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The main parameters and characteristics of LED

  日期:2011-11-18  浏览次数:84

  LED is the use of compound materials pn junction photovoltaic devices. Its junction with the pn junction electrical properties of devices: IV characteristics, CV characteristics and optical properties: spectral response, light-emitting properties of light intensity point, the time characteristics and thermal characteristics.
      1, LED electrical characteristics of 1.1 IV characterization of pn junction LED chip performance of the main parameters of preparation. Nonlinear IV characteristics of LED, rectifier properties: one-way conductivity, plus a positive bias that the performance of low contact resistance, and vice versa for the high contact resistance. .
     (1) positive dead: (Figure oa or oa 'section) a point for the turn-on voltage for V0, when V <Va, the applied electric field is still a lot to overcome because of carrier diffusion barrier formed by the electric field, then R is large; its turn-on voltage for different values ​​of the different LED, GaAs is 1V, red GaAsP to 1.2V, GaP is 1.8V, GaN is 2.5V. .
     (2) forward the work area: current IF exponential relation between the applied voltage.
IF = IS (e qVF / KT -1) ------------------------- IS is the reverse saturation current V> 0 时, V> VF IF the forward work area index increased with VF IF = IS e qVF / KT

     (3) Reverse dead: V <0 时 pn junction plus anti-bias V = - VR, the reverse leakage current IR (V =-5V) when, GaP is 0V, GaN is 10uA.
     (4) the reverse breakdown region V <- VR, VR called the reverse breakdown voltage; voltage corresponding to the IR for the reverse leakage current. When the reverse bias has been increased so that V <- VR, then a sudden increase in IR and breakdown phenomenon. As used in different types of compound materials, various LED's reverse breakdown voltage VR is also different.
     1.2 CV characteristics in view of the LED chips are 9 × 9mil (250 × 250um), 10 × 10mil, 11 × 11mil (280 × 280um), 12 × 12mil (300 × 300um), so the pn junction of various sizes, to end capacitor (zero bias) C ≈ n + pf or so. C-V characteristics showed a quadratic function (Figure 2). 1MHZ AC signal from the C-V characteristics with the tester measured.
     1.3 The maximum allowable power dissipation PF m when the current flowing through the LED IF, the voltage drop across the power consumption for the UF P = UF × IF. LED work, plus a bias, bias must be issued to promote carrier recombination, light, and partly into heat, the junction temperature rise. If the junction temperature Tj, the external ambient temperature Ta, then when Tj> Ta, the internal heat with the tube socket outward heat transfer, heat dissipation (power), can be expressed as P = KT (Tj - Ta).
     1.4 Response time characterization of a monitor to track changes in the speed of external information. Several existing display LCD (liquid crystal display) of about 10-3 ~ 10-5S, CRT, PDP, LED have reached 10-6 ~ 10-7S (us level).
     ① Response time from the use of point of view, is the LED light up and off the delay time, that figure tr, tf. Figure t0 value is very small, can be ignored.
     ② response time depends on carrier lifetime, the junction capacitance of the device and circuit impedance.
     LED's light up time - the rise time tr is the power to reach the normal brightness of 10%, has been to reach the normal brightness of 90% of its time.
     LED off time - fall time tf is reduced to the normal light 10% of its original time.
     Different materials of the LED response time was different; such as GaAs, GaAsP, GaAlAs its response time <10-9S, GaP is 10-7 S. So they can be used in 10 ~ 100MHZ frequency system.
     2 LED optical properties of infrared light-emitting diode (not visible) and two series of visible light, the former available radiometric, which can be used photometry to measure its optical properties.
     2.1 luminescence to light intensity and angular distribution Iθ
     Luminous intensity (normal light intensity) is characterized by an important strength of light-emitting device performance. LED large-scale application requirements are cylindrical, spherical package, due to the role of convex lens, the ancient capital has a strong directional: the maximum intensity in the normal directional light, with the horizontal angle of 90 °. When a deviation from a different angle θ to the Dhamma, the light intensity also changes. Light intensity with different packaging shapes and intensity dependent angular direction.
     Angular distribution of luminous intensity LED light Iθ is described in all directions in space intensity distribution of glazing. It mainly depends on the package of technology (including bracket, die grain head scattering agent added to epoxy resin or not)
     ⑴ order to obtain a high point of the angular distribution:
     ① LED die location farther away from the die head capsules;
     ② the use of cone-shaped (bullet) of mold particles head;
     ③ Do not add epoxy encapsulated scattering agent.
