In semiconductor design, the p-doped substrate, often referred to as “ground” (GND or VSS), serves as the reference potential for all circuits on a chip. However, an unwanted effect known as substrate currents can lead to a localized increase in the ground potential. This increase is caused by current flow (Isub) through the parasitic track resistor (Rsub) in the substrate, resulting in an undesired voltage rise.
Key Points about Substrate Currents
- Unwanted Voltage Rise: Substrate currents cause a local increase in the ground potential, which can be calculated using the equation V = Rsub * Isub (Equation 7.1). This voltage rise is undesirable and can affect circuit performance.
- Parasitic Nature: Substrate currents are always parasitic, meaning they are unintended and unwanted. Designers should aim to avoid current flows in the substrate whenever possible.
- Current Injection and Discharge: Isub is injected into the substrate by a current source, often referred to as a “substrate current injector.” It is then discharged through contacts in the “SUB” net to the ground bond pad.
- Resistance Components: The resistance (Rsub) in the substrate current path is composed of parallel track resistances (Rsub1, Rsub2) and contact resistances (Rcont). Minimizing these resistances is essential to reduce IR drops.
- Doped Substrate: To minimize voltage drop (IR drop), a heavily doped (low-resistance) p-substrate is often used. This helps maintain a more stable ground potential. However, an additional lightly doped layer is required to facilitate the fabrication of devices.
- Causes of Substrate Currents: Substrate currents can occur for various reasons. One common scenario is in NPN transistors operated in “saturation mode,” where the collector potential is minimized. In such cases, forward-biased diodes can act as parasitic PNP transistors, emitting holes into the substrate, leading to substrate currents.
Impact of Substrate Currents
Increased IR Drop
- Substrate currents, when present, can lead to an increased IR drop. This occurs because carriers (holes) travel longer distances in the substrate, causing a higher potential rise. In extreme cases, this can affect the proper functioning of devices and even lead to junction isolation (JI) failures.
Parasitic Junction Capacitances
- Substrate currents can also result in parasitic junction capacitances. These capacitances introduce crosstalk and ground bounce effects, modulating adjacent circuitry. Ground bounce is an unwanted voltage rise on the circuit ground, which can introduce noise and interfere with nearby components.
In summary, substrate currents are parasitic effects that can lead to undesired voltage rises in the ground potential of semiconductor devices. Designers must take measures to minimize these effects, including optimizing resistance components, using appropriately doped substrates, and avoiding scenarios that lead to substrate currents. Failure to address substrate currents can impact the performance and reliability of integrated circuits.
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