Why is MOS tube broken down by static electricity, how should we deal with it?

MOS tube is an ESD sensitive device. Its input resistance is very high, and the capacitance between the gate and source is very small, so it is easily charged by external electromagnetic field or static electricity. A small amount of charge may form a high level on the interelectrode capacitance. The voltage (think U=Q/C) damages the tube, and it is difficult to discharge the charge in the case of strong static electricity, which is easy to cause electrostatic breakdown. There are two ways of electrostatic breakdown: one is voltage type, that is, the gate The thin oxide layer breaks down, forming pinholes, shorting between the gate and the source, or shorting between the gate and the drain; the second is the power type, that is, the metallized film aluminum strip is blown, causing the gate to open or The source is open. Like the MOS transistor, the JFET has a high input resistance, but the input resistance of the MOS transistor is higher.

The electrostatic discharge forms a short-time large current, and the time constant of the discharge pulse is much smaller than the time constant of the heat dissipation of the device. Therefore, when the ESD current passes through a small pn junction or Schottky junction, a large instantaneous power density will be generated, resulting in local overheating, which may cause the local junction temperature to reach or exceed the intrinsic temperature of the material (such as silicon). The melting point of 1415 ° C), the partial or multiple melting of the junction caused the pn junction to short circuit, the device completely failed. The occurrence of such failure depends mainly on the power density of the internal region of the device. The smaller the power density, the less susceptible the device is to damage.

The reverse biased pn junction is more prone to thermal failure than the positively biased pn junction. The energy required to damage the junction under reverse bias conditions is only about one tenth of that under positive bias conditions. This is because most of the power is consumed in the center of the junction when reverse biasing, and more is consumed in the bulk resistor outside the junction. For bipolar devices, the area of ​​the emitter junction is typically smaller than the area of ​​the other junctions, and the junction is also closer to the surface than the other junctions, so degradation of the emitter junction is often observed. In addition, pn junctions with breakdown voltages above 100V or leakage currents less than 1nA (such as JFET gate junctions) are more sensitive to electrostatic discharge than conventional pn junctions of similar size.

All things are relative, not absolute. MOS tubes are only sensitive to other devices. ESD has a big feature that is random, and it can not be broken without touching the MOS tube. In addition, even if ESD is generated, the tube will not necessarily be broken down. The basic physical characteristics of static electricity are: (1) there is a force of attraction or repulsion; (2) there is an electric field, and there is a potential difference with the earth; (3) a discharge current is generated. Usually these three cases (ESD) will affect the electronic components in the following three cases: (1) element to attract dust, the change in impedance between the lines, affect the functionality and lifetime of the element; (2) element by an electric field or a current insulating layer destruction And the conductor, so that the component can not work (complete destruction); (3) due to the instantaneous electric field soft breakdown or current overheating, the component is injured, although still working, but the life is impaired. Therefore, ESD damage to the MOS tube may be one or three. It is not always the second case. In the above three cases, if the components are completely destroyed, they must be detected and eliminated in the production and quality tests with less impact. If the component is slightly damaged, it is not easy to be found in normal tests. In this case, it is often found to be damaged after repeated processing or even when it is used. Not only is the inspection difficult, but the loss is also difficult to predict. Static electricity is as harmful to electronic components as serious fire and explosion accidents.

Under what circumstances will electronic components and products suffer from electrostatic damage? It can be said that electronic products are subject to electrostatic damage from the production to the use. From device manufacturing to plug-in soldering, machine assembly, packaging and transportation to product application, it is under the threat of static electricity. In the entire electronic product production process, each small step in each stage, electrostatic sensitive components may be affected or damaged by static electricity, but in fact the most important and easily overlooked is the transmission and transportation of components. the process of. In this process, transportation is easily damaged by the static electricity generated by the external electric field (such as passing near high-voltage equipment, frequent worker movement, rapid vehicle movement, etc.), so the transmission and transportation process need special attention to reduce losses and avoid indifferent Dispute. Protected by a Zener regulator.

The current mos tube is not so easy to be broken down, especially the high-powered vmos, mainly with diode protection. The vmos gate capacitance is large and no high voltage can be sensed. Unlike the dry north, the south is not sensitive to static electricity. There is now an increase in IO port protection inside most CMOS devices. However, it is not a good habit to directly touch the CMOS device pins by hand. At least the solderability of the pins is deteriorated.

First, the input resistance of the MOS transistor itself is very high, and the capacitance between the gate and the source is very small, so it is easily charged by the external electromagnetic field or static electricity, and a small amount of charge can form a relatively high voltage on the interelectrode capacitance. (U=Q/C), the tube is damaged. Although the MOS input has antistatic protection measures, it should be treated with care. It is best to use metal containers or conductive materials for storage and transportation. Do not put them in chemical materials or chemical fiber fabrics that are prone to static electricity. Tools, instruments, workbench, etc. should be well grounded during assembly and commissioning. To prevent damage caused by static interference from the operator, if it is not suitable to wear nylon or chemical fiber clothes, it is best to pick up the ground before touching the manifold. When straightening or manually soldering the device leads, the equipment used must be well grounded.

Second, the protection diode at the input end of the MOS circuit generally has a current tolerance of 1 mA. When an excessive transient input current (more than 10 mA) may occur, the input protection resistor should be connected in series. Therefore, a MOS tube with a protective resistor inside can be selected for application. Also, due to the limited instantaneous energy absorbed by the protection circuit, too large an instantaneous signal and an excessively high electrostatic voltage will disable the protection circuit. Therefore, the soldering iron must be grounded reliably during soldering to prevent leakage current from penetrating the input end of the device. In general use, after the power is turned off, the residual heat of the soldering iron can be used for soldering, and the grounding pin is soldered first.

MOS is a voltage-driven component that is sensitive to voltage. The floating G is easy to accept external interference to turn on the MOS. The external interference signal charges the GS junction capacitor. This tiny charge can be stored for a long time. In the test, G is very dangerous. Many of them are bursting like this. G is connected to a pull-down resistor to the ground. The bypass interference signal will not pass through. Generally, it can be 10~20K. This resistor is called the gate resistor. It acts as a bias voltage for the FET; it acts as a bleeder resistor (protects the gate G~source S). The first function is well understood. Here is the principle of the second action: protection gate G~source S: the resistance between the GS poles of the FET is very large, so that as long as there is a small amount of static electricity, The equivalent capacitance between his GS poles generates a very high voltage at both ends. If these small amounts of static electricity are not released in time, the high voltage at both ends may cause the FET to malfunction, or even break through it. GS pole; at this time, the resistance between the gate and the source can discharge the above-mentioned electrostatic diarrhea, thereby protecting the field effect tube.