     Such measures will enable LED 2θ1 / 2 = 6 ° or so, greatly improving the directivity.
     ⑵ the current package of several commonly used scattering angle (2θ1 / 2 angle) circular LED: 5 °, 10 °, 30 °, 45 °
2.2 light-emitting peak wavelength and spectral distribution
     ⑴ LED luminous intensity, or optical power output changes with different wavelengths, coming up with a distribution curve - spectral distribution curve. When this curve is established, the main components of the wavelength, colorimetric purity, and other related parameters has also come to be.
     LED spectral distribution and preparation of compound semiconductors used in the type, nature and the pn junction structure (epitaxial layer thickness, doping impurities), etc., but not with the device geometry, the package has nothing to do.
     Here are several drawn from the different compounds obtained by doping the semiconductor and LED spectral response curve. Among
LED spectral distribution curve
     A blue InGaN / GaN 2 green GaP: N 3 red GaP: Zn-O 4 infrared GaAs 5 Si 6 standard incandescent light sensitive photocell
     ① is a blue InGaN / GaN light-emitting diodes, peak is λp = 460 ~ 465nm;
     ② green GaP: N of the LED, peak is λp = 550nm;
     ③ red GaP: Zn-O of the LED, peak is λp = 680 ~ 700nm;
     ④ is the use of GaAs infrared LED materials, peak is λp = 910nm;
     ⑤ Si photodiode is usually used for the optical receiver.
     Thus, no matter what the material of the LED, there is a relative light intensity at the highest (maximum light output), there should be a corresponding wavelength, the wavelength is called the peak wavelength, with λp said. Have only monochromatic light wavelength λp.
     ⑵ line width: both sides of the LED spectrum peak at ± △ λ, there are two light intensity equal to the peak (maximum intensity) half-way point, the two points corresponding to λp-△ λ, λp + △ λ the width between called the line width, also known as half-power width or half-height width. Width half-height width of the reflection spectrum, ie the parameters of monochromatic LED, LED half-width of less than 40 nm.
     ⑶ main wavelength: there is not just a single color LED light, that not only has a peak wavelength; even more than one peak, is not monochromatic. This introduction describes the main characteristics of LED color wavelength. Dominant wavelength is the human eye can observe, mainly monochromatic light by the wavelength of the LED. Color the better, then that is the main wavelength λp. Materials such as GaP may issue more than one peak wavelength, dominant wavelength and the only one, it will work long term with the LED junction temperature rise and long-wave dominant wavelength bias.
2.3 Flux
     F is characterized by the total flux LED light output of the radiation energy, it marks the device performance advantages and disadvantages. F is the LED light in all directions and the energy of it directly related to current work. As the current increases, LED luminous flux increases. Visible LED's luminous flux in lumens (lm).
      Radiated power LED - luminous flux and chip materials, packaging technology level and the size of the external current source. The most current color LED luminous flux of about 1 lm, white LED's F ≈ 1.5 ~ 1.8 lm (small chip), for 1mm × 1mm made of the power-on-chip white LED, F = 18 lm.
2.4 light-emitting efficiency and visual sensitivity
      ① LED efficiency of internal efficiency (pn junction near the electrical energy into light energy efficiency) and external efficiency (the efficiency of radiation to the outside). The former is used to analyze and evaluate the merits of the characteristics of the chip. The most important feature LED optical radiation of light energy is used (light volume) and the ratio of input energy, light-emitting efficiency.
      ② the use of lighting and visual sensitivity in some of the photometric parameters. Human visual sensitivity in the λ = 555nm at a maximum 680 lm / w. If the visual acuity recorded as Kλ, the visible light energy flux F P and the relationship between the P = ∫ Pλdλ; F = ∫ KλPλdλ
      ③ luminous efficiency - quantum efficiency η = number of photons emitted / pn junction carrier number = (e / hcI) ∫ λPλdλ If the input energy W = UI, then the luminous energy efficiency ηP = P / W if the photon energy hc = ev, then η ≈ ηP, the total luminous flux F = (F / P) P = KηPW where K = F / P
      ④ lumen efficiency: LED luminous flux F / plus the power consumption W = KηP
      It is to evaluate the characteristics of a foreign package LED, LED lumen efficiency refers to the same applied current, the larger the energy of visible light radiation, it is also known as visible light-emitting efficiency.
