Analysis of the Causes of MOS Tube Breakdown by Static Electricity

VBsemi

Analysis of the Causes of MOS Tube Breakdown by Static Electricity [Copy link]

 

MOS tube is an ESD sensitive device. Its input resistance is very high, and the gate-source capacitance is very small. Therefore, it is very easy to be charged by the induction of external electromagnetic field or static electricity. It is difficult to discharge the charge in the presence of strong static electricity, which 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, causing a short circuit between the gate and the source, or between the gate and the drain;

The second is the power type, that is, the metallized film aluminum strip is melted, causing the gate to be open or the source to be open.

Nowadays, MOS tubes are not so easy to be broken down, especially high-power VMOS tubes, mainly because many of them are protected by diodes. VMOS gate capacitance is large and cannot sense high voltage. If you encounter a 3DO type MOS tube in winter without an anti-static ring, basically one will be broken.

Unlike the dry north, the south is humid and less prone to static electricity. In addition, most CMOS devices now have added IO port protection. But it is not a good habit to touch the pins of CMOS devices directly with your hands. At least it will make the pins less solderable.

Electrostatic discharge forms a short-term 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 passes through a small pn junction or Schottky junction, it will generate a large instantaneous power density, resulting in local overheating, which may cause the local junction temperature to reach or even exceed the intrinsic temperature of the material (such as the melting point of silicon 1415℃), causing the junction area to melt locally or multiple places, resulting in a pn junction short circuit and complete device failure. Whether this failure occurs or not mainly depends on the power density in the internal area of the device. The smaller the power density, the less susceptible the device is to damage.

Reverse biased pn junctions are more susceptible to thermal failure than forward biased pn junctions. The energy required to damage the junction under reverse bias conditions is only about one-tenth of that under forward bias conditions. This is because most of the power is consumed in the center of the junction area when reverse biased, while it is consumed in the body resistance outside the junction area when forward biased. For bipolar devices, the emitter junction area is usually smaller than the area of other junctions, and the junction surface is closer to the surface than other junctions, so the 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 the gate junction of a JFET) are more sensitive to electrostatic discharge than conventional pn junctions of similar size.

Everything is relative, not absolute. MOS tubes are only more sensitive than other devices. One of the major characteristics of ESD is randomness. It does not mean that MOS tubes can be broken down without being touched. In addition, even if ESD is generated, it does not necessarily break down the tube.

The basic physical characteristics of static electricity are:

(1) Having the power to attract or repel;

(2) There is an electric field, which has a potential difference with the earth;

(3) A discharge current will be generated.

These three situations, namely ESD, generally cause the following three situations to affect electronic components:

(1) The components absorb dust, changing the impedance between circuits and affecting the function and life of the components;

(2) The electric field or current destroys the insulation layer and conductor of the component, making the component unable to work (complete destruction);

(3) Due to instantaneous soft breakdown of the electric field or overheating caused by the current, the component is damaged. Although it can still work, its life is shortened.

Therefore, ESD may damage MOS tubes in one, three or two cases, not necessarily in the second case every time. Among the above three cases, if the component is completely damaged, it will be detected and eliminated during production and quality testing, and the impact will be less. If the component is slightly damaged, it is not easy to be found in normal testing. In this case, it is often discovered that it has been damaged after multiple processing or even in use. Not only is it difficult to check, but the loss is also difficult to predict. The harm caused by static electricity to electronic components is no less than the loss caused by serious fire and explosion accidents.

Under what circumstances will electronic components and products be damaged by static electricity? It can be said that the entire process of electronic products from production to use is threatened by static electricity. From device manufacturing to plug-in welding, whole machine assembly, packaging and transportation to product application, they are all under the threat of static electricity. In the entire electronic product production process, every small step in every stage, static sensitive components may be affected or damaged by static electricity, but in fact, the most important and easily overlooked point is the process of component transmission and transportation. In this process, transportation is easily exposed to external electric fields (such as passing near high-voltage equipment, frequent workers, rapid vehicle movement, etc.) due to movement, which generates static electricity and is damaged. Therefore, special attention should be paid to the transmission and transportation process to reduce losses and avoid unnecessary disputes. If you want protection, add Zener voltage regulator 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 and cannot sense high voltage. Unlike the dry north, the south is humid and not easy to generate static electricity. In addition, most CMOS devices now have added IO port protection. But it is not a good habit to touch the pins of CMOS devices directly with your hands. At least it will make the pins less solderable.

Causes and solutions for MOS tube breakdown:

First, the input resistance of the MOS tube itself is very high, and the gate-source capacitance is very small, so it is very easy to be charged by the induction of external electromagnetic fields or static electricity, and a small amount of charge can form a very high voltage (U=Q/C) on the inter-electrode capacitance, which will damage the tube. Although the MOS input end has anti-static protection measures, it still needs to be treated with caution. 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. During assembly and debugging, tools, instruments, workbenches, etc. should be well grounded. To prevent damage caused by electrostatic interference of operators, it is not advisable to wear nylon or chemical fiber clothes, and it is best to connect hands or tools to the ground before touching the integrated block. When straightening and bending the device leads or manually welding, 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 1mA when it is on. When there may be excessive transient input current (over 10mA), an input protection resistor should be connected in series. Therefore, a MOS tube with an internal protection resistor can be selected for application. In addition, since the instantaneous energy absorbed by the protection circuit is limited, too large instantaneous signals and excessively high static voltage will render the protection circuit ineffective. Therefore, the soldering iron must be reliably grounded during welding to prevent leakage from breaking through the input end of the device. In general use, the residual heat of the soldering iron can be used for welding after the power is turned off, and the ground pin should be welded first.

MOS is a voltage-driven element and is very sensitive to voltage. The suspended G is easily susceptible to external interference, causing the MOS to conduct. The external interference signal charges the GS junction capacitance, and this tiny charge can be stored for a long time. In the experiment, it is very dangerous to leave G suspended. 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 directly. It can generally be 10~20K. This resistor is called the gate resistor. Function 1: Provide bias voltage for the field effect tube; Function 2: Act as a discharge resistor (protect the gate G~source S). The first function is easy to understand. Here is an explanation of the principle of the second function: Protect the gate G~source S: The resistance value between the GS poles of the field effect tube is very large, so as long as there is a small amount of static electricity, a very high voltage can be generated at both ends of the equivalent capacitor between its GS poles. If these small amounts of static electricity are not discharged 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 resistor added between the gate and the source can discharge the above-mentioned static electricity, thereby protecting the field effect tube.

FR5305irf3205IRF540 IRF630 IRF840

langtuodianzi

It is related to the structure and material of the MOS tube. The input impedance of the MOS tube is particularly high, and the gate insulation layer is very thin. Under the combined effect of these two factors, MOS is very sensitive to ESD.