Why does high input resistance of MOS tube fail against static electricity

MOS transistor has a very high input resistance. However, it is also very sensitive to electrostatic discharge (ESD) due to its high input resistance and very small gate-source capacitance. The MOS transistor can easily become charged when exposed to external electromagnetic fields or static electricity. Moreover, in situations with strong static electricity, it is difficult to discharge the accumulated charge, which can lead to static discharge breakdown.

MOS transistor

There are generally two types of electrostatic breakdown:
The first one is voltage type, where the thin oxide layer of the gate electrode breaks down, forming pinholes and short-circuiting between the gate electrode and the source electrode, or between the gate electrode and the drain electrode.

The second one is power type, where the metalized thin film aluminum strip is melted, causing either an open circuit between the gate electrode and the source electrode or an open circuit between the gate electrode and the drain electrode.

Reasons and solutions for the breakdown of MOSFET?
Firstly, the input resistance of the MOSFET is very high, while the capacitance between the gate and source terminals is very small. Therefore, it is highly susceptible to the induction of external electromagnetic fields or static electricity, and even a small amount of charge can cause a significant voltage to form across the capacitance (U=Q/C), leading to damage to the transistor.

Although the MOSFET’s input terminal has protection against static electricity, it still requires careful handling. It is best to use metal containers or packaging materials with conductive properties for storage and transportation, and avoid placing them in environments with materials or fabrics that can generate static high voltage, such as chemical materials or synthetic fibers.

During assembly and debugging, tools, instruments, workbenches, etc., should all be properly grounded. It is important to prevent damage caused by static interference from the operator. It is not advisable to wear nylon or synthetic fiber clothing. It is also recommended to ground the hand or tool before touching the integrated circuit. When straightening or bending the device leads or performing manual soldering, the equipment used must be properly grounded.

Second, the protection diode on the input end of the MOS circuit has a current limit of generally 1mA when it is conducting. When there is a possibility of excessive transient input current (more than 10mA), an input protection resistor should be connected in series. Therefore, when applying, a MOS tube with an internal protection resistor can be selected.

Furthermore, since the protection circuit can only absorb a limited amount of instantaneous energy, excessive instantaneous signals and excessively high static voltages will render the protection circuit ineffective. Therefore, during soldering, the soldering iron must be reliably grounded to prevent the device’s input end from being damaged by leakage current. When using, the soldering can be done using the residual heat of the soldering iron after powering off, and the grounding pins should be soldered first.

What is the role of the MOS gate-source (G-S) pull-down resistor?
MOS is a voltage-driven device that is sensitive to voltage. The floating gate (G) is easily influenced by external interference, causing the MOS to conduct. The external interference signal charges the G-S junction capacitance, and this small charge can be stored for a long time.

MOS gate-source (G-S) connect way

In the experiment, it is very dangerous for G to be suspended, as many pipes have burst due to this reason. By adding a pull-down resistor to the ground, bypass interference signals will not pass through directly. The resistor is typically around 10~20K and is called a gate resistor.
Function 1: Provides bias voltage for field-effect transistors.
Function 2: Acts as a bleeder resistor to protect the gate G and the source S.

The first function is easy to understand. Here, let’s explain the principle of the second function. The resistance between the gate G and the source S of a field-effect transistor is very large. Therefore, even a small amount of static electricity can generate a very high voltage across the equivalent capacitance between the G-S terminals.

If these small amounts of static electricity are not discharged in a timely manner, the high voltage at both ends can cause the field-effect transistor to malfunction or even breakdown the G-S terminals. The resistor added between the gate and the source can discharge the static electricity mentioned above, thereby protecting the field-effect transistor.

LISUN ESD simulator guns (Electrostatic Discharge Generator/Electrostatic Gun/ESD Guns) is in full compliance with IEC 61000-4-2, EN61000-4-2, ISO10605, GB/T17626.2, GB/T17215.301 and GB/T17215.322.

ESD61000-2_Electrostatic Discharge Simulator

What is a ESD test?
The static electricity generated by the human body to the object or between two objects may cause the electrical and electronic equipment circuits to malfunction or even be damaged. The ESD generator is designed for the withstanding ESD performance measurement for the assessment of electrical and electronic equipment. ESD61000-2/ESD61000-2A has a LCD touch screen in both English and Chinese. 

What is an esd simulator gun used for?
Electrostatic discharge simulator is highest electrostatic voltage can be up to 30 kv, which is enough to cover the most severe standard grade of electrostatic voltage requirement (the voltage requirements of the grade 4 air discharge electrostatic is 15 KV). The ESD test gun can be used for the most of electrical and electronic equipment for electrostatic discharge test, and also can ensure comparability and reproducibility of test.