Why does static electricity break down the MOS tube?

In fact, the MOS tube is an ESD sensitive device. Its input resistance is very high, and the capacitance between the gate and the source is very small, so it is very easy to be charged by the external electromagnetic field or static electricity. Discharging charges can easily cause electrostatic breakdown.

There are two ways of electrostatic breakdown:

One is the voltage type , that is, the thin oxide layer of the gate breaks down, forming pinholes, shorting the gate and the source, or shorting the gate and the drain; the other is the power type, that is, the metallized thin

film aluminum strip Fuse, resulting in open gate or open source.

The basic physical characteristics of static electricity are:

1) have the power of attraction or repulsion;

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

3) A discharge current will be generated.

These three situations, that is, ESD, generally affect electronic components in the following three situations:

1) The component absorbs dust, changes the impedance between lines, and affects the function and life of the component;

2) The insulation layer and conductor of the component are damaged due to the electric field or current, so that the component cannot work (completely destroyed);

3) Due to the instantaneous soft breakdown of the electric field or overheating caused by the current, the components are injured. Although they can still work, their lifespan is damaged.

Therefore, the damage to the MOS tube by ESD may be one or three 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 during production and quality testing, and the impact is relatively small.

If the component is slightly damaged, it is not easy to be found in normal testing. In this case, it is often found that the damage is due to repeated processing, even when it is already in use. Not only is it difficult to inspect, but the loss is also unpredictable. The harm of static electricity to electronic components is no less than the loss of serious fire and explosion accidents.

Under what circumstances will electronic components and products be damaged by static electricity?

It can be said that electronic products are threatened by electrostatic damage in the whole process from production to use. From device manufacturing to plug-in assembly and welding, complete machine assembly, packaging and transportation to product application, they are all under the threat of static electricity.

In the entire production process of electronic products, every small step in every stage, electrostatic sensitive components may be affected or damaged by static electricity, but in fact the most important and easily overlooked point is the transmission and transportation of components the process of.

During this process, transportation is damaged due to the static electricity generated by the movement easily exposed to the external electric field (such as passing near high-voltage equipment, frequent movement of workers, rapid movement of vehicles, etc.), so special attention should be paid to the transmission and transportation process to reduce losses and avoid indifference disputes. For protection, add a Zener voltage regulator tube for protection.

The current mos tubes are not so easy to be broken down, especially the high-power vmos, mainly because many of them have diode protection.

The vmos gate has a large capacitance and cannot sense high voltage. Unlike the dry north, the wet south is not prone to static electricity. In addition, IO port protection has been added inside most CMOS devices. But it is not a good habit to directly touch the pins of CMOS devices with your hands. At the very least it makes the pins less solderable.
 

Reasons and solutions for the breakdown of MOS tubes

First, the input resistance of the MOS tube itself is very high, and the capacitance between the gate and the source is very small, so it is very easy to be charged by the induction of the external electromagnetic field or static electricity, and a small amount of charge can form a fairly high voltage on the inter-electrode capacitance (U=Q/C), damage the tube.

Although the MOS input terminal has anti-static protection measures, it still needs to be treated with care. It is best to use metal containers or conductive materials for packaging during storage and transportation, and do not place it in chemical materials or chemical fiber fabrics that are prone to static high voltage.

When assembling and debugging, tools, instruments, workbenches, etc. should be well grounded. To prevent the operator from being damaged by electrostatic interference, it is not advisable to wear nylon or chemical fiber clothes, and it is best to connect the hands or tools to the ground before touching the integrated block. When straightening and bending device leads or manually soldering, the equipment used must be well grounded.

Second, the protection diode at the input end of the MOS circuit has an on-time current tolerance of 1mA. When an excessive transient input current (over 10mA) may occur, an input protection resistor should be connected in series. Therefore, a MOS tube with an internal protection resistor can be selected during application.

Also, due to the limited instantaneous energy absorbed by the protection circuit, too large instantaneous signal and excessive electrostatic voltage will make the protection circuit useless.

Therefore, the soldering iron must be reliably grounded during soldering to prevent leakage from breaking down the input end of the device. In general use, the residual heat of the soldering iron can be used for soldering after the power is turned off, and the grounding pin should be soldered first.

MOS is a voltage-driven element, which is very 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 capacitance, and this tiny charge can be stored for a long time.

In the test, G hanging in the air is very dangerous. Many tubes burst because of this. G is connected to a pull-down resistor to the ground, and the bypass interference signal will not pass through. Generally, it can be 10~20K.

This resistance is called gate resistance, function 1: provide bias voltage for the field effect tube; function 2: play the role of discharge resistance (protect gate G ~ source S).

The first function is easy to understand. Here is an explanation of the principle of the second function: protection gate G~source S: the resistance value between the GS electrodes of the field effect transistor is very large, so that as long as there is a small amount of static electricity, it can make the GS electrodes of the FET equal to each other. A high voltage is developed across the effective capacitor.

If these small amounts of static electricity are not released in time, the high voltage at both ends may cause the field effect tube to malfunction, and may even break down its GS pole; at this time, the resistance added between the gate and the source can be The above-mentioned electrostatic discharge is released, thereby playing the role of protecting the field effect tube.