Electronic LED display is the use of compound materials made of the pn junction photovoltaic devices. It has pn junction end of the electrical characteristics of devices: IV characteristics, CV features and optical properties: spectral response characteristics of the light-emitting properties of light point, time and the characteristics of thermal characteristics.
1, LED electrical properties

1.1 IV Characterization of pn junction LED chips Preparation of the main performance parameters. LED's have a nonlinear IV characteristics, the nature of rectification: a one-way electrical conductivity, which is applied bias performance of the low contact resistance, contrary to the high contact resistance.
If the left:

(1) of the dead are: (oa map or oa 'section) a point for the voltage V0 to open, when V <Va, applied electric field is still a lot to overcome as a result of the proliferation of carrier barrier formed by the electric field, at this time is R Large; open LED voltage for different values in different, GaAs as 1V, red GaAsP for 1.2V, GaP to 1.8V, GaN to 2.5V.

(2) are working: IF current and voltage exponential relationship
IF = IS (e qVF / KT -1) ------------------------- IS for the reverse saturation current.
V> 0 when, V> VF forward with the work of the VF area IF index rose IF = IS e qVF / KT

(3) Reverse Dead: V <0 when pn junction to increase anti-bias
V = - VR, the reverse leakage current IR (V =-5V) when, GaP to 0V, GaN to 10uA.

(4) Reverse breakdown District V <- VR, VR is called reverse breakdown voltage; VR voltage corresponding to the reverse leakage current IR. When the reverse bias has been increased so that the V <- VR, while there is a sudden increase in IR and the emergence of the phenomenon of breakdown. As the compounds used by different types of materials, all kinds of LED reverse breakdown voltage VR is also different.

1.2 C-V characteristics
In view of the LED chips have a 9 × 9mil (250 × 250um), 10 × 10mil, 11 × 11mil (280 × 280um), 12 × 12mil (300 × 300um), it is pn junction area of large and small, so junction capacitance (zero Bias) C ≈ n + pf about.
C-V characteristics were secondary function (Figure 2). 1MHZ by the exchange of signals with C-V characteristics of the measured test.

1.3 maximum allowable power PF m

When the flow of LED current IF, drop tube for UF is power consumption for P = UF × IF

LED work, in addition to bias, bias to a certain carrier issued a composite light, and become part of the heat, so that the junction temperature rise. If the junction temperature for Tj, the external environment temperature Ta, when Tj> Ta, with internal heat pipe block out heat, heat dissipation (power) can be expressed as P = KT (Tj - Ta).

1.4 response time

Characterization of a response time monitor to track changes in the external information processing speed. Several existing display LCD (liquid crystal display) about 10-3 ~ 10-5S, CRT, PDP, LED reached 10-6 ~ 10-7S (us level).

① the use of response time from the point of view, the LED is lit and extinguished by the time delay, that is, the map tr, tf. Figure t0 in a very small value can be ignored.

② response time depends largely on carrier life, the device's junction capacitance and impedance circuit.

The LED light - is the rise time tr connected to the power LED brightness so that the normal 10% of the year, until reaching the normal brightness of 90% experienced.

LED off - fall time tf is the normal light-emitting reduced to 10% of the original experience.

Different materials have the LED system response time varies; such as GaAs, GaAsP, GaAlAs its response time <10-9S, GaP for the 10-7 S. Therefore, they can be used in the 10 ~ 100MHZ frequency system.



2 LED optical properties

There are infrared light-emitting diode (non-visible) and two series of visible light, which can be used radiation, which can be used photometry to measure the optical properties of its.

2.1 assay to the light intensity and angular distribution Iθ

2.1.1 luminous intensity (normal light) is a characterization of light-emitting devices of the important properties of strength. LED large number of applications is cylindrical, spherical package, as a result of the convex lens, one-time capital has a strong point: at light Method to the direction of the largest, and its horizontal angle to 90 °. To deviate Fa-rectification when θ different point of view, the light also changes. As the luminous intensity in different packaging shapes and intensity dependent on the direction angle.

2.1.2 luminous intensity of the angular distribution Iθ is described in the LED light-emitting space in all directions polishing distribution. It depends largely on the technology package (including the stent, the first module tablets, the addition of epoxy resin scattering agent or not)

⑴ for the high point of the angular distribution (Figure 1)

① LED die from the location of the tablets in the first mode is far more;
② the use of cone-shaped (bullet) The first module tablets;
③ epoxy resin in the package not to increase scattering agent.
To take such measures to get the LED 2θ1 / 2 = 6 ° or so, greatly improved directional.

