LEDs are optoelectronic devices that make pn junctions from compound materials. It has the electrical properties of a pn junction device: IV, CV, and optical: spectral response, luminescence, directional, time, and thermal properties.
LED electrical characteristics
A. IV Characterization The main parameters of the LED chip pn junction preparation performance. The IV characteristics of LEDs have non-linear, rectifying properties: unidirectional conductivity, that is, the application of a positive bias exhibits low contact resistance, and vice versa.
(1) Positive dead zone: (Fig. oa or oa' segment) Point a is the turn-on voltage for V0. When V < Va, the applied electric field overcomes a lot of carrier field diffusion due to carrier diffusion. Very large; the turn-on voltage is different for different LEDs, GaAs is 1V, red GaAsP is 1.2V, GaP is 1.8V, and GaN is 2.5V.
(2) Forward working area: current IF is exponential with applied voltage
IF = IS (e qVF/KT –1) ------------------------- IS is the reverse saturation current.
When V>0, the forward working area IF of V>VF rises with the VF index IF = IS e qVF/KT
(3) Reverse dead zone: pn junction plus reverse bias when V<0
When V = - VR, when the reverse leakage current IR (V = -5V), GaP is 0V and GaN is 10uA.
(4) Reverse breakdown region V<- VR, VR is called reverse breakdown voltage; VR voltage corresponds to IR is reverse leakage current. When the reverse bias voltage is increased such that V < - VR, a sudden increase in IR occurs and a breakdown occurs. The reverse breakdown voltage VR of various LEDs is also different due to the variety of compound materials used.
B. CV characteristics
The LED chip has 9×9mil (250×250um), 10×10mil, 11×11mil (280×280um), 12×12mil (300×300um), so the pn junction area is different, which makes the junction capacitance (zero offset). Pressure) C≈n+pf or so.
The CV characteristics are quadratic (see Figure 2). The 1 MHz electromagnetic signal was measured with a CV characteristic tester.
C. Maximum allowable power consumption PF m
When the current flowing through the LED is IF and the tube voltage drop is UF, the power consumption is P=UF×IF.
When the LED is working, the applied bias voltage and bias current must cause the carrier to recombine and emit a part of the heat, which causes the junction temperature to rise. If the junction temperature is Tj and the external ambient temperature is Ta, then when Tj>Ta, the internal heat is transferred to the outside through the stem, and the heat dissipation (power) can be expressed as P = KT(Tj – Ta).
C. Response Time Response Time is a representation of how quickly a particular display tracks changes in external information. There are several display LCDs (liquid crystal displays) of about 10-3~10-5S, and CRT, PDP, and LEDs all reach 10-6~10-7S (us level).
LED optical properties
The light-emitting diode has two series of infrared (non-visible) and visible light. The former can use radiance, and the latter can measure its optical characteristics by photometry.
A. Luminescence normal light intensity and its angular distribution Iθ
Luminous intensity (normal light intensity) is an important property for characterizing the luminescence of a light-emitting device. A large number of LED applications require cylindrical and spherical encapsulation. Due to the action of the convex lens, they have strong directivity: the light intensity in the normal direction is the largest, and the angle of intersection with the horizontal plane is 90°. When deviating from the normal θ angle, the light intensity also changes. The intensity of the illumination varies with the package shape and the intensity depends on the angular direction. The angular distribution Iθ of the luminous intensity is a measure of the intensity distribution of the LED illumination in all directions of the space. It depends mainly on the packaging process (including the addition of scattering agents in the stent, the pellet head, and the epoxy resin).
B. Luminescence peak wavelength and its spectral distribution
The LED luminous intensity or optical power output varies with wavelength, and is plotted as a distribution curve—the spectral distribution curve. When this curve is determined, the relevant colorimetric parameters of the device, such as the dominant wavelength and purity, are also determined.
The spectral distribution of the LED is related to the type and nature of the compound semiconductor used in the preparation, and the pn junction structure (epitaxial layer thickness, doping impurities), etc., regardless of the geometry and packaging of the device.
C. LED spectral distribution curve
1 blue InGaN/GaN 2 green light GaP: N 3 red light GaP: Zn-O
4 Infrared GaAs 5 Si photosensitive photocell 6 Standard tungsten filament lamp 1 is a blue InGaN/GaN light-emitting diode with a peak of λp = 460-465 nm;
2 is a green GaP: N LED with a spectral peak λp = 550 nm;
3 is a red GaP: Zn-O LED, the emission spectrum peak λp = 680 ~ 700nm;
4 is the infrared LED using GaAs material, the emission spectrum peak λp = 910nm;
5 is a Si photodiode, usually used for photoelectric reception.
It can be seen from the figure that the LED made of any material has the strongest relative light intensity (the largest light output), and has a wavelength corresponding to it. This wavelength is called the peak wavelength and is represented by λp. Only monochromatic light has a λp wavelength.
Line width: At the ±Δλ on both sides of the peak of the LED line, there are two points whose light intensity is equal to half of the peak (maximum light intensity). These two points correspond to λp-△λ, and the width between λp+△λ is called Line width, also known as half power width or half height width.
The half-height width reflects the narrow line width, that is, the parameter of the monochromaticity of the LED, and the half width of the LED is less than 40 nm.
Main wavelength: Some LEDs emit light not only in a single color, that is, not only have a peak wavelength; there are even multiple peaks, not monochromatic light. The dominant wavelength is introduced for this purpose by describing the LED chromaticity characteristics. The dominant wavelength is what the human eye can observe, and the wavelength of the main monochromatic light emitted by the LED. The better the monochromaticity, the λp is also the dominant wavelength.
For example, GaP material can emit multiple peak wavelengths, and there is only one dominant wavelength. It will work with the LED for a long time, the junction temperature will rise and the dominant wavelength will be biased toward long wavelength.
