channel doping

**Channel Doping** is the **controlled introduction of dopant atoms into the transistor channel region** — to set the threshold voltage and control short-channel effects in planar CMOS devices. **What Is Channel Doping?** - **Purpose**: Adjust the work function difference between gate and channel to set $V_t$. - **NMOS**: P-type doping (boron) in the channel increases $V_t$. - **PMOS**: N-type doping (arsenic/phosphorus) in the channel increases $|V_t|$. - **Concentration**: Typically $10^{17}$ to $10^{18}$ cm$^{-3}$ for planar devices. **Why It Matters** - **Mobility Degradation**: Channel dopants act as scattering centers, reducing carrier mobility. - **Variability**: Random Dopant Fluctuation (RDF) causes $V_t$ mismatch — the #1 variability source in planar CMOS. - **Undoped Channels**: FinFET and FD-SOI use undoped (intrinsic) channels to eliminate RDF, controlling $V_t$ by work function metal instead. **Channel Doping** is **the classical knob for threshold voltage** — effective but increasingly problematic at advanced nodes due to the statistical variability of individual dopant atoms.