⑵ Several of the current package of scattering angle (2θ1 / 2 c.) circular LED: 5 °, 10 °, 30 °, 45 °

2.2 light-emitting peak wavelength and spectral distribution

⑴ LED luminous intensity or power output as the wavelength of light changes, are ready a distribution curve - spectrum distribution curve. When the curve is established, devices related to the dominant wavelength, color purity, and other related parameters are followed.

LED preparation and distribution of the spectrum used by the type of compound semiconductors, the nature and structure of the pn junction (layer thickness, doping impurities), and so on, with the device geometry, the package has nothing to do.

Under the plans drawn from several different compound semiconductor doping system and a LED spectral response curve. One LED spectral distribution curve

1 blue InGaN / GaN 2 green GaP: N 3 red GaP: Zn-O
4 infrared photosensitive GaAs 5 Si phototube 6 standard tungsten
① blue InGaN / GaN light-emitting diodes, the spectral peak λp = 460 ~ 465nm;
② green GaP: N of the LED, spectra peak λp = 550nm;
③ red GaP: Zn-O of the LED, spectra peak λp = 680 ~ 700nm;
④ is the use of GaAs infrared LED materials, the spectral peak λp = 910nm;
⑤ Si photodiode is usually used for optical receiver.

Can be seen from the chart, no matter what material the LED, has a relatively light intensity of the strongest Agency (maximum light output), there should be a corresponding wavelength, the wavelength is called peak wavelength, λp said. Λp can only monochromatic light wavelengths.

⑵ width of the spectrum: the peak on both sides of the spectrum LED ± △ λ, the existence of two equal to the peak intensity (maximum intensity) of 50 points, two points corresponding to λp-△ λ, λp + △ λ between the width Called line width, also known as the width of half-power or high-width and a half.

Half the width of lines reflect the width, that is, the monochrome LED of the parameters, LED and a half width of less than 40 nm.

⑶ dominant wavelength: Some light-emitting LED is not only a single color, that is not only a peak wavelength; even more than the peak is not monochromatic. This description of the LED color and the introduction of the main characteristics of the wavelength. Is the dominant wavelength of the human eye can be observed from the main issue of monochromatic LED's wavelength. And monochrome as possible, λp is the main wavelength.

GaP material may be issued if the number of peak wavelength, dominant wavelength and only one, it will work with the long-term LED, while the main junction temperature rise toward long-wave wavelength.

2.3 flux

F is the luminous flux LED characterization of the total output of radiation energy, it is a beautiful device performance advantages and disadvantages. F for the LED in all directions and the light-emitting energy, and it is directly related to the current work. With the increase in current, LED luminous flux increases followed. LED luminous flux of the visible light unit lumens (lm).

LED radiation from outside the power - and the luminous flux chip materials, packaging technology and the level of current source plus size. At present, single-color LED luminous flux of the approximately 1 lm, white LED of F ≈ 1.5 ~ 1.8 lm (small chips), 1mm × 1mm for the power-on-chip made of white LED, its F = 18 lm.

2.4 light-emitting efficiency and visual sensitivity

① LED efficiency of the internal efficiency (pn junction in the vicinity of the light energy into electrical energy efficiency) and external efficiency (to the external radiation efficiency). The former is used to analyze and evaluate the advantages and disadvantages of chip features.

LED photo of the most important features is a ray radiation energy (light-emitting volume) and the input power ratio, that is, the light-emitting efficiency.

② visual sensitivity is the use of lighting and photometry in a number of parameters. Human visual sensitivity at λ = 555nm there is a maximum of 680 lm / w. If the visual sensitivity in mind for Kλ, while light-emitting energy P and F flux can be seen as the relationship between P = ∫ Pλdλ; F = ∫ KλPλdλ

③ light-emitting efficiency - quantum efficiency η = launch of the photon number / pn junction carrier number = (e / hcI) ∫ λPλdλ

If the energy input for the W = UI, while light-emitting energy efficiency ηP = P / W
If the photon energy hc = ev, while η ≈ ηP, while the total optical F = (F / P) P = KηPW where K = F / P

④ lumens efficiency: LED luminous flux of the F / plus power consumption W = KηP
It is the evaluation of properties outside the LED package, LED lumens of high-efficiency means the same in the current plus radiation energy more visible, it is also known as visible light-emitting efficiency.

