What is electrostatic discharge (ESD)?

ESD refers to the discharge of electric charges that occurs when substances charged with charges of different polarities come close to each other or come into contact with each other.

Figure 1 Triboelectric series
Figure 1 Triboelectric series

All matter is composed of atoms, each of which consists of a positively charged nucleus and negatively charged electrons, making it electrically neutral. When an atom loses electrons, for example, as a result of two objects coming into contact with each other, it becomes a positively charged ion (called a cation). Conversely, an atom that gains electrons becomes a negatively charged ion (called an anion).
A transfer of electrons does not occur randomly; some materials tend to become positively charged whereas others tend to become negatively charged. Figure 1 shows a triboelectric series, a list of materials that are empirically ordered according to their tendency to acquire a positive or negative charge. Electrostatic discharge (ESD) is a sudden flow of electric charge between two electrically charged objects caused by contact.

When two different materials come into contact, electrons are transferred from one material to the other. However, while these materials are in contact with each other, positive and negative ions are in equilibrium. In this state, static electricity cannot be detected. You experience static electricity when you separate these differently charged materials. Suppose, for example, that you are wearing a polyester down jacket on top of a wool sweater. Then, the sweater becomes negatively charged because friction causes electrons to move from the down jacket to the sweater. At this time, the down jacket becomes positively charged. When you then take off the sweater, you might experience electrostatic sparks as some electrons on the sweater are discharged to the down jacket.
Look at Figure 1. Polyvinyl chloride and human hair are far away from each other in the triboelectric series as highlighted by magenta arrows. The farther away two materials are from each other, the greater the charge they build up. This means that more ESD energy is discharged when they are separated. You rarely get static shocks when you take off a cotton shirt.

Static electricity is akin to the charging of a capacitor. To prevent ESD events, it is important to avoid the buildup of charge.　There are several ways to prevent static electricity:

  1. In the case of a metal, connect it to ground.
  2. Increase humidity to reduce the surface impedance of a material.
  3. Use an ionizer to ionize the atmosphere with the opposite polarity to that of the charged object in order to neutralize the electrostatic charge.　

Electronics manufacturers implement these measures to prevent static electricity from being produced.
However, there is no way to eliminate static electricity. So, ICs generally incorporate ESD protection circuitry at I/O pins because they could be damaged or destroyed by ESD energy.
Although such ESD protection circuitry provides protection against relatively small ESD transients that occur in a device under protection (device-level ESD), it cannot withstand ESD transients that are introduced into ICs via USB and other ports (system-level ESD). Keep in mind that electronic devices are constantly exposed to system-level ESD in our daily life.
To protect ICs from system-level ESD, it is necessary to add external ESD protection diodes. For details, visit the following FAQs:

FAQs:

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