Generation mechanism of static electricity 

Usually, static electricity is generated due to friction or induction.

Frictional static electricity is generated by the movement of electrical charges generated during contact, friction, or separation between two objects. The static electricity left by friction between conductors is usually relatively weak, due to the strong conductivity of the conductors. The ions generated by friction will quickly move together and neutralize during and at the end of the friction process. After friction of the insulator, a higher electrostatic voltage may be generated, but the amount of charge is very small. This is determined by the physical structure of the insulator itself. In the molecular structure of an insulator, it is difficult for electrons to move freely free from the binding of the atomic nucleus, so friction results in only a small amount of molecular or atomic ionization.

Inductive static electricity is an electric field formed by the movement of electrons in an object under the action of an electromagnetic field when the object is in an electric field. Inductive static electricity can generally only be generated on conductors. The effect of spatial electromagnetic fields on insulators can be ignored.

Electrostatic discharge mechanism

What is the reason why 220V mains electricity can kill people, but thousands of volts on people can’t kill them? The voltage across the capacitor meets the following formula: U=Q/C. According to this formula, when the capacitance is small and the amount of charge is small, a high voltage will be generated. “Usually, the capacitance of our bodies and objects around us is very small. When an electric charge is generated, a small amount of electric charge can also generate a high voltage.”. Due to the small amount of electric charge, when discharging, the generated current is very small, and the time is very short. The voltage cannot be maintained, and the current drops in an extremely short time. “Because the human body is not an insulator, the static charges accumulated throughout the body, when there is a discharge path, will converge. Therefore, it feels like the current is higher and there is a sense of electric shock.”. After static electricity is generated in conductors such as human bodies and metal objects, the discharge current will be relatively large.

For materials with good insulation properties, one is that the amount of electric charge generated is very small, and the other is that the generated electric charge is difficult to flow. Although the voltage is high, when there is a discharge path somewhere, only the charge at the contact point and within a small range nearby can flow and discharge, while the charge at the non contact point cannot discharge. Therefore, even with a voltage of tens of thousands of volts, the discharge energy is also negligible.

Hazards of static electricity to electronic components

Static electricity can be harmful to LEDs, not just LED’s unique “patent”, but also commonly used diodes and transistors made of silicon materials. Even buildings, trees, and animals can be damaged by static electricity (lightning is a form of static electricity, and we won’t consider it here).

So, how does static electricity damage electronic components? I don’t want to go too far, just talking about semiconductor devices, but also limited to diodes, transistors, ICs, and LEDs.

The damage caused by electricity to semiconductor components ultimately involves current. Under the action of electric current, the device is damaged due to heat. If there is a current, there must be a voltage. However, semiconductor diodes have PN junctions, which have a voltage range that blocks current both in the forward and reverse directions. The forward potential barrier is low, while the reverse potential barrier is much higher. In a circuit, where the resistance is high, the voltage is concentrated. But for LEDs, when the voltage is applied forward to the LED, when the external voltage is less than the threshold voltage of the diode (corresponding to the material band gap width), there is no forward current, and the voltage is all applied to the PN junction. When the voltage is applied to the LED in reverse, when the external voltage is less than the reverse breakdown voltage of the LED, the voltage is also applied to the PN junction entirely. At this time, there is no voltage drop in either the faulty solder joint of the LED, the bracket, the P area, or the N area! Because there is no current. After the PN junction is broken down, the external voltage is shared by all the resistors on the circuit. Where the resistance is high, the voltage borne by the part is high. As far as LEDs are concerned, it is natural that the PN junction bears most of the voltage. The thermal power generated at the PN junction is the voltage drop across it multiplied by the current value. If the current value is not limited, excessive heat will burn out the PN junction, which will lose its function and penetrate.

Why are ICs relatively afraid of static electricity? Because the area of each component in an IC is very small, the parasitic capacitance of each component is also very small (often the circuit function requires very small parasitic capacitance). Therefore, a small amount of electrostatic charge will generate a high electrostatic voltage, and the power tolerance of each component is usually very small, so electrostatic discharge can easily damage the IC. However, ordinary discrete components, such as ordinary small power diodes and small power transistors, are not very afraid of static electricity, because their chip area is relatively large and their parasitic capacitance is relatively large, and it is not easy to accumulate high voltages on them in general static settings. Low power MOS transistors are prone to electrostatic damage due to their thin gate oxide layer and small parasitic capacitance. They usually leave the factory after short-circuiting the three electrodes after packaging. In use, it is often required to remove the short route after welding is completed. Due to the large chip area of high-power MOS transistors, ordinary static electricity will not damage them. So you will see that the three electrodes of power MOS transistors are not protected by short circuits (early manufacturers still short circuited them before leaving the factory).

An LED actually has a diode, and its area is very large relative to each component within the IC. Therefore, the parasitic capacitance of LEDs is relatively large. Therefore, static electricity in general situations cannot damage LEDs.

Electrostatic electricity in general situations, especially on insulators, can have a high voltage, but the amount of discharge charge is extremely small, and the duration of the discharge current is very short. The voltage of the electrostatic charge induced on the conductor may not be very high, but the discharge current may be large and often continuous. This is very harmful to electronic components.

Why does static electricity damage LED chips not often occur

Let’s start with an experimental phenomenon. A metal iron plate carries 500V static electricity. Place the LED on the metal plate (pay attention to the placement method to avoid the following problems). Do you think the LED will be damaged? Here, to damage an LED, it should usually be applied with a voltage greater than its breakdown voltage, which means that both electrodes of the LED should simultaneously contact the metal plate and have a voltage greater than the breakdown voltage. As the iron plate is a good conductor, the induced voltage across it is equal, and the so-called 500V voltage is relative to the ground. Therefore, there is no voltage between the two electrodes of the LED, and naturally there will be no damage. Unless you contact one electrode of an LED with an iron plate, and connect the other electrode with a conductor (hand or wire without insulating gloves) to ground or other conductors.

The above experimental phenomenon reminds us that when an LED is in an electrostatic field, one electrode must contact the electrostatic body, and the other electrode must contact the ground or other conductors before it can be damaged. In actual production and application, with the small size of LEDs, there is rarely a chance that such things will happen, especially in batches. Accidental events are possible. For example, an LED is on an electrostatic body, and one electrode contacts the electrostatic body, while the other electrode is just suspended. At this time, someone touches the suspended electrode, which may damage the LED Light.

The above phenomenon tells us that electrostatic problems cannot be ignored. Electrostatic discharge requires a conductive circuit, and there is no harm if there is static electricity. When only a very small amount of leakage occurs, the problem of accidental electrostatic damage can be considered. If it occurs in large quantities, it is more likely to be a problem of chip contamination or stress.