metal gate work function

**Metal Gate Work Function** is the **effective work function ($Phi_{m,eff}$) of the metal gate electrode** — which directly sets the threshold voltage ($V_t$) of the transistor in a High-k/Metal Gate (HKMG) stack, replacing the traditional role of polysilicon doping. **What Is Metal Gate Work Function?** - **$Phi_m$ Requirement**: - **NMOS**: $Phi_m approx 4.0-4.2$ eV (near Si conduction band edge). - **PMOS**: $Phi_m approx 5.0-5.2$ eV (near Si valence band edge). - **Materials**: TiN ($Phi_m approx 4.6-4.8$, mid-gap), TiAl ($Phi_m approx 4.2$, NMOS), TiAlC. - **Tuning**: Achieved by adjusting metal composition, thickness, and dipole engineering at the high-k/metal interface. **Why It Matters** - **$V_t$ Setting**: Unlike poly-Si (where $V_t$ was set by implant doping), in HKMG the gate metal defines $V_t$. - **Multi-$V_t$**: Multiple TiN/TiAl layer combinations provide different $V_t$ flavors (LVT, SVT, HVT) on the same die. - **EOT Scaling**: Work function tuning must be done without degrading the effective oxide thickness. **Metal Gate Work Function** is **the tuning dial for threshold voltage** — the metal property that replaced polysilicon doping as the primary $V_t$ control knob in modern transistors.