It is well known that integrated circuits (ICs) are comprised of microelectronic components such as transistors, diodes, resistors, capacitors, etc., in a substrate, and metal interconnects which connect the components in circuits. It is furthermore well known that some internal nodes of an IC are connected to external nodes. Internal nodes of an IC are connected to external nodes by metal elements, known as Input-Output (IO) pads or bondpads, among other designations, on the top surface of an IC. IO pads are connected to external functions by various means, including wirebonding, bumpbonding, beamlead bonding, etc. By necessity, internal nodes of an IC that are connected to IO pads are exposed to the electrical environment of manufacturing processes and packaging and assembly processes. Moreover, in some applications, internal nodes are exposed to user electrical environments during the life of a product in which an IC is included. Electrical environments consist of static and time-varying electric and magnetic fields, and electric charge sources, such as humans with a static charge or electrically charged objects in close proximity to ICs. These features of electrical environments, especially charge sources, pose a significant danger to ICs. Charge sources are often at a much higher electrical potential than ICs. When charge sources come into close proximity to ICs, large potential differences between charge sources and ICs, typically between a few tens of volts and a few thousand volts, can cause a gap between them to become electrically conducting as a result of avalanche ionization and breakdown of a separating medium, usually air. The result of breakdown of a gap between a charge source and an IC is to partially or completely discharge the charge source into the IC, and is known as electrostatic discharge (ESD). Electrical currents associated with ESD are typically very high and can damage components of an IC.
In typical IC manufacturing and packaging and assembly operations, great care is taken to avoid exposing ICs to ESD hazards. Nevertheless, ICs are subject to ESD incidents. ESD protection devices and circuits in ICs are connected to bondpads to provide sufficient ESD immunity from typical IC manufacturing, packaging and assembly, and product end use environments.
Common electrical components in ESD protection circuits are bipolar transistors. Bipolar transistors are used to provide a low resistance shunt to ground during ESD events. In order to handle large currents associated with ESD events, bipolar transistors used in ESD protection circuits have large emitter-base junction areas, compared to transistors used for logic or signal processing circuits. Large junction areas are necessary to sustain the high power dissipated during ESD events and to minimize the voltage increase across the device at high current. Contacts and optional metal silicide uniformly distributed across the surface of the emitter, base and collector are used for uniform junction biasing and low resistive current conduction to the metal interconnects.
Current inhomogeneity and current crowding are well known to be issues in bipolar transistors in ESD protection circuits. Current crowding occurring during ESD events can result in permanent damage to bipolar transistors, causing loss of circuit functionality or shortened operating life of ESD protection circuit and IC.