Electrical characteristics of LED chip

  LEDs are optoelectronic devices that use compound materials to make pn junctions. It has the electrical characteristics of pn junction junction devices: I-V characteristics, C-V characteristics and optical characteristics: spectral response characteristics, luminous intensity directivity characteristics, time characteristics and thermal characteristics.

     1. LED electrical characteristics

     1.1 I-V characteristics Characterize the main parameters of LED chip pn junction fabrication performance. The I-V characteristics of LEDs have nonlinear and rectifying properties: unidirectional conductivity, that is, low contact resistance with positive bias, and high contact resistance on the contrary.

     As shown on the left:

     (1) Forward dead zone: (Fig. oa or oa' section) point a is the turn-on voltage for V0. When V

     (2) Forward working area: the current IF has an exponential relationship with the applied voltage

     IF = IS (e qVF/KT –1) ------------------------- IS is the reverse saturation current .

     When V>0, the forward working region IF of V>VF increases with the VF index IF = IS e qVF/KT

     (3) Reverse dead zone: when V<0, the pn junction is reverse biased

     When V = - VR, when the reverse leakage current IR (V = -5V), GaP is 0V, GaN is 10uA.

     (4) Reverse breakdown region V <- VR, VR is called reverse breakdown voltage; VR voltage corresponding to IR is reverse leakage current. When the reverse bias voltage keeps increasing so that V<- VR, the IR suddenly increases and the breakdown phenomenon occurs. Due to the different types of compound materials used, the reverse breakdown voltage VR of various LEDs is also different.

     1.2 C-V Characteristics

     In view of the LED chips are 9×9mil (250×250um), 10×10mil, 11×11mil (280×280um), 12×12mil (300×300um), the size of the pn junction is different, so that the junction capacitance (zero) Bias voltage) C≈n+pf or so.

     The C-V characteristic is a quadratic function relationship (Figure 2). Measured by a 1MHZ AC signal with a C-V characteristic tester.

     1.3 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 external bias voltage and bias current will definitely promote the recombination of the carriers to emit light, and some of them will become heat, which will increase the junction temperature. 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 tube base, and the heat (power) is dissipated, which can be expressed as P = KT (Tj – Ta).

     1.4 Response time

     Response time describes how quickly a display can track changes in external information. There are several kinds of display LCD (liquid crystal display) about 10-3~10-5S, CRT, PDP, LED all reach 10-6~10-7S (us level).

     ① Response time, from the point of view of use, is the delay time between LED lighting and extinguishing, namely tr and tf in the figure. The value of t0 in the figure is very small and can be ignored.

     ② The response time mainly depends on the carrier lifetime, the junction capacitance of the device and the circuit impedance.

     The lighting time of the LED - the rise time tr refers to the time from when the power is turned on to make the luminous brightness reach 10% of the normal value, until the luminous brightness reaches 90% of the normal value.

     LED off time - fall time tf refers to the time it takes for the normal light emission to decrease to 10% of the original.

     LEDs made of different materials have different response times; for example, GaAs, GaAsP, and GaAlAs have a response time of <10-9S, and GaP is 10-7S. So they can be used in 10~100MHZ high frequency system.

     2 LED optical characteristics

     There are two series of light-emitting diodes: infrared (non-visible) and visible light. The former can be used for radiance, and the latter can be measured by photometry.

     2.1 Luminous normal light intensity and its angular distribution Iθ

     2.1.1 The luminous intensity (normal luminous intensity) is an important performance to characterize the luminous intensity of a light-emitting device. A large number of LED applications require cylindrical and spherical packages. Due to the effect of the convex lens, they all have strong directivity: the light intensity in the normal direction is the largest, and the angle between it and the horizontal plane is 90°. When deviating from the normal direction at different θ angles, the light intensity also changes accordingly. The luminous intensity depends on the angular direction of the intensity with different package shapes.

     2.1.2 The angular distribution of luminous intensity Iθ is to describe the luminous intensity distribution of LED luminescence in all directions of space. It mainly depends on the process of encapsulation (including bracket, die head, adding scattering agent to epoxy resin or not)

     (1) In order to obtain the angular distribution of high directivity (as shown in Figure 1)

     ① The position of the LED die is farther from the die head;

     ② Use a conical (bullet) die head;

     ③ Do not add scattering agent to the encapsulated epoxy resin.

