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Electrostatic discharge (ESD) is caused by the discharge of an excess or deficiency of electrons on one surface with respect to another surface or to ground. When a static charge is present on an object, the molecules are electrically imbalanced. ESD takes place when equilibrium is attempted via the transfer of electrons between one object and another that is at a different voltage potential. When an ESD-sensitive device, such as a MOSFET or other discrete component becomes part of the discharge path or is exposed to an electrostatic field, it can be permanently damaged. ESD damage is thermal in nature resulting in melted material or dielectric rupture due to excess electric field. Hence, ESD protection measures involving on-chip ESD protection structures or ESD enhanced fabrication processes are usually the first line of defense. While on-chip ESD protection circuit design has been an integral part of development of almost every single IC chip, inclusion of such protection on most discrete components has been considered optional by many semiconductor and system manufacturers using these products in their systems. ESD protected versions of discrete components have been made available, but minimal risk of ESD damage due to the ESD protection measures utilized during manufacture and an appropriate PCB design mitigating ESD damage to the discrete component, coupled with demand for low cost components with high volume most often results in the purchase of unprotected discrete components.
For a variety of reasons, there has been a recent move by some large system manufacturers to require inclusion of ESD protection on discrete components designed into their systems. Just as with integrated circuits (ICs), ESD protection of some types of discrete components can be challenging. For MOSFETs, the gate oxide between the gate and channel utilizing modern CMOS processes is extremely thin resulting in varying degrees of ESD sensitivity. The electric field Eox across the gate oxide of thickness tox when voltage V is applied is Eox = V / tox. For very small tox, the electric field Eox can exceed the breakdown field Ebk for relatively small voltages. [1] ESD damage of the MOSFET occurs when the gate-to-source voltage is high enough to damage the gate dielectric. A leakage path through the gate oxide can result in a catastrophic failure. Besides the MOSFET, IGBT power modules are also sensitive to ESD because the thickness of the gate isolation is on the order of one hundred nanometers.
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