TVS Diodes (ESD Protection Devices)What is Static Electricity? Basic Knowledge and ESD Countermeasures

Here, we will provide an easy-to-understand explanation of the basics of static electricity, including its fundamental mechanism, the strength of static electricity voltage, and its relationship with humidity, etc.

All matter is made of atoms, which consist of nuclei (mainly protons) carrying a positive (+) electric charge^*1 and electrons carrying a negative (-) electric charge. Electric charge refers to the amount of electricity (quantity of electricity) possessed by matter or particles.

As shown in Figure 1, when the electric charge amounts of the nuclei (+) and the electrons (-) are equal, that substance is electrically neutral (neutralized state). However, if electrons (-) migrate and change in number—for example, due to contact with a different substance—the substance gains a positive (+) electric charge, that is to say, it becomes electrified.
Static electricity refers to the state in which electric charges are unevenly accumulated in an object due to this migration of electrons.

Different substances vary in terms of whether they acquire a positive (+) or negative (-) polarity and in terms of the degree of electrification. See Triboelectric Series - Electrification Properties by Substance below for details.

Most people have probably experienced an electric shock due to static electricity when touching a metal doorknob or similar object. This occurs because, when a human body that is charged with static electricity (electrified) touches a substance such as metal that easily conducts electricity (a conductor), the accumulated electric charge is discharged all at once. It is said that people feel an electric shock starting from around 3.0 kV, and examples of how an electric shock feels depending on the voltage are provided below.

Humidity can become extremely low in indoor locations such as offices and homes where air conditioning is used and especially in manufacturing environments. When relative humidity decreases, the electrification voltage increases, which can lead to static electricity problems such as electrostatic discharge (ESD) damage (explained in Problems due to Static Electricity and ESD Damage below). Therefore, caution is needed.

In the relationship between relative humidity and electrification described above, various actions, substances, and states were given as examples. We will explain here what kinds of factors cause static electricity, differences in electrification polarity among substances, and combinations of substances that are prone to becoming charged with a large amount of electricity.

As explained in Mechanism of Static Electricity at the start of this article, when different substances come into contact with each other, electrons (-) within the substances migrate, and the change in their distribution produces electrification.
In addition to contact between substances, static electricity is also produced for various reasons, as shown in Figure 2. Typical causes are shown below.

While there are various causes of static electricity as described above, the electrification polarity and the magnitude of the voltage differ depending on the substances and their combination. Figure 3 "Triboelectric series" shows the electrification tendency of various substances.

In the triboelectric series shown in Figure 3, substances positioned on the left side are susceptible to positive (+) electrification, and substances positioned on the right side are susceptible to negative (-) electrification. In addition, in combinations of substances that come into contact with each other, a greater distance between the positions on the triboelectric series tends to result in a greater electrification voltage.

As explained in "Strength (Voltage) of Electric Shocks Caused by Static Electricity and Environments where Electrification Easily Occurs" above, even the static electricity felt when a person touches a metal doorknob can generate voltage of 3.0 kV or more. Furthermore, in a low-humidity environment, merely walking on a carpet can electrify a person with a voltage as high as 35.0 kV. Such static electricity risks causing problems in various contexts in daily life, as well as on manufacturing sites. Here, we will introduce examples of problems due to static electricity, the causes of damage due to electrostatic discharge (ESD), and methods for preventing such problems.

Like magnets, substances electrified positively (+) and substances electrified negatively (-) exert an attractive force on each other, and substances electrified with the same polarity exert a repulsive force on each other. These forces are called Coulomb forces.
Static electricity can also lead to malfunction or damage to delicate electronic components such as ICs. Examples of problems caused by this characteristic of static electricity are provided below.

In the processing of items such as films, or before painting a frame or casing, foreign matter (dust) in the air may adhere to the item, items being processed may stick to each other, or automatic parts feeders may become clogged, which can lead to defects or decreased yield.

Misalignment of tiny parts can result in assembly defects, and capsules may be ejected from plastic containers in packaging processes for products such as medicines, thereby leading to defects.

In electronic device assembly processes, electrostatic discharge (ESD) may be applied to circuit boards and damage the ICs. In addition, if ESD is generated from a user's body when they press a button on an electrical product or insert and remove plugs to and from various interfaces such as USB, the ICs and other semiconductor components inside the device may be damaged.

In addition to damage caused by factors such as IC burning due to ESD intrusion, as shown in the photo above, there are also cases where electromagnetic noise generated during electrostatic discharge causes electronic devices to malfunction, or where static electricity leads to ignition, explosion, or fire, etc.

