Some people suggest that the temperature span should be larger than 50 degrees when measure the PN junction temperature by using voltage method. The voltage-temperature relation will be linear in normal working temperature range of LED Lights, it is useless. The room temperature is 25 degree. According to the suggestion mentioned above, next measuring temperature be around 75 degree. However, the upper limit of the normal working temperature of LED PN junction is 70 degree. Is there any reason for us to measure the voltage-temperature factor in 75 degree: a cannot-be-reached temperature under normal use? It is impossible for a LED manufacturer to build a very sophisticated temperature-controlled lab. Thermostatic equipments can only keep the temperature within a range. It is also impossible to obtain accurate measuring results by using them.

 

 

However, if we are unable to fix the surrounding temperature, we can obtain accurate voltage-temperature factors through the relative temperature derivative, and consequently working temperature of LED PN junction. Firstly, we need to record the room temperature, namely the pre-working PN junction temperature. This temperature can be treated as the initial junction temperature corresponding to the initial voltage. Secondly, apply a voltage on both sides of LED Products, wait until the entire PN junction working in a stable condition, then read the voltage value. In accordance with measured voltage and the predefined voltage-temperature factor, we can calculate the increase value. Add this value together with the room temperature measured previously, then we have the final LED PN junction temperature, which is quite closed to the real value. Overall, voltage method is much more precise than thermal image method.

 

Formulas for thermal resistance method given as followed:

R_θ = (T_j-T_c)/P

R_θ is the thermal resistance of PN junction. T_c is the package temperature. P is the power consumption. R_θ andT_c are fixed values given by the production specification. Once we know them, we can calculate the LED PN junction temperature T_j through this formula. There is another problem, how can LED chip manufacturers measure thermal resistances? They need to know junction temperature at first! In fact, this method is not used to calculate PN junction temperature. On the contrary, it is used to calculate thermal resistance though junction temperature. Most LED High Bay manufacturers still use voltage method to measure junction temperature.

More and more people realize that estimating the specific temperature of LED chips by measuring LED lamps covers temperature is inaccurate. However, how to get the exact temperature of LED PN junction? At present, there are three commonly used PN junction temperature measuring methods: thermal resistance method, voltage method and thermal imagine method.

 

 

Thermal imagine method requires very expensive instruments which cannot be afforded by ordinary people. However, using thermal imagine method to measure PN junction temperature cannot guarantee the accuracy of results. Thermal imagines can only display the general heat distribution. There are many other things need to be taken into account to get the accurate temperature value. Surface emissivity is the main factor. In order to measure accurate temperatures though thermal image, we need to set a surface emissivity value at first. However, in the surface of LED Street Light chip, there are several materials. Different materials have different surface emissivity values. Even if every exact surface emissivity value was known by us, which one should be chosen as the reference value? Meanwhile, if we successfully obtain the accurate temperature by thermal images, it is a surface temperature, what about the temperature of LED PN junction? No matter what kind of optimization is taken, thermal imagine method can only get an approximate value. Therefore, it is a not commonly used method to measure temperatures of PN junctions. In the next step, we are going to do some introductions about thermal resistance method and voltage method.

 

Voltage method:

According to semiconductor theory, the voltage of PN junction is not only a function of temperature, but also a function of current. And the relation between junction voltage and junction current is not linear, which means voltage-temperature relations are different under in different current. Thus, if we intend to measure the temperature of LED PN junction, the working current needs to be known at first. Before measuring PN junction temperature using voltage method, we need to set a fixed constant working current for LED High Bay. Then put them into a thermostatic equipment to offer a constant working temperature. The last step is reading the measuring voltage value. Change into another temperature, repeat this operation once again. Normally, 3-5 measuring results can give out a relatively accurate relation between voltage and working temperature of LED PN junction, namely voltage-temperature factor.

As we all know, light color of LED can be determined by materials used in n-type region and p-type region, and light intensity is determined by the number of carriers within LED Street Light. Carriers’ number is not fixed in diodes. Temperature can exert a significant influence on it. With the increase of temperature, the intrinsic excitation effect will be enhanced. More electron-hole pairs will be generated in n-type region and p-type region respectively. This additional electron-hole pairs will change the width of LED PN junction as well as internal current.

 

 

Apply a forward bias voltage on LED High Bay, a certain amount of heat will be generated. Thus more electron-hole pairs will be created during this process. This intrinsic excitation can happen within p-type region, n-type region and space charge region.

 

Intrinsic excitation within p-type region: electrons created by intrinsic excitation combine with holes provided by the positive potential connected to the other side of p-type section, leading the reduction of PN junction. This kind of combination can generate heat energy.

