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.