LED is the use of compound material pn junction made of optoelectronic devices. It has the electrical characteristics of the pn junction type device: I-V characteristics, C-V characteristics and optical characteristics: spectral response characteristics, luminous intensity pointing characteristics, time characteristics and thermal properties.
LED electrical characteristics
A. I-V characteristics
Characterization of LED chip pn junction preparation performance of the main parameters. LED I-V characteristics of non-linear, rectifier nature: one-way conductivity, that is, plus positive bias performance of low contact resistance, and vice versa for high contact resistance.
(1) positive dead zone: (Figure oa or oa 'section) a point for V0 for the open voltage, when V <Va, plus electric field is still overcome by the carrier diffusion and the formation of potential barrier electric field, then R Large; open voltage for different values of different LEDs, GaAs 1V, red GaAsP 1.2V, GaP 1.8V, GaN 2.5V.
(2) positive work area: current IF and the applied voltage is exponential relationship
IF = IS (e qVF / KT -1) ------------------------- IS is the reverse saturation current.
V> 0, the forward operating area of V> VF rises with the VF index IF = IS e qVF / KT
(3) reverse dead zone: V <0 pn plus anti-bias
When V = - VR, the reverse leakage current IR (V = -5V), GaP is 0V, and GaN is 10uA.
(4) Reverse breakdown area V <- VR, VR is called reverse breakdown voltage; VR voltage corresponds to IR as reverse leakage current. When the reverse bias is increased so that V <- VR, there is a sudden increase in IR and the breakdown phenomenon occurs. Due to the different types of compounds used, the reverse breakdown voltage VR of the various LEDs is also different.
B. C-V characteristics
LED chip has 9 × 9mil (250 × 250um), 10 × 10mil, 11 × 11mil (280 × 280um), 12 × 12mil (300 × 300um), so pn junction area of different sizes, so that the junction capacitance (zero Pressure) C ≈ n + pf.
C-V characteristics of a quadratic function (Figure 2). The AC signal from 1MHZ is measured with a C-V characteristic tester.
C. Maximum allowable power consumption PF m
When the current flowing through the LED is IF, the tube voltage drop to UF is the power consumption of P = UF × IF
LED work, the bias, bias current must cause the carrier compound light, and some become hot, so that the junction temperature rise. If the junction temperature is Tj and the external ambient temperature is Ta, then when Tj> Ta, the internal heat is swept outward by means of the tube, and the heat dissipation (power) can be expressed as P = KT (Tj - Ta).
C. Response time
Response time to represent a monitor to track the speed of changes in external information. There are several display LCD (liquid crystal display) about 10-3 ~ 10-5S, CRT, PDP, LED are up to 10-6 ~ 10-7S (us level).
LED optical characteristics
Light-emitting diodes are infrared (non-visible) and visible light two series, the former can be used radiation, which can be used to measure the optical properties of optical properties.
A. Normal light intensity and its angular distribution
Luminous intensity (normal light intensity) is an important feature that characterizes the luminous intensity of a light emitting device. LED large application requirements are cylindrical, ball package, due to the role of convex lens, it has a strong directivity: located in the normal direction of the largest light intensity, and the horizontal angle of 90 °. When the deviation from the positive to different θ angle, light intensity also changes. The luminous intensity depends on the angular direction depending on the shape of the package. The angular distribution Iθ of the luminous intensity is a description of the intensity distribution of the LED light emission in all directions of the space. It depends mainly on the packaging process (including the stent, mold beads, epoxy resin added in the scattering agent or not)
B. Luminescence peak wavelength and its spectral distribution
LED luminous intensity or optical power output varies with the wavelength, drawn a distribution curve - spectral distribution curve. When this curve is determined, the relevant dominant chromaticity parameters such as the dominant wavelength, purity, etc. of the device are also dependent.
The spectral distribution of the LED is related to the type and properties of the compound semiconductor used for the preparation, the structure of the pn junction (epitaxial layer thickness, doping impurities), and so on, regardless of the geometry of the device and the encapsulation mode.
C. 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 photosensitive photoelectric tube 6 standard tungsten lamp
① is blue InGaN / GaN light emitting diodes, emission peak λp = 460 ~ 465nm;
② is green GaP: N LED, emission peak λp = 550nm;
③ is red GaP: Zn-O LED, emission peak λp = 680 ~ 700nm;
④ is the use of infrared LED GaAs materials, emission peak λp = 910nm;
⑤ is a Si photodiode, usually used for photoelectric reception.
As can be seen from the figure, no matter what material made of LED, there is a strongest relative intensity of light (light output maximum), with a corresponding wavelength, the wavelength is called the peak wavelength, with λp said. Only monochromatic light only λp wavelength.
Spectral line width: At the peak of both sides of the peak of the LED spectrum, 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 λp + Line width, also known as half power width or half width.
The half-height width reflects the width of the line, that is, LED monochrome parameters, LED half-width less than 40 nm.
Main wavelength: Some LED light is not only a single color, that is not only a peak wavelength; even a number of peaks, not monochromatic light. This describes the LED chrominance characteristics and introduces the dominant wavelength. The main wavelength is the human eye can be observed by the LED to send the main monochromatic light wavelength. The better the monochromaticity, then λp is the dominant wavelength.
Such as GaP material can send multiple peak wavelengths, and the main wavelength is only one, it will work with the LED long-term, junction temperature and the main wavelength of the long wave.