     Taking the above measures can make the LED 2θ1/2 = about 6°, which greatly improves the directivity.

     (2) The scattering angle (2θ1/2 angle) circular LED of several commonly used packages: 5°, 10°, 30°, 45°

     2.2 Luminescence peak wavelength and its spectral distribution

     (1) The luminous intensity or optical power output of the LED varies with the wavelength, and is drawn into a distribution curve—spectral distribution curve. After this curve is determined, the relevant colorimetric parameters such as the dominant wavelength and purity of the device are also determined accordingly.

     The spectral distribution of LEDs is related to the type, properties and pn junction structure (epitaxial layer thickness, doping impurities) of the compound semiconductor used in the preparation, and has nothing to do with the geometry and packaging of the device.

     The figure below plots the spectral response curves of several LEDs made from different compound semiconductors and doping. in

     LED spectral distribution curve

     1 Blue light InGaN/GaN 2 Green light GaP:N 3 Red light GaP:Zn-O

     4 Infrared GaAs 5 Si photocell 6 Standard tungsten lamp

     ① It is a blue InGaN/GaN light-emitting diode with a spectral peak λp = 460～465nm;

     ② It is a green GaP:N LED with a spectral peak λp = 550nm;

     ③ It is a red GaP:Zn-O LED with a spectral peak λp = 680-700nm;

     ④ The infrared LED uses GaAs material, and the emission spectral peak λp = 910nm;

     ⑤ is a Si photodiode, usually used for photoelectric reception.

     It can be seen from the figure that no matter what material LEDs are made of, there is a place where the relative light intensity is the strongest (the light output is the largest), and there is a wavelength corresponding to it. This wavelength is called the peak wavelength and is represented by λp. Only monochromatic light has λp wavelength.

     (2) Spectral line width: At ±△λ on both sides of the peak of the LED spectral line, there are two points with light intensity equal to half of the peak (maximum light intensity), and these two points correspond to the width between λp-△λ and λp+△λ respectively. It is called spectral line width, also known as half-power width or half-height width.

     The width at half maximum reflects the narrow spectral line width, that is, the parameter of the monochromaticity of the LED, and the half width of the LED is less than 40 nm.

     (3) Dominant wavelength: Some LEDs emit not only a single color, that is, not only one peak wavelength; even multiple peaks, not monochromatic light. The dominant wavelength is introduced to describe the LED chromaticity characteristics. The dominant wavelength is the wavelength at which the main monochromatic light emitted by the LED can be observed by the human eye. The better the monochromaticity, the λp is also the dominant wavelength.

     For example, GaP material can emit multiple peak wavelengths, but only one main wavelength. It will shift to long wavelengths as the junction temperature increases as the LED works for a long time.

     2.3 Luminous flux

     The luminous flux F is the radiant energy that characterizes the total light output of the LED, and it marks the performance of the device. F is the sum of the energy emitted by the LED in all directions, which is directly related to the working current. As the current increases, the LED luminous flux increases. The unit of luminous flux of visible light LEDs is lumens (lm).

     The power radiated from the LED - the luminous flux is related to the chip material, the level of packaging technology and the size of the external constant current source. At present, the maximum luminous flux of monochromatic LED is about 1 lm, and the F ≈ 1.5~1.8 lm (small chip) of white LED. For a 1mm × 1mm power stage chip made of white LED, its F=18 lm.

     2.4 Luminous efficiency and visual acuity

     ① LED efficiency has internal efficiency (the efficiency of converting electrical energy into light energy near the pn junction) and external efficiency (the efficiency of radiation 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 radiated light energy (luminous amount) to the input electrical energy, that is, the luminous efficiency.

     ② Visual sensitivity is the use of some parameters in lighting and photometry. Human visual acuity has a maximum value of 680 lm/w at λ = 555nm. If the visual acuity is recorded as Kλ, then the relationship between the luminous energy P and the visible luminous flux F is P=∫Pλdλ; F=∫KλPλdλ

     ③ Luminous efficiency - quantum efficiency η = number of photons emitted / 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 / additional power consumption W = KηP

     It is to evaluate the characteristics of LEDs with external packaging. The high lumen efficiency of LEDs means that the energy of radiating visible light is larger under the same applied current, so it is also called visible light luminous efficiency.