ESD is an abbreviation of "electrostatic discharge" and refers to the phenomenon in which, when static electricity discharges, high voltage and large current are applied to the semiconductor devices in IC circuits, causing malfunction or damage and leading to device failure.
Electrostatic discharges that result in ESD damage are mainly classified into the following three models.

When an electrically charged person's hand touches a conductive part of a circuit board or a semiconductor device, a sudden discharge current occurs and is applied to semiconductor devices such as ICs, causing ESD damage. In addition, discharge from the human body to internal circuits can also occur via the buttons or interfaces of electrical devices.

When a semiconductor device or similar component is in an electrified state due to friction, etc., and a conductive part is grounded^*3, a sudden discharge current flows and ESD damage occurs.

This is a model in which a rapid discharge current occurs when an electrically charged conductor comes into contact with a semiconductor device, leading to ESD damage. For example, when the device side is grounded^*3, discharge occurs if the conductive part of an electrically charged device comes into contact with it. This phenomenon also requires attention in the design of factory automation equipment.

Here, we will describe some measures to combat static electricity problems, with a focus on environmental or object electrification and ESD damage.

Measures to combat static electricity problems based on the above-mentioned Human Body Model generally consist of workers wearing wrist straps grounded with earthing wires to prevent electrification and the use of anti-static mats. This enables prevention of the occurrence of ESD damage.
In addition, a method for preventing ESD damage based on the Charged Device Model or the Machine Model is the use of ionizers (static eliminators) that neutralize accumulated electric charges. By also controlling the humidity, the risk of electrification can be further reduced, leading to the prevention of static electricity problems.

When static electricity is discharged from various input/output interfaces such as power ports and USB, operation buttons, and other points that people touch, that overvoltage can be applied to internal IC circuits, leading to malfunction or damage. This means that there is a risk of ESD damage based on the Human Body Model, even in usage environments such as ordinary households and offices.
A typical countermeasure method is the use of "ESD protection elements" that suppress and divert excessive voltage inflow due to ESD, thereby protecting semiconductor devices such as ICs from the application of overvoltage. By installing ESD protection elements on signal lines leading from various interfaces and operation buttons to ICs, ICs are protected from high-voltage pulses caused by ESD.
"What are ESD Protection Elements? Types of ESD Protection Devices and How They Work" provides a detailed explanation of basic knowledge, types, and advantages of ESD protection elements.

Here, we will explain the circumstances behind the increasing risk of ESD damage in our modern technological environment, and the importance of ESD countermeasures.

In recent years, electronic devices such as PCs, smartphones, and tablets have become essential items for people around the world. These devices are equipped with various interfaces such as USB ports for charging and data communications.
In addition, peripheral gadgets are also becoming more affordable and functional, and products equipped with lithium-ion batteries that support fast charging and high-speed data communications are increasing in number at a rapid pace.
With the spread of such electronic devices, the risk of ESD damage is increasing not only in manufacturing processes but also in consumer use environments.

For example, if mass-produced, mass-consumption smartphones and various gadgets experience ESD damage during consumer use, this will incur costs such as human resource costs for customer support, transportation costs, and warranty repair costs. Furthermore, if products with insufficient ESD countermeasures manifest themselves as problems in the market and result in suspension of sales or recalls, this will lead to major losses for companies.

For this reason, ESD-resistant design is becoming increasingly important in electronic device design. Under these circumstances, the adoption of "TVS diodes" as a type of ESD protection element is expanding.

In addition to TVS diodes, ESD protection elements also include other types such as varistors and suppressors, each differing in cost and in characteristics such as allowable energy, response speed, parasitic capacitance^*4, and clamping voltage^*5. It is important to select the appropriate element according to the application and interface conditions. (See "Differences Between TVS Diodes, Varistors, and Suppressors" in "Features of TVS Diodes and Differences from Other ESD Protection Elements" for details.)

Among ESD protection elements, semiconductor-based TVS diodes have a fast response speed and can easily keep parasitic capacitance low, making them suitable for ESD protection on high-speed data lines (USB, HDMI, etc.) and high-frequency signal lines. They are also commonly used on charging power lines in combination with other protection elements and filters, depending on the circuit conditions.
By locating appropriately designed TVS diodes next to interfaces, overvoltage and overcurrent due to ESD can be bypassed, enabling reduced stress on ICs and other semiconductor devices. This helps to reduce malfunctions and latent failure risks caused by ESD in field environments after products are shipped.

Murata Manufacturing (hereafter, "Murata") provides a lineup of TVS diodes as ESD protection elements that achieve high static electricity suppression ability in a compact device to meet modern needs such as high-speed transmission and DC power.

The content thus far can be summarized as follows.