contact resistance

**Contact Resistance** is the **resistance at the interface between two different materials in an electrical connection** — typically the interface between a metal contact plug and the silicon (or silicide) at the transistor source/drain, and the critical parameter limiting transistor external resistance. **What Determines Contact Resistance?** - **Specific Contact Resistivity ($ ho_c$)**: Intrinsic property of the metal-semiconductor interface. Units: $Omega cdot cm^2$. - **Contact Area**: $R_c = ho_c / A_{contact}$. As contacts shrink, $R_c$ increases. - **Barrier Height**: Lower Schottky barrier height -> lower $ ho_c$. - **Doping**: Higher S/D doping -> thinner barrier -> more tunneling -> lower $ ho_c$. **Why It Matters** - **Performance Limiter**: At 7nm and below, contact resistance contributes > 50% of the total transistor series resistance ($R_{ext}$). - **Scaling**: Contact area shrinks quadratically with node, driving $R_c$ up dramatically. - **Solutions**: Ultra-high S/D doping (>$10^{21}$ cm$^{-3}$), metal-insulator-semiconductor (MIS) contacts, wrap-around contacts (WAC). **Contact Resistance** is **the bottleneck at the transistor doorstep** — the resistance penalty that increasingly dominates transistor performance as contacts shrink.