     The following lists several common LED lumen efficiencies (visible light luminous efficiency):

     LED emission color λp (nm) material visible light luminous efficiency (lm/w) external quantum efficiency

     highest value average

     Red light 660-620 GaP:Zn-OGaAlAsGaAsP

      Orange light 615-600 GaP:Zn-OGaAlAsGaAsP

     Yellow light 595-590 GaP:N-N 0.45 0.1

     Green light 555-500 GaP:N 4.2 0.7 0.015~0.15

     Blu 485-410 GaN 10

     Violet 400-280 GaN 10

White band GaN+YAG small chip 1.6, large chip 18

     High-quality LEDs require a large amount of light energy radiated to the outside, and as much light as possible, that is, the external efficiency should be high. In fact, the outward light emitted by the LED is only a part of the internal light emission, 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 recombination efficiency of minority and multi-carrier in the barrier region, and ηe is the external light extraction (light extraction efficiency) efficiency.

     Due to the high refractive index of the LED material, ηi≈3.6. When the chip emits light at the interface between the crystal material and the air (without epoxy encapsulation), if it is vertically incident, 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%. , in view of the fact that the crystal itself absorbs a considerable part of the light, so 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 Cover the chip surface with a transparent material with high refractive index (epoxy resin n=1.55 is not ideal); ② Process the chip crystal surface into a hemispherical shape;

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

     2.5 Luminous brightness

     Brightness is another important parameter of LED luminous performance, which has strong directionality. The brightness in the positive normal direction is BO=IO/A, and the brightness of the surface 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 within the unit solid angle, and the unit is cd/m2 or Nit.

     If the surface of the light source is an ideal diffuse reflection surface, the brightness BO is constant regardless of the direction. The surface brightness of a clear blue sky and fluorescent lights is about 7000Nit (nits), and the surface brightness of the sun from the ground is about 14×108Nit.

     The brightness of LED is related to the applied current density. For general LEDs, JO (current density) increases and BO also increases approximately. In addition, the brightness is also related to the ambient temperature. As the ambient temperature increases, the ηc (recombination efficiency) decreases, and the BO decreases. When the ambient temperature remains unchanged, the current increases enough to cause the junction temperature of the pn junction to rise. After the temperature rises, the brightness is saturated.

     2.6 Lifespan

     Aging: LED luminous brightness appears the phenomenon of light intensity or light brightness attenuation 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 e-t/τ, where Bt is the brightness after t time, and BO is the initial brightness.

     Usually the time t that the brightness takes to drop to Bt=1/2BO is called the life of the diode. It takes a long time to measure t, and the life is usually calculated by estimation. Measurement method: Pass a certain constant current source to the LED, after 103 ~ 104 hours of ignition, measure BO, Bt=1000~10000, substitute Bt=BO e-t/τ to find τ; then substitute Bt=1/2BO, The lifetime t can be obtained.

     For a long time, it is always believed that the LED life is 106 hours, which means that a single LED is under IF=20mA. With the development and application of power LEDs, foreign scholars believe that the light attenuation percentage value of LEDs is used as the basis for life. For example, the light attenuation of LED is 35% of the original, and the lifespan is more than 6000h.

     3 Thermal properties

     The optical parameters of the LED have a great relationship with the junction temperature of the pn junction. Generally, the temperature rise of the LED is not obvious when it works at a small current IF < 10mA, or when the LED is continuously lit for a long time of 10~20 mA. If the ambient temperature is high, the dominant wavelength or λp of the LED will drift to a long wavelength, and the BO will also decrease. Especially, the temperature rise of the dot matrix and large display screen will affect the reliability and stability of the LED. Specially designed scattering ventilation device .

     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°C, the wavelength shifts to the long wavelength by 1 nm, and the uniformity and consistency of light emission deteriorate. This is for the design of the light source of lamps used for lighting that requires miniaturization and dense arrangement to improve the light intensity and brightness per unit area, especially attention should be paid to the use of lamp housings with good heat dissipation or special general equipment to ensure long-term operation of LEDs.