LEDs are optoelectronic devices that make pn junctions from compound materials. It has the electrical properties of a pn junction device: IV, CV, and optical: spectral response. This article will give you a detailed introduction.
1, LED electrical characteristics
1.1 IV characteristics
Characterize the main parameters of the LED chip pn junction preparation performance. The IV characteristics of LEDs have non-linear, rectifying properties: unidirectional conductivity, that is, the application of a positive bias exhibits low contact resistance, and vice versa.
As shown above:
(1) Positive dead zone: (Fig. oa or oa' segment) Point a is the turn-on voltage for V0. When V < Va, the applied electric field overcomes a lot of potential field due to carrier diffusion, and R is very Large; turn-on voltage is different for different LEDs, GaAs is 1V, red GaAsP is 1.2V, GaP is 1.8V, and GaN is 2.5V.
(2) Forward working area: current IF is exponential with applied voltage
IF = IS (e qVF/KT –1) -------------------------IS
Is the reverse saturation current. When V>0, the positive working area IF of V>VF rises with the VF index.
IF = IS e qVF/KT
(3) Reverse dead zone: When V<0, pn junction plus reverse bias voltage V= - VR, when reverse leakage current IR (V= -5V), GaP is 0V and GaN is 10uA.
(4) Reverse breakdown region V<- VR, VR is called reverse breakdown voltage; VR voltage corresponds to IR is reverse leakage current. When the reverse bias voltage is increased such that V < - VR, a sudden increase in IR occurs and a breakdown occurs. The reverse breakdown voltage VR of various LEDs is also different due to the type of compound material used.
1.2 CV characteristics
Since the LED chip has 9×9mil (250×250um), 10×10mil, 11×11mil (280×280um), 12×12mil (300×300um), the pn junction area is different, which makes the junction capacitance (zero). Bias) C≈n+pf or so. The CV characteristics are quadratic (see Figure 2). The 1 MHZ AC signal was measured with a CV characteristic tester.
1.3 Maximum allowable power consumption PFm
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 applied bias voltage and bias current must cause the carrier to recombine and a part of the heat becomes The junction temperature rises. 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 stem, and the heat dissipation (power) can be expressed as P = KT(Tj – Ta).
1.4 Response time
Response time characterizes how fast a particular display tracks changes in external information. There are several display LCDs (liquid crystal displays) of about 10-3~10-5S, and CRT, PDP, and LEDs all reach 10-6~10-7S (us level).
1. The response time is from the point of view of use, which is the time delayed by the LED lighting and extinction, namely tr and tf in Figure 3. The value of t0 in the figure is small and can be ignored.
2. The response time depends mainly on the carrier lifetime, the junction capacitance of the device, and the circuit impedance. The lighting time of the LED - the rise time tr is the time elapsed since the power is turned on to make the luminance of the light reach 10% of normal, until the luminance of the light reaches 90% of the normal value. LED off time - Fall time tf is the time it takes for normal illumination to diminish to 10% of the original. The response time of LEDs made by different materials is different; for example, GaAs, GaAsP, GaAlAs have a response time of <10-9S and a GaP of 10-7 S. Therefore they can be used in 10~100MHZ high frequency systems.
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