Well Proximity Effect (WPE) is a layout-dependent effect where transistors near the edge of a well implant exhibit different threshold voltages — because the angled implant ions scatter laterally near the well boundary, altering the channel doping profile.

What Causes WPE?

• Mechanism: During well implantation, ions near the edge of the photoresist mask scatter laterally into the channel region.
• Effect: Devices near the well edge have higher or lower doping -> $V_t$ shifts of 20-50 mV.
• Distance Dependence: Effect diminishes exponentially with distance from the well edge (negligible beyond ~2-3 $mu m$).

Why It Matters

• Analog Mismatch: Current mirrors and differential pairs placed near well edges exhibit offset.
• SRAM: Bit cells near the N-well boundary have different $V_t$ -> different noise margins.
• Mitigation: Place matched devices far from well edges; use dummy devices at boundaries.

WPE is the edge effect of doping — where transistors near the well boundary receive an unintended dose of scattered ions, shifting their characteristics.