Well Proximity Effect (WPE)

The well proximity effect (WPE) is a crucial phenomenon in analog circuit design, particularly in the context of modern highly scaled bulk CMOS (Complementary Metal-Oxide-Semiconductor) technologies. This effect arises due to the use of high-energy ion implants to create deep retrograde well profiles, which serve functions such as latch-up protection and suppression of lateral punch-through.

The WPE introduces complexities in analog circuit design by causing shifts in the bias points of Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) located near the well edges. These shifts can be substantial, often ranging from 20% to 30%. Such shifts have the potential to lead to catastrophic failures in circuits. To understand the impact of WPE on MOSFETs, it’s essential to consider the implant process.

During the ion implantation process, atoms can scatter laterally from the edge of the photoresist mask and become embedded in the silicon surface near the well edge. This lateral scattering results in a concentration gradient within the well, changing with distance from the mask edge. This gradient can extend over several micrometers, influencing MOSFET characteristics over this range.

Primary effect of WPE

The primary effect of WPE is on the threshold voltage (Vt) of MOSFETs. As MOSFETs are placed closer to the well edge, their Vt increases. This shift in Vt is more pronounced for nMOS (n-type MOSFETs) compared to pMOS (p-type MOSFETs). This difference in Vt shifts between nMOS and pMOS devices can lead to problems like current mirrors shifting out of saturation, which can result in circuit failures.

The impact of WPE is further exacerbated by the orientation of the source and drain regions of the MOSFETs relative to the well edge. Devices with the source oriented towards the well edge exhibit different behavior compared to devices with the drain oriented towards the well edge. This source/drain orientation-dependent effect can cause variations in drain current (Id) and, consequently, impact circuit performance.

To mitigate the effects of WPE, analog designers may consider various layout configurations, such as using symmetric source and drain connections or arranging MOSFETs in common wells. However, it’s important to note that these layout choices come with their own trade-offs and may not completely eliminate the WPE-induced shifts in performance.

Conclusion

In summary, the well proximity effect (WPE) is a critical consideration in analog circuit design using modern CMOS technologies. It introduces significant shifts in MOSFET characteristics, particularly Vt, which can lead to circuit failures if not properly accounted for in the design. Designers must carefully select layout configurations and consider source/drain orientation to mitigate the impact of WPE and ensure reliable circuit operation

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