D. Luminous flux
The luminous flux F is the radiant energy that characterizes the total light output of the LED, which characterizes the performance of the device. F is the sum of the energy of the LED that emits light in all directions, which is directly related to the operating current. As the current increases, the LED luminous flux increases. The luminous flux of the visible LED is in lumens (lm).
LED outward radiation power - luminous flux and chip materials, packaging technology and the level of constant current source. The current single-color LED luminous flux of about 1 lm, white LED F ≈ 1.5 ~ 1.8 lm (small chip), for 1mm × 1mm power chip made of white LED, the F = 18 lm.
E. luminous efficiency and visual sensitivity
① LED efficiency has internal efficiency (pn junction near the energy conversion from light energy efficiency) and external efficiency (radiation to external efficiency). The former is only used to analyze and evaluate the characteristics of the chip merits and demerits.
The most important characteristic of LED optoelectronics is the ratio of radiant energy (luminous quantity) to input power, ie, luminous efficiency.
② visual sensitivity is the use of lighting and photometry in some of the parameters. The human visual sensitivity has a maximum value of 680 lm / w at λ = 555 nm. If the visual sensitivity is denoted by 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 / pn junction carrier number = (e / hcI) ∫λPλdλ
If the input energy is W = UI, the luminous energy efficiency ηP = P / W
If the photon energy hc = ev, η ≈ η P, the total luminous flux F = (F / P) P = KηPW where K = F / P
④ lumen efficiency: LED luminous flux F / plus power consumption W = KηP
It is to evaluate the characteristics of LED package with outer package, LED lumen efficiency refers to the same current in the same radiation under the visible energy of the visible light, it is also called visible light luminous efficiency. High quality LED requires outward light energy, the light emitted as much as possible, that is, the external efficiency is higher. In fact, LED light is only part of the internal light, the total luminous efficiency should be
Ηc is the efficiency of the external light (light extraction efficiency), and ηc is the efficiency of the external light (light extraction efficiency).
As the LED material refractive index is high ηi ≈ 3.6. When the chip emits light in the crystal material and air interface (no epoxy package) if the vertical incident, reflected by the air, the reflectivity (n1-1) 2 / (n1 + 1) 2 = 0.32, reflecting the 32% , In view of the crystal itself has a considerable part of the absorption of light, so greatly reduce the external light efficiency.
In order to further improve the external light efficiency ηe can take the following measures: ① with a higher refractive index of transparent material (epoxy resin n = 1.55 is not ideal) to cover the chip surface; ② chip crystal surface processed into hemispherical;
③ use Eg large compound semiconductor as substrate to reduce the crystal light absorption. It has been used with n = 2.4 ~ 2.6 low melting point glass [composition As-S (Se) -Br (I)] and a large thermoplastic cap, the infrared GaAs, GaAsP, GaAlAs LED efficiency increased 4 to 6 times The
F. Luminous brightness
Brightness is another important parameter of LED luminous performance, with strong directional. Its normal direction of the brightness BO = IO / A, specify a direction of the surface brightness of the luminous body is equal to the surface area of the luminous body unit projection area in the unit solid angle of the radiation flux, the unit is cd / m2 or Nit.
If the surface of the light source is the ideal diffuse reflection surface, the brightness BO is independent of the direction. Clear blue sky and fluorescent lamp surface brightness of about 7000Nit (Nitide), from the ground to see the sun surface brightness of about 14 × 108Nit.
LED brightness and the current density, the general LED, JO (current density) increase BO also increased approximately. In addition, the brightness is also related to the ambient temperature, the ambient temperature increases, ηc (compound efficiency) decreases, BO decreases. When the ambient temperature does not change, the current increases enough to cause pn junction temperature rise, temperature rise, the brightness was saturated.
G. Life
Aging: LED luminous brightness with a long time working light intensity or brightness attenuation phenomenon. 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 e-t / τ, Bt is the brightness after t time, BO is the initial brightness.
Usually the brightness is reduced to Bt = 1 / 2BO experienced time t called the life of the diode. It takes a long time to measure t, usually to calculate life. Measurement method: to the LED through a constant current source, ignite 103 ~ 104 hours later, has measured BO, Bt = 1000 ~ 10000, into the Bt = BO et / τ τ; then Bt = 1 / 2BO into, Can be obtained life t.
For a long time, LED life is always considered 106 hours, which refers to a single LED at IF = 20mA. With the power LED development and application, foreign scholars believe that the LED light attenuation percentage value as the basis for life. Such as LED light attenuation of the original 35%, life> 6000h.
Thermal characteristics
LED optical parameters and pn junction temperature has a great relationship. The general work in the small current IF <10mA, or 10 ~ 20 mA long continuous light LED temperature is not obvious. If the ambient temperature is high, the main wavelength or λp of the LED will drift to the long wavelength, BO will drop, especially the lattice, large display of the temperature rise of the LED can be *, the stability of the design should be designed specifically for scattered ventilation Device.
The main wavelength of the LED with the temperature relationship can be expressed as λp (T ') = λ0 (T0) + △ Tg × 0.1nm / ℃
As can be seen from the equation, when the junction temperature increases by 10 ° C, the wavelength drifts to the long wave by 1 nm, and the uniformity and uniformity of the emission are deteriorated. This is for the lighting of the light source requirements for miniaturization, intensive arrangement to improve the intensity per unit area, the brightness of the design should pay attention to the use of a good cooling of the lamp or special equipment to ensure long-term work.