 

Intrinsic excitation within LED PN junction: if an electron-hole pair is generated within PN junction, the electron will be impelled into p-type section due to external electric field. This electron will combine with a hole provide by the positive potential. Since this kind of combination cannot provide enough potential energy to generate visible light but infrared rays. Some parts of these infrared rays are absorbed by LED and consequently converted into heat energy, the other parts radiated into the air. Scientists also name this type of combination as invalid combination.

 

From the analysis of above two intrinsic excitations, a part of holes provided by power supply will be combined by electrons created by intrinsic excitations. These holes cannot pass PN junction. Similarly, a part of electrons provided by power supply cannot pass PN junction due to intrinsic excitations. Intrinsic excitation within LED PN junction is the chief culprit for this invalid combination. Thus, only a part of carriers provide by power supply can pass though PN junction containing enough potential energy (reach the band gap) to emit visible lights.

 

However, if the working temperature is higher than a certain threshold value, optical quenching phenomenon will be caused. Except for invalid combination caused by intrinsic excitations within LED PN junction and LED Panel, tunnel effect is another factor. When carriers concentration is too high, the Fermi level of p-type region can reach up to or even exceed the Fermi level of n-type region. So electrons have no need to pass PN junction and reach to p-type region directly. The energy released by the combination of electrons and holes will be lower than the band gad, leading to the optical quenching phenomenon.

3.  Electrons in the edge of p-type section. Electrons pass LED PN junction and finally reach the edge of p-type section. Most of them will be captured by the large amount of holes within p-type section. Since electrons have higher potential energy than holes, after being captured by holes, this potential energy will be released as the form of electromagnetic wave (LED Street Light). The light color all depends on the material of diodes. Different diodes has different band gap (different potential energy), which can emit lights with different wavelengths. As we all know, wavelength determines the color of the light. Wavelength within visible range can be seen by human eyes. Electrons from n-type section originally have a higher potential energy than holes within p-type section. In addition that electrons’ potential energy is raised when they pass though the space charge region. It is quite easily for them to reach up to the band gap potential energy. Notice that, this potential energy gained by electrons has nothing to do with external electric field, only decided by LED Projects materials used in n-type region and p-type region.

 

 

4.  Electrons within p-type section. Most electrons will be combined by holes when they enter p-type region. This process will cause the reduction of holes in the edge of p-type region. Since we add a forward bias voltage on LED PN junction, this reduction can be immediately supplemented by holes given by the positive potential connected to the other side of p-type region. External electric field exerts a forward force on electrons and make them accelerate, collide with nucleus and other electrons within p-type region. During this process, energy created by external electric field is converted into heat energy.

 

According to the above analysis we can see that the light energy generated by electron-hole combination is not decided by external electric field by materials used in p-type section and n-type section. It is a fixed characteristic of LED PN junction. On the other hand, all of heat energy created by PN junction is a conversion of electric energy from external electric field. It is an inevitable outcome of LED. Thus a good cooling system is very important for LED lighting applications, especially for those LED lamps with high power consumptions. For the reason that LED cannot work in high temperatures.

 

LED (light-emitting diode) is a kind of one-way conductive electronic component. Generally, diode is a PN junction composed by a p-type semiconductor and an n-type semiconductor. An equilibrium condition is reached within the space charge region in the condition of zero bias.

 

 

At room temperature, why do carriers within LED PN junction generate heat along with light?

1. Electrons in n-type section. Under positive bias (the normal operating condition for LED Street Light), the negative potential connected to n-type section will continuously provide electrons to PN junction. In matter of fact, most of the electrons provided by the negative potential will be “swallowed” in n-type section. 80%-90% electrons will be captured by nucleus or collide with other electrons. Once electrons were caught by other nucleus, they would lose the original speed given by external electric field. Part of them convert into heat energy, the other part is transferred to adjacent electrons as kinetic energy. Then these “new freed” electrons will repeat their predecessors’ destiny. A substantial amount of heat will be generated during the constant repetition of this process. Thus we can see that most of the energy provided by external electric field converts into heat energy of nucleus at last.
2. Electrons in space charge region (PN junction). There is an internal electric field in space charge region which is opposite to external electric field. Therefore,LED Tube internal electric field will created an inverse force on electrons as opposite as external electric field dose. However, since the forward force generated by external electric field is much greater than the backward force generated by internal electric field. Electrons are still impelled in the original direction in LED PN junction. In the view of LED Products energy transmission, internal electric field creates negative work on electrons. In other words, PN junction absorbs kinetic energy from electrons and converts them into heat energy. The source of electrons’ kinetic energy is the external electric field. On the both sides of space charge region, there exists an energy level difference. Electrons need to increase their energy level as well as overcome the PN junction resistance. The energy applied to increasing the energy level is from nucleus within LED PN junction. Since energy is absorbed by electrons, the vibration amplitude of nucleus will reduce, leading to a temperature reduction. However this “cooling down” is trivial compare with the heat generated by the negative work from the internal electric field. On the whole, LED PN junction will generate a large amount of heat in their work.