree of the device 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: A constant current source is passed through the LED. After 103~104 hours of ignition, BO is measured successively, Bt=1000~10000, and τ is obtained by substituting Bt=BO et/τ; then Bt=1/2BO is substituted to obtain 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 light attenuation percentage of LEDs can be used as the basis for life. For example, if the light attenuation of LEDs is 35% of the original, the life is >6000 h.

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