In semiconductor devices, particularly in CMOS and BiCMOS technologies, the proper functioning of junction-isolation (JI) diodes is essential to isolate different regions and prevent unwanted current flow. However, several scenarios can lead to the unintended forward biasing of JI diodes, resulting in parasitic effects and leakage currents. This section explores these scenarios and their consequences.
Substrate currents can raise the ground potential above the well potential. When this occurs, the diode becomes forward-biased, allowing current to flow from the chip ground contact through the substrate contacts, into the substrate, and finally into the well. This current flow is undesirable and can lead to potential issues within the well.
External Inductance-Induced Transients
Another scenario involves external inductances connected to the well. Transients induced by these inductances can “pull” the well’s potential below the ground potential. In this case, the diode becomes forward-biased, and an “invalid” current (denoted as I) flows from the chip’s ground contact through the nearest substrate contacts, into the substrate, and eventually into the affected well. This forward-biased diode acts as a switch that turns on this unwanted current. While the effect of this hole current is generally localized to the affected well, it can have broader consequences.
Unwanted Electron Emission
When the diode is forward-biased, it emits electrons into the p-doped region, which are considered minority carriers in this context. These minority carriers, primarily through diffusion, can reach neighboring n-wells. In those neighboring n-wells, the electric fields of the correctly reverse-biased JI diodes transport these injected electrons into the wells. This phenomenon results in unwanted leakage current in the n-wells.
Parasitic NPN Transistor Effect
The effect of unwanted electron emission and leakage current can be likened to a parasitic NPN transistor. In this “parasitic” transistor, the n-well with a potential pulled below ground serves as the emitter, the p-doped region functions as the base, and other neighboring n-wells act as the collectors. This parasitic NPN transistor can exhibit a current gain (denoted as B), which quantifies its parasitic effect.
The impact of this parasitic effect can be significant, especially when adjacent wells are involved, leading to a small parasitic base width and a higher current gain (B). Additionally, even a small fraction of the emitter current reaching a distant well with a larger parasitic base width can disrupt sensitive analog circuits operating at low current levels.
Addressing these issues during chip and system design is challenging, particularly due to the fast signal rise times in modern applications that amplify these effects. Instead, the focus shifts to minimizing these effects during physical design. Strategies to weaken the parasitic NPN transistor in the layout are explored to mitigate the consequences of unwanted electron emission, leakage current, and diode forward biasing. These physical design measures aim to improve the reliability and performance of semiconductor devices in the presence of parasitic effects.
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