length of diffusion (lod) effect

LOD (Length of Diffusion) Effect is a layout-dependent effect where the distance from a transistor's channel to the nearest STI edge affects its performance — because the compressive stress from STI changes carrier mobility, and this stress depends on the active area (OD) length.

What Causes the LOD Effect?

• Mechanism: STI (SiO₂) has a different thermal expansion coefficient than Si. After anneal, the STI exerts compressive stress on the active silicon.
• Short OD: More stress (STI edges closer to channel) -> mobility change.
• Long OD: Less stress (STI edges far from channel) -> different mobility.
• Asymmetry: SA (source-side OD length) and SB (drain-side OD length) affect stress independently.

Why It Matters

• Analog Design: Two transistors with different OD lengths have different $I_{on}$ and $V_t$ even if $W/L$ is identical.
• Standard Cells: Different logic cells have different SA/SB -> systematic performance variation.
• Modeling: BSIM models include SA, SB parameters to capture LOD in SPICE simulation.

LOD Effect is the stress fingerprint of layout — where the geometry of the active area directly controls the mechanical stress felt by the channel.