【 Weak current College 】 LED parameters and characteristics 】

LED (light emitting diode) is the use of compound material pn junction of optoelectronic devices. It features a pn junction type devices for electrical characteristics: I-v characteristics, c-v characteristics and optical properties: spectral response characteristics, luminous intensity point to properties, time characteristics and thermal characteristics.

1. LED electrical characteristics
1.1I-V characterization LED chip performance PN. preparation for the main parameter LED I-v characteristics of nature with nonlinear, Rectifier, namely: one-way conductivity are biased performance plus low contact resistance, on the contrary as high contact resistance.
Figure:

(1) to the dead zone: (oa or oa ') a point for V0 to turn on voltage, V "Va, have overcome many applied electric field due to carrier diffusion and form a barrier electric field, where R is very big; open voltage for different LED its values are different, GaAs to 1V, red, GaAsP-GaP for 1.8V 1.2V, GaN as 2.5V.
(2) forward workspace: current IF and applied voltage exponential relationships
IF = IS (eqVF/KT-1)--------------------------IS to reverse saturation current.
V > 0 V ＞ VF forward workspace IF with VF index increased IF = ISeqVF/KT
(3) reverse dead zone: V "0 pn junction with reverse bias
V =-VR, reverse leakage current IR (V =-5V), GaP to 0V, GaN is 10uA.
(4) reverse breakdown District V "-VR, VR is called reverse Breakdown voltage, voltage corresponding IR VR to reverse leakage current. When the reverse bias has been increased to V "-VR, an IR a sudden increase in the breakdown. As a result of the use of different kinds of compounds, various materials LED reverse Breakdown voltage VR.

1.2C-V properties
In view of the LED chip has 9 × 9mil (250 × 250um), 10 × 11 × 11mil 10mil, (280 × 280um), 12 × 12mil (300 × 300um), the pn junction area sizes to junction capacitance (zero bias) C ≈ n + pf or so. C-v characteristics is quadratic function relationships (as in Figure 2). By 1MHZ AC signal using c-v characteristics measured test instrument.


1.3 PFm maximum allowable power consumption
When current flows through the LED as IF, you tube pressure drop for UF P = power consumption for UF×IF
LED work, plus bias, bias must promote the light carrier compound, and some become hot, high temperature rise of the knot. If the junction temperature for Tj, external ambient temperature Ta, Ta when Tj ＞, internal heat by heat to the outside of the tube socket, dissipated heat (power), can be expressed as P = KT (Tj-Ta).

1.4 response time
Response time characterization of a monitor to track how changes in the external information. Existing several display LCD (liquid crystal display) for approx. 10-3 ~ 10-5S, CRT, PDP, LED all reach 10-6 ~ 10-7S (us-grade).
① response time from the point of view, is the LED is lit and extinguished the delay time, tr, tf in the figure. Map t0 value is very small and can be ignored.
② response time depends on carrier lifetime, device-capacitance and circuit impedance.

Led light up time-rise time tr refers to the power that glow brightness to achieve normal 10% starting to glow brightness to achieve 90% of the normal experience of time.
LED off time — fall time tf is the normal glow subsided to its 10% of the time.
Different materials obtained by LED response times vary; such as GaAs, GaAsP, GaAlAs its response time 10-9S, the GaP is 10-7S. They are available in 10 ~ 100MHZ RF System.

2LED optical properties
Light-emitting diodes have infrared (not visible) and visible light two series, the former available radiation degree, the latter available photometry to measure its optical properties.
2.1 luminous law to light intensity and angle θ distribution I
2.1.1 luminous intensity (normal light intensity) is the characterization of electroluminescence strength of important performance. LED a large number of applications require a cylinder, ball packaging, because of the role of a convex lens, the ancient capital with strong links to sex: located in the normal direction of light intensity maximum angle between the horizontal plane, and is 90 °. When deviations from the correct angle θ to different light intensity changes with it. Luminous intensity with different package shape and strength depends on the angle direction.
2.1.2 the luminous intensity distribution I in the angle θ is the description of the led in the space of all directions light intensity distribution. It mainly depends on the package process (including the brackets, die-head, add scattering agent for epoxy resin or not)
⑴ for high directivity of angular distribution (Figure 1)


