well proximity effect (wpe)

**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.