1. I-V characteristics. The I-V characteristic is the main parameter that characterizes the performance of the PN junction of the LED chip. The I-V characteristic of the LED has nonlinearity and unidirectional conductivity, that is, the applied positive bias voltage is low resistance, and vice versa, it is high resistance, as shown in Figure 1.
(1) Positive dead zone. In Figure 1, point a is the turn-on voltage for Va. When V<Va, the applied electric field has not overcome the barrier electric field formed by the diffusion of minority carriers, and the resistance R is very large at this time; the turn-on voltage has different values for different LEDs, GaAs is 1V, red GaAsP is 1.2V, GaP is 1.8V, and GaN is 2.5V.
(2) Positive working area. The working current IF has an exponential relationship with the applied voltage
In the formula: Is is the reverse saturation current.
In the positive working area of V>VF, IF rises with the VF index
The positive working current IF refers to the positive current value when the LED normally emits light. In actual use, the IF should be selected below 0.6 IFm according to the needs.
The positive working voltage VF is obtained under a given positive current. It is generally measured when IF=20mA. LED positive working voltage VF is 1.4~3V. When the ambient temperature rises, the positive working voltage VF will drop.
When the positive voltage is less than a certain value (called the threshold), the current is extremely small and the LED does not emit light. When the voltage exceeds a certain value, the positive current increases rapidly with the voltage to make the LED glow. From the V-I curve, the LED’s positive voltage, reverse current and reverse voltage, reverse leakage current (IR<10μA) and other parameters can be obtained. The positive volt-ampere characteristics of the LED are shown in Figure 2. The LED volt-ampere characteristic model can be expressed by the following formula
VF=Vturn-on + RsIF+(ΔVF/ΔT)(T－25℃)
In the formula: Vturn-on is the starting voltage of the LED; R is the slope of the volt-ampere curve; T is the ambient temperature; ΔVF/ΔT is the temperature coefficient of the LED positive voltage, and the typical value for most LEDs is -2V/℃.
From the LED’s V-I curve and model, a small change in the positive voltage of the LED after the positive conduction will cause a large change in the LED current, and factors such as ambient temperature and LED aging time will also affect the electrical performance of the LED. Because the light output of the LED is directly related to the LED current, it is necessary to control the change in the output parameters of the drive circuit when the input voltage and ambient temperature change in the LED application. Otherwise, the light output of the LED will change with changes in factors such as input voltage and temperature, and if the LED current is out of control, long-term operation of the LED under high current will affect the reliability and life of the LED, and even cause the LED to fail.
(3) Reverse dead zone. When V<0, the PN junction is reverse biased; the reverse leakage current IR (V=-5V) of GaP-LED is 0A, and the reverse leakage current IR (V=-5V) of GaN-LED is 10μA.
(4) Reverse breakdown area 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, IR suddenly increases and breakdown occurs. Due to the different types of compound materials used, the reverse breakdown voltage VR of various LEDs is also different.
2. C-V characteristics. The LED chip has several specifications of 9×mil (250×50μm), 10×10mil, 11×11mil (280×280μm), 12×12mil (300×300μm), so the size of the PN junction area is different, so that the junction capacitance (zero bias) is approximately C≈n+pF. The C-V characteristic of the LED is a quadratic function, as shown in Figure 3 (the C-V characteristic shown in Figure 3 is measured with a C-V characteristic tester on a 1MHz AC signal).
3. Allow power consumption P. When the current flowing through the LED is IF and the tube voltage drop is VF, the power consumption of the LED is P=VF×IF. When the LED is working, an external bias voltage and a certain bias current cause a part of the carriers in the PN junction to recombine and emit light, and some of them become heat, which increases the junction temperature. If the junction temperature is Tj and the external ambient temperature is Ta, when Tj>Ta, the internal heat of the LED is transferred to the outside through the tube base, and the heat dissipation (power) can be expressed as
P= KT(Tj－Ta)
4. Response time. The response time of the LED is an important parameter for marking the response speed, especially when it is pulsed or electrically modulated. Response time refers to the time it takes for the LED to start to glow (rise) and turn off (decay) after the forward current is input. From the point of view of use, the response time of the LED is the delay time between LED lighting and extinguishing, such as tr , tf in Figure 4. In Figure 4, the value of t0 is very small and can be ignored. The response time of the LED mainly depends on the carrier lifetime, the junction capacitance of the device and the circuit impedance.
1) The lighting time tr (rising time) of the LED. tr refers to the time from when the power is turned on to make the LED light-emitting brightness reach 10% of the normal value, until the light-emitting brightness reaches 90% of the normal value.
2) LED off time tf (fall time). tf refers to the time elapsed from the normal luminescence attenuation to 10% of the original.
LEDs made of different materials have different response times; for example, the response time of GaAs, GaAsP, and GaAlAs is less than 10-9s, and the response time of GaP is 10-7s.