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.