Why does static electricity penetrate a MOS tube?

   Electrostatic breakdown can be performed in two ways:   

One is voltage type, that is, the thin oxide layer of the grid breakdown, the formation of pinhole, short circuit between the grid and the source, or short circuit between the grid and the drain;

The second is power type, that is, the metallized film aluminum strip is fused, resulting in open gate or open source.
 

Nowadays MOS tubes are not so easy to be broken down, especially high-power Vmos, mainly because many of them are protected by diodes. Vmos gate capacitance is large, no high voltage induction. If you encounter 3DO MOS tube in winter without antistatic ring try, basically touch one hanging one.

Unlike the dry north, the wet south is less prone to static electricity. Also, most CMOS devices now have added IO port protection inside. However, it is not a good habit to touch CMOS device pins directly with your hands. At the very least, it makes the pins less solderable.

Electrostatic discharge generates a short-time large current, and the time constant of the discharge pulse is much smaller than the time constant of the device heat dissipation. Therefore, when the electrostatic discharge current through a small area of PN junction or Schottky junction, will produce a large instantaneous power density, the formation of local overheating, it is possible to make the local junction temperature reach or even exceed the intrinsic temperature of the material (such as silicon melting point 1415℃), the junction area of local or multiple melting lead to pn junction short circuit, the device completely failure. Whether this failure occurs or not depends on the power density of the internal region of the device. The smaller the power density is, the less vulnerable the device is to damage.

Thermal failure of reverse biased PN junction is more likely than that of positive biased PN junction, and the energy required to damage the junction under reverse biased condition is only about one tenth of that under positive biased condition. This is because most of the power is consumed in the center of the junction region in the case of negative bias, while most of the power is consumed in the volume resistance outside the junction region in the case of positive bias. For bipolar devices, the area of the emitting junction is usually smaller than that of the other junctions, and the junction is closer to the surface than the other junctions, so degradation of the emitting junction is often observed. In addition, PN junctions with breakdown voltages higher than 100V or leakage currents less than 1nA (such as JFET gate junctions) are more sensitive to electrostatic discharges than conventional PN junctions of similar size.

All things are relative, not absolute. MOS tube is just more sensitive than other devices. ESD has a great feature of randomness, and it can not be broken down without touching MOS tube. In addition, even if it produces ESD, it will not necessarily break through the pipe.

The basic physical characteristics of static electricity are as follows:

(1) have the power to attract or repel;

(2) there is an electric field, and there is a potential difference with the earth;

(3) Discharge current will be generated.

These situations, namely ESD, will generally affect electronic components in the following three situations:

(1) Components absorb dust, change the impedance between lines, affect the function and life of components;

(2) The insulation layer and conductor of the element are damaged by electric field or current, so that the element cannot work (complete damage);

(3) Due to the instantaneous soft breakdown of the electric field or overheating of the current, the component is injured, although it can still work, but the life is damaged. So ESD damage to MOS tube may be one, three or two cases, not necessarily the second case every time. In the above three cases, if the component is completely damaged, it must be detected and eliminated in production and quality testing with less impact. If the component is slightly damaged, it is not easy to detect in normal testing. In this case, it is often discovered after repeated processing, even when it is in use. It is not only difficult to inspect, but also difficult to predict the loss. The damage caused by static electricity to electronic components is no less than that caused by serious fire and explosion accidents.

Under what circumstances are electronic components and products subject to electrostatic damage? It can be said that electronic products are threatened by static electricity from production to use. From device manufacturing to plug-in assembly and welding, machine assembly, packaging and transportation until product application, are under the threat of static electricity. In the whole process of electronic product production, every small step in every stage, electrostatic sensitive elements may be affected or damaged by static electricity, but in fact, the most important and easy to neglect is in the process of component transmission and transportation. In this process, transportation is damaged by static electricity generated by mobile exposure to external electric fields (such as near high-voltage equipment, frequent movement of workers, rapid movement of vehicles, etc.). Therefore, special attention should be paid to the transmission and transportation process to reduce losses and avoid unnecessary disputes. Protection plus zener regulator pipe protection.
 

Nowadays MOS tubes are not so easy to be broken down, especially high-power Vmos, mainly because many of them are protected by diodes. Vmos gate capacitance is large, no high voltage induction. Unlike the dry north, the wet south is less prone to static electricity. Also, most CMOS devices now have added IO port protection inside. However, it is not a good habit to touch CMOS device pins directly with your hands. At the very least, it makes the pins less solderable.

   MOS tube breakdown causes and solutions   
 

First, MOS tube itself has a very high input resistance, and the inter-gate source capacitance is very small, so it is easy to be charged by the induction of external electromagnetic field or static electricity. However, a small charge can form a very high voltage (U=Q/C) on the inter-pole capacitance, which will damage the tube. Although MOS input terminal has antistatic protection measures, it still needs to be treated with care. It is best to use metal containers or conductive materials in storage and transportation, and do not put them in chemical materials or chemical fiber fabrics that are prone to electrostatic high voltage. During assembly and debugging, tools, meters and workstations should be well grounded. To prevent the damage caused by the electrostatic interference of the operator, such as nylon, chemical fiber clothes should not be worn, hands or tools should be connected to the ground before contacting the integrated block. When straightening and bending device leads or soldering them manually, the equipment used must be well grounded.

 

Second, MOS circuit input protection diode, its current tolerance is generally 1mA, in the possibility of large transient input current (more than 10mA), should be connected to the input protection resistance. Therefore, a MOS tube with internal protection resistance can be selected for application. Because of the limited instantaneous energy absorbed by the protection circuit, too large instantaneous signal and too high electrostatic voltage will make the protection circuit lose its effect. Therefore, during welding, the electric iron must be reliably grounded to prevent leakage from breaking down the input end of the device. In general use, the waste heat of the electric iron can be used for welding after power failure, and the grounding pin can be welded first.

MOS is a voltage driving element, which is very sensitive to voltage. The suspended G can easily accept external interference to make the MOS turn on, and the external interference signal can charge the G-S junction capacitor, and this tiny charge can be stored for a long time. In the test, G hanging is very dangerous, many because of such tube explosion, G is connected with a pulldown resistance to the ground, the bypass interference signal will not be straight through, generally can be 10~20K. This resistor is called the gate resistor, and its function is 1: to provide bias voltage for the MOSFEts; Function 2: play the role of discharge resistance (protection grid G~ source S). The first function is easy to understand, here to explain the principle of the second function: protection gate G~ source S: The resistance value between g-S poles of the MOSFEtt is very large, so as long as there is a small amount of static electricity, it can produce a very high voltage at both ends of the equivalent capacitance between g-S poles. If the small amount of static electricity is not discharged in time, the high voltage at both ends of the MOSFEtt may cause the misoperation of the MOSFEtt, and even may break down its G-S pole. At this time, the resistance between the gate and the source can discharge the electrostatic discharge, thus playing a role in protecting the field effect tube.

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