D. Luminous flux Luminous flux F is the radiant energy that characterizes the total light output of the LED, which indicates the performance of the device. F is the sum of the energy of the LEDs emitting light in all directions, which is directly related to the operating current. As the current increases, the LED luminous flux increases. The luminous flux unit of visible light LED is lumens (lm).
The power radiated by the LED - the luminous flux is related to the chip material, the packaging process level and the size of the applied constant current source. At present, the luminous flux of a single-color LED is about 1 lm, and that of a white LED is 1.5 to 1.8 lm (small chip). For a power chip of 1 mm × 1 mm, a white LED is made, and its F=18 lm.
E. Luminous Efficiency and Visual Sensitivity 1 LED efficiency has internal efficiency (efficiency of converting electrical energy into light energy near the pn junction) and external efficiency (efficiency radiated to the outside). The former is only used to analyze and evaluate the characteristics of the chip.
The most important characteristic of LED optoelectronics is the ratio of the emitted light energy (amount of luminescence) to the input electrical energy, ie the luminous efficiency.
2 Visual acuity is the use of some parameters in illumination and photometry. The human visual sensitivity has a maximum value of 680 lm/w at λ = 555 nm. If the visual sensitivity is recorded as Kλ, the relationship between the luminous energy P and the 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) ∫ λP λdλ
If the input energy is W=UI, the luminous energy efficiency is 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 to evaluate the LED characteristics of the outer package. The high lumen efficiency of the LED means that the energy of radiating visible light is larger under the same applied current, so it is also called visible light luminous efficiency. High-quality LEDs require large amounts of light to be radiated outwards, and as much as possible, the external efficiency is high. In fact, the outward luminescence of the LED is only a part of the internal luminescence, and the total luminous efficiency should be η=ηiηcηe, where ηi is the p and n junctions with less sub-injection efficiency, and ηc is the sub- and multi-sub-composite efficiency in the barrier region. , ηe is the efficiency of external light extraction (light extraction efficiency).
Since the refractive index of the LED material is very high ηi ≈ 3.6. When the chip emits light in the crystal material and the air interface (no epoxy package), if it is incident perpendicularly, it is reflected by the air, and the reflectivity is (n1-1)2/(n1+1)2=0.32, which reflects 32%. In view of the considerable absorption of light by the crystal itself, 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 covering the surface of the chip with a transparent material having a relatively high refractive index (epoxy resin n=1.55 is not ideal); 2 processing the surface of the chip crystal into a hemispherical shape;
3 A compound semiconductor having a large Eg is used as a substrate to reduce light absorption in the crystal. Some people used n=2.4~2.6 low melting glass [component As-S(Se)-Br(I)] and made a large thermoplastic cap, which can improve the LED efficiency of infrared GaAs, GaAsP and GaAlAs by 4-6 times. .
F. Luminance Brightness Luminance is another important parameter of LED luminescence performance and has strong directionality. The brightness of the positive normal direction is BO=IO/A, which specifies that the surface brightness of the illuminant in a certain direction is equal to the luminous flux radiated by the unit projection area on the surface of the illuminator in unit solid angle, and the unit is cd/m2 or Nit.
If the surface of the light source is an ideal diffuse surface, the brightness BO is constant regardless of the direction. The surface brightness of the clear blue sky and fluorescent lights is about 7000 Nit, and the brightness of the sun surface is about 14×108 Nit from the ground.
The brightness of the LED is related to the applied current density. The general LED, JO (current density) increase BO also increases approximately. In addition, the brightness is also related to the ambient temperature, the ambient temperature rises, ηc (composite efficiency) decreases, and BO decreases. When the ambient temperature is constant, the current increases enough to cause the pn junction temperature to rise. After the temperature rise, the brightness is saturated.
G. Life aging: The brightness of the LED illuminates with light intensity or brightness decay with long hours of operation. The degree of aging of the device is related to the size of the applied constant current source, which can be described as Bt=BO et/Ï„, Bt is the brightness after t time, and BO is the initial brightness.
The time t that is typically experienced when the brightness is reduced to Bt = 1/2 BO is called the lifetime of the diode. It takes a long time to measure t, and the life is usually calculated by calculation. Measurement method: the LED is connected to a constant current source. After igniting for 103 to 104 hours, BO, Bt=1000~10000 is measured successively, and Ï„ is obtained by substituting Bt=BO et/Ï„; then Bt=1/2BO is substituted. The life t can be found.
LED lifetime has long been considered to be 106 hours, which means that a single LED is at IF = 20 mA. With the development and application of power LEDs, foreign scholars believe that the percentage of light attenuation of LEDs is the basis of life. For example, the light attenuation of the LED is 35% and the lifetime is >6000h.
Thermal characteristics
The optical parameters of the LED have a great relationship with the pn junction temperature. Generally work in small current IF <10mA, or 10~20 mA continuous lighting for a long time LED temperature rise is not obvious. If the ambient temperature is high, the main wavelength or λp of the LED will drift to long wavelengths, and BO will also drop. Especially the temperature rise of the dot matrix and large display screen should be specially designed for the sag and stability of the LED. Device.
The dominant wavelength of the LED can be expressed as λp(T')=λ0(T0)+ΔTg×0.1nm/°C with temperature.
It can be seen from the formula that each time the junction temperature is increased by 10 ° C, the wavelength shifts to 1 nm for a long wave, and the uniformity and uniformity of light emission are deteriorated. This is a design that requires miniaturization and dense arrangement as a light source for illumination to increase the light intensity and brightness per unit area. In particular, attention should be paid to the use of a heat-dissipating lamp housing or special-purpose equipment to ensure long-term operation of the LED.
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