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Product Knowledge

Uncovering the working conditions of photodiodes - little knowledge


Semiconductor light-emitting devices include a semiconductor light-emitting diode (LED), digital tube, symbol tube, meter-shaped tube and dot matrix display screen (matrix tube) and so on. Each light-emitting unit in the digital tube, symbol tube, meter-shaped tube, and matrix tube is a light-emitting diode.


LED Luminescence Principle

Light-emitting diodes (LED) are made of III-IV compounds, such as GaAs (GaAs), GaP (GaP), GaAsP (GaAs) and so on, whose core is PN junction. Therefore, it has I-N characteristics of general P-N junctions, i.e. forward conduction, reverse cutoff and breakdown characteristics. Also, it has luminescent properties under certain conditions. Under forward voltage, electrons are injected into P region from N region and holes into N region from P region. The minority carriers (minority carriers) entering the opposite region are combined with the majority carriers (poly carriers) to emit light, as shown in Figure 1.

Assuming that luminescence occurs in the P region, the injected electrons and valence band holes directly recombine to emit light or are captured by the luminescence center before recombining with the holes. In addition to this luminescent recombination, some electrons are captured by non-luminescent centers (which lie in the middle of the conduction band and the intermediate band) and then recombined with holes. The energy released at each time is not large enough to form visible light. The larger the proportion of luminous recombination to non-luminous recombination, the higher the quantum efficiency of light. Since the recombination is luminous in the minority carrier diffusion region, the light is generated only within a few microns near the PN junction surface. It has been proved theoretically and practically that the peak wavelength of light is related to the bandgap width Eg of semiconductor materials in the luminous region, i.e. the unit of Eg in the form of lambda_1240/Eg(mm) is electronic volt(eV). If visible light can be produced (the wavelength is 380 nm violet light to 780 nm red light), the Eg of semiconductor materials should be between 3.26 and 1.63 eV. Infrared light is longer than the wavelength of red light. At present, there are infrared, red, yellow, green and blue light-emitting diodes, but blue light-emitting diodes are not widely used because of their high cost and price.

Basic working conditions of LED
LED is a current-driven low-voltage single-guide electrical device. To ensure the normal operation of the LED, it must meet the following basic requirements.
(1) The input DC voltage must be no less than the forward voltage drop of the LED, otherwise, the LED will not turn on and emit light.
(2) DC or unidirectional pulse current drive is used. When driving parallel LED or LED series, a constant current is required instead of the constant voltage power supply.
Fig. 1 shows the relationship between the forward current IF and the forward voltage TIE of six white-light LEDs randomly selected, of which 3 are trademark A and 3 are trademark B. If these six (parallel) LEDs are driven by a constant voltage, the forward currents between them are quite different. For example, they are driven by 3.4V and the forward currents range from L 0 to 44mA, which makes the brightness and chroma of each LED have great differences.
The rated current (30mA) of the LED decreases with the increase of temperature, as shown in Figure 2. According to the figure, when the ambient temperature rises to 50C, the rated current drops to 20mA. In this case, to prevent the burnout of the LED, the driving current must be limited to 20mA. Therefore, to avoid driving current exceeding the maximum rated value and affect its reliability, and to obtain the expected brightness requirements and ensure the consistency of brightness and chroma of each LED, the constant current driving mode should be adopted instead of constant voltage driving mode.

Working Conditions of Light Emitting Diodes

Fig. 16 Relation Curves of forwarding Current with forwarding Voltage of White LED
Significance of Electrical Parameters of Light Emitting Diodes
(1) Spectral Distribution and Peak Wavelength: The light emitted by a light-emitting diode is not a single wavelength, and its wavelength is roughly shown in Figure 2. It can be seen from the figure that the intensity of a certain wavelength of lambda 0 is the highest in the light emitted by the luminescent tube, and the wavelength is the peak wavelength.
(2) Luminescent intensity IV: Luminescent intensity of light-emitting diodes usually refers to the luminescent intensity in the direction of the normal (to the cylindrical light-emitting tube refers to its axis). If the radiation intensity in this direction is (1/683) W/sr, the luminescence will be 1 Candela (symbol cd). Because of the low light-emitting intensity of the general LED, the light-emitting intensity is usually measured by Candela (mcd).
(3) Semi-width of the spectrum lambda: It represents the spectral purity of the luminescent tube. It refers to the distance between the two wavelengths corresponding to the 1/2 peak intensity in Figure 3.
(4) Half-value angle theta 1/2 and angle of view: theta 1/2 refers to the angle between the direction where the luminous intensity value is half of the axial intensity value and the luminous axis (normal direction). Two times the half-value angle is the angle of view (or half-power angle).

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