      Here are some common LED luminous efficiency (luminous efficiency of visible light):
      LED light color λp (nm) material visible light luminous efficiency (lm / w) external quantum efficiency
Highest average
      Red 700660650 GaP: Zn-OGaAlAsGaAsP 120.50.5
      1 - 30.30.2
      Yellow 590 GaP: N-N 0.45 0.1
      Green 555 GaP: N 4.2 0.7 0.015 ~ 0.15
      Blu-ray 465 GaN 10
Small white band GaN + YAG chip 1.6, 18 large chips
       Quality of the LED light requires large amount of external radiation, the outgoing light as much as possible, namely, the external efficiency is higher. In fact, LED interior light out light is only part of the total luminous efficiency should be η = ηiηcηe, where ηi to as p, n junction minority carrier injection efficiency, ηc barrier zone in the minority carrier recombination efficiency and multi- , ηe out external light (light extraction efficiency) efficiency.
       As a high refractive index LED ηi ≈ 3.6. When the chip emits light in the crystal material and the air interface (without epoxy encapsulation) if normal incidence, is reflective air, reflection rate (n1-1) 2 / (n1 +1) 2 = 0.32, reflecting 32% , in view of the crystal itself, a considerable portion of the absorption of light, so greatly reducing the external out external light effects to further improve the optical efficiency ηe may take the following measures: ① transparent material with high refractive index (n = 1.55 is not ideal epoxy resin ) covering the chip surface; ② processed into the chip crystal surface hemispherical; ③ compound semiconductor with Eg for large substrate to reduce the absorption of light within the crystal. Some people have used n = 2.4 ~ 2.6 of the low melting point glass [component As-S (Se)-Br (I)] and for sealing thermoplastic large cap, can infrared GaAs, GaAsP, GaAlAs LED efficiency of 4 to 6 times .
2.5 brightness
      Brightness of the LED light is another important performance parameters, with a strong direction. The Dhamma line direction of the brightness BO = IO / A, specify a direction of light-emitting surface brightness equal to projected surface area of ​​the surface units in the unit solid angle of the radiation flux, in units of cd/m2 or Nit.
If the light source is an ideal diffuse reflector surface brightness BO has nothing to do with the direction of the constant. Fluorescent clear blue sky and the surface brightness of about 7000Nit (nits) from the ground to see the sun's surface brightness is about 14 × 108Nit.
       LED brightness and applied current density, the average LED, JO (current density) increased BO approximation is also increased. In addition, the brightness with ambient temperature, ambient temperature, ηc (composite efficiency) decreased, BO reduced. When the ambient temperature constant, current increases enough to cause the pn junction junction temperature rise, temperature rise, the intensity was saturated.
       2.6 life of aging: LED brightness with long working hours appear light intensity or brightness attenuation. Aging device and the size of the external current source can be described as Bt = BO et / τ, Bt t time for the brightness, BO initial brightness. Usually the brightness down to Bt = 1/2BO experienced by time t is called the life of the diode. Determination of t take a long time, usually obtained projected life. Measuring method: to a certain constant current through the LED source, 103 to 104 hours after the fire, has measured BO, Bt = 1000 ~ 10000, into Bt = BO et / τ obtained τ; then Bt = 1/2BO into, Life can be obtained t. For a long time that the LED lifetime total of 106 hours, which is a single LED at IF = 20mA under. With the power LED development and application of foreign scholars think that the percentage of the LED light attenuation values ​​as a basis for life. If the LED light attenuation of the original 35%, life expectancy> 6000h. 3 thermal characteristics of the optical parameters of LED junction temperature with the pn junction has a great relationship. Generally work in small current IF <10mA, or 10 ~ 20 mA for a long time continuously lit LED temperature is not obvious. If the ambient temperature is high, LED's dominant wavelength or long wavelength λp will drift to, BO will decline, especially dot-matrix, a large display of temperature on the LED reliability, stability of the ventilation devices should be designed to scatter . LED main wavelength with temperature can be expressed as λp (T ') = λ0 (T0) + △ Tg × 0.1nm / ℃ from the equation, whenever the junction temperature rise 10 ℃, the wavelength shift to longer wavelengths 1nm, and the light uniformity, consistency variation. This lighting of the lamp as light source requires small, densely arranged in order to improve the light intensity per unit area, the light intensity of the design with particular attention to good lighting enclosure with heat or special general equipment, ensure the LED long-term work.

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