① L E D Wick location away from the die-head away;
② the use of cone-shaped (bullet) die-head;
③ package epoxy resin don't add scattering agent.
These measures will enable L E D 2 θ 1/2 = 6 ° left, dramatically improves directional.
⑵ currently several common package scattering angle (2 1/2 angle θ) circular LED: 5 °, 10 °, 30 °, 45 °

2.2 peak wavelength and spectral light distribution
⑴ L E D shine strength or power output with the wavelength change, into a distribution curve – spectral distribution curve. When this determination, the devices of the dominant wavelength and purity, and other relevant colorimetric parameters also along with it.
LED spectral distribution and preparation of compound semiconductors used in the type, nature and pn junction structure (epitaxial layer thickness, doped impurities), and the geometry of the device, Regardless of the package.
The following figure drawn several different compound semiconductor and doped with a system of LED spectral response curve. Which
① is blue InGaN/GaN LEDs, spectral peak λ p = 460 ~ 465nm;
② is the Green LED of the GaP: N, spectral peak λ = 550nm; p
③ is red GaP: Zn-O led, spectral peak λ p = 680 ~ 700nm;
④ is infrared LED GaAs material used,the spectral peak 910nm; p = λ
⑤ is Si photodiode, typically used for optical receiver.

LED spectral distribution curve
1 blue InGaN/GaN2 green GaP: N3 red GaP: Zn-O4 infrared photocell GaAs5Si photosensitive 6 standard tungsten
The figure shows that whatever material LED, has one of the strongest relative light intensity (maximum optical output), as opposed to a wavelength, this wavelength is called peak wavelength, expressed with λ p. Only the monochromatic light has a wavelength λ p.
⑵ spectral line width: LED spectral lines on either side of the peak, ± △ λ exist two intensity equal to peak (maximum light intensity) half of the points, the two corresponding p-△ λ λ, λ λ p + △ called spectral lines between width width, also known as half-width or half-power high-width.
Half-height width reflect the spectral line width, that is, the parameters of the monochrome LED, LED a half-width is less than the 40nm.
⑶ dominant wavelength: have led glow is not just a single color, that is, not only has a peak wavelength; even more peak, is not a monochromatic light. As this description LED color characteristics and introduces the main wavelength. Primary wavelength is the human eye can observed, given by the LED mainly monochromatic wavelength. Monochrome-the better, λ p is the dominant wavelength.
If the GaP materials can issue multiple peak wavelength, but there is only one main wavelength, it will work with the LED permanent temperature high and the main junction wavelength in favor of the long-wave.

2.3 luminous flux
Luminous flux f is the characterization of the total light output LED radiation energy, it flags the performance of the device. F is LED in all directions of energy and that it is directly linked with the operating current. With the current increase in LED luminous flux with increased.
Visible led luminous flux in lumen (lm).
LED to radiation power — the luminous flux and chip material, packaging technology standard and plus size constant current source.
Currently monochrome led luminous flux at approximately 1lm, white led's F ≈ 1.5 ~ 1.8lm (small chip), for the power-on-chip 1mm×1mm made of white led, F = 18lm.

2.4 luminous efficiency and Visual sensitivity
① L E D efficiency internal efficiency (pn junction near the energy efficiency of a solar energy) and external efficiency (radiation to external efficiency). The former is used to analyze and evaluate the merits of the chip.
LED photoelectric most important characteristic is the use of radiation light energy (light weight) and enter the power ratio, i.e. the luminous efficiency.
② Visual sensitivity is used for lighting and photometry in some parameters. Human visual sensitivity at λ = 555nm Department have a maximum value of 680lm/w. If Visual sensitivity recorded as K, glow energy λ P and visible light flux F relationship between ∫ P p = λ λ d; F = λ λ ∫ P K d λ
③ luminous efficiency — quantum efficiency η = emission of photon number/pn knot carrier number = (e/hcI) ∫ P d λ λ λ
If the input energy is W = UI, glow energy efficiency η P = P/W
If the photon energy hc = ev, η η ≈ P, total luminous flux F = (F/P) P = η P W K-K = F/P
④ lumen efficiency: led luminous flux F/plus power consumption P K W = η
It is the evaluation characteristic with the foreign package LED, led lumens efficient means equally impressed current radiation visible light energy is large, it is also called the visible light emitting efficiency.
The following lists some common LED luminous efficiency (visible light emitting efficiency):