The following is a list of some common lumens efficiency LED (light-emitting efficiency of visible light):
LED luminous color λp (nm) light-emitting materials, the efficiency of visible light (lm / w) external quantum efficiency
Highest average
Red 700660650 GaP: Zn-OGaAlAsGaAsP 2.40.270.38 120.50.5 1 ~ 30.30.2
Huang 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
White band GaN + YAG small 1.6-chip, chip, 18

Quality of radiation outside the requirements of the LED light energy, optical outward to issue as much as possible, that is external to the high efficiency. In fact, LED light-emitting out only within the light-emitting part of the overall light-emitting efficiency should be

η = ηiηcηe, where the ηi to p, n minority carrier injection efficiency of the end zone, ηc for the barrier in the area of minority carrier with more complex sub-efficiency, ηe for external light (light out efficiency) efficiency.

LED materials because of the high refractive index ηi ≈ 3.6. When issued by the chip and optical crystal materials in the air when the interface (no epoxy package) if the normal incidence, reflective air reflectivity for the (n1-1) 2 / (n1 +1) 2 = 0.32, reflecting the 32 %, In view of the lens itself, a considerable portion of the light absorbed, so greatly reduce the external light efficiency.

In order to further improve the efficiency of external light ηe can take the following measures: ① with a higher refractive index of transparent material (n = 1.55 epoxy resin is not satisfactory) covering the surface of the chip; ② the crystal surface processing chip into hemispherical; ③ with Eg The largest compound semiconductor substrate to make in order to reduce the absorption of light inside the crystal. Some people have used n = 2.4 ~ 2.6 glass of low melting point [component As-S (Se)-Br (I)] and thermoplastic closure for large caps, will enable the infrared GaAs, GaAsP, GaAlAs improve the efficiency of the LED's 4 to 6 Times.

2.5-brightness light-emitting

LED brightness light-emitting performance is another important parameter, has a strong direction. The actual direction of the line of brightness BO = IO / A, designated the direction of a light-emitting surface brightness light-emitting surface equal to the surface of projection units in the area of unit solid angle in the radiation flux, the unit cd/m2 or Nit.

If the light source is the ideal surface diffuse reflection surface brightness BO has nothing to do with the constant direction. The clear blue sky and the surface brightness of the fluorescent lamp is about 7000Nit (Bennett), from the ground to see the surface brightness of the sun is about 14 × 108Nit.

LED brightness and external current density, general LED, JO (current density) increase in the BO approximation is also increasing. In addition, the brightness with ambient temperature, ambient temperature rise, ηc (composite efficiency) decreased, BO reduced. When the ambient temperature change, the current increases enough to cause colorectal pn junction temperature rise, temperature rise, the brightness was saturated.

Life 2.6

Aging: LED brightness light-emitting With the emergence of long working hours and light intensity or brightness decay phenomenon. And the degree of aging of the device plus the size of the current source, can be described as Bt = BO et / τ, Bt for t time the brightness, BO for the initial brightness.

Usually the brightness down to Bt = 1/2BO experienced t the time of life known as the diode. T determined to spend a long time, usually in order to achieve the projected life expectancy. Measurement: to pass a certain LED current source, the light 103 ~ 104 hours, one after another measured BO, Bt = 1000 ~ 10000, into the Bt = BO et / τ obtained τ; then Bt = 1/2BO into, Life can be obtained t.

For a long time that the total LED life of 106 hours, which means a single LED in the next IF = 20mA. With the power LED-based development and application of foreign scholars to believe that the LED light attenuation as a percentage of the value of life based on. If the LED light for the decay of the original 35%, life expectancy> 6000h.




3 thermal characteristics
LED optical parameters of the pn junction and the junction temperature significantly. In general the work of a small current IF <10mA, or 10 ~ 20 mA for a long time in a row LED to light up the temperature rise was not obvious. If the ambient temperature high, LED main wavelength λp or will shift to long wavelength, BO will decline, particularly in dot-matrix, the temperature rise on the LED display reliability, stability should be specially designed ventilation scattering device .

LED main wavelength with temperature can be expressed as the relationship between λp (T ') = λ0 (T0) + △ Tg × 0.1nm / ℃


By type, we can see that when the junction temperature rise 10 ℃, while the long-wavelength drift 1nm, and the luminous uniformity, consistency variation. This is used as lighting lamps light of the requirements of small, densely arranged in order to increase the per unit area on the light intensity, brightness with particular attention paid to the design used dissipation good shell, or devoted to general lighting equipment, to ensure the long-term work of the LED.

Reference link: http://www.17led.com/LEDdisplay/2008-04/57p3.html