Quality-of-the-art LED to the outer radiation of light energy, to light as much as possible, i.e. external efficiency. In fact, LED to the outer glow is only part of the inner glow, total luminous efficiency η η = expected I c e, η η η-I to p, n-zone, minority carrier injection efficiency η c as a potential barrier area little child and multiple child composite efficiency η eto external light (optical removal efficiency) efficiency.
Due to the high refractive index LED material I ≈ 3.6 η. When chip light in the crystal material with the air interface (no epoxy encapsulation) if perpendicular incidence, air reflection, reflectivity-(n1-1) 2/(n1 + 1) 2 = 0.32, reflecting accounted for 32%, in view of the lens itself to light a considerable part of absorption, thus significantly reducing external light efficiency.
In order to further improve the external light efficiency η e can take the following measures:
① high refractive index with transparent materials (epoxy resin n = 1.55 is not ideal) coverage in the chip surface;
② the chip Crystal surface processing into half-sphere;
③ used Eg large compound semiconductors for substrate to reduce Crystal light absorption. Someone has used the n = 2.4 ~ 2.6 of low melting point glass [constituents As-S (Se)-Br (I)] and made of thermoplastic large cap, so that the infrared GaAs, GaAsP, GaAlAs led efficiency 4 ~ 6 times.

2.5 glow brightness
Brightness is LED luminescent properties of another important parameter that has a very strong direction. The brightness of the reinforcing bar direction BO = IO/A, specify a direction light emitting surface brightness equal to glow on the surface of the projection area on the units in the unit solid angle of radiation of light flux, cd/m2 or Nit.
If the lamp surface is ideal diffuse reflectance surface brightness has nothing to do with the direction to BOConstant. The clear blue sky and fluorescent surface brightness about 7000Nit (nits), from the ground at the Sun's surface brightness about 14 × 108Nit.
LED brightness and impressed current density, General LED, JO (current density) increase BO also approximate.
In addition, the brightness is also concerned with the environment temperature, ambient temperature rise, η c (composite efficiency), BO decrease. When the ambient temperature does not change the current increase enoughto cause high pn junction temperature, temperature, brightness is saturated.

2.6 life
Aging: LED glow brightness with long hours and intensity or brightness of decay. Device aging degree and plus the size of the constant current source, can be described as Bt = B O e-t/τ, Bt-t time brightness, BO to initial brightness. Usually the brightness down to Bt = 1/2BO experience time t called diode life. Determination of t take a long time, typically to seek life. projected Measurement method: to pass to a constant current LED, lit 103 ~ 104 hours after successively measured BO, Bt = 1000 ~ 10000, substitution of a Bt = B O e-t/τ τ; seeking out and then put the Bt = 1/2BO substitution, you can find the life t.
Has long LED life always think to 106 hours, this means a single LED under the IF = 20mA. With the development and application of power led, foreign scholars think that LED light attenuation percentage value as the basis for life. If the LED light attenuation up to 35%, life ＞ 6000h.

3 thermal characteristics
LED optical parameters and pn junction temperature has a lot to do. Generally work in small currents Ang 10mA IF, or 10 ~ 20mA long continuous lights LED temperature rise is not obvious. If the ambient temperature is higher, LED by the dominant wavelength or λ p to the long wavelength drift, BO will decline, especially in the dot-matrix, the temperature rise on the big screen LED reliability, stability should specifically designed ventilation. scattering
LED by the dominant wavelength with temperature relationship can be expressed as λ p (T ') = λ 0 (T0) + △ Tg×0.1nm/¡æ
By type, high temperature whenever the knot, 10 ° c to-wave drift 1nm wavelength, and luminous uniformity, consistency variation. This is as for lighting requirements miniaturization of lamp light, dense arrangement to increase the unit size of light intensity, the brightness of design particular attention with thermal good lamp housing or specialized generic device,ensure that the LED permanent jobs.