FinFET 2010s Manage Fin Body Depletion Effects in Lightly Doped Channel

# FinFET (2010s): Step 13 — Manage Fin-Body Depletion Effects in the Lightly Doped Channel

## 1. The Consequence of Going Undoped

Step 5 noted that FinFETs use lightly doped or undoped fin channels to avoid the random-dopant-fluctuation mismatch planar transistors suffered from. That same choice has its own consequence: with so little fixed charge in the channel body, the fin's depletion region can extend through the entire fin width even in the off state, a behavior distinct from, and harder to control through doping alone than, a planar transistor's partial-depletion behavior.

## 2. Fully-Depleted Behavior and Its Benefit

When a fin is thin enough that the depletion region from both gate sidewalls meets in the middle even at zero gate bias, the device operates in a fully-depleted mode: there is no neutral, undepleted body region left for leakage paths to hide in, which is part of why thin fins deliver the steep subthreshold swing Step 2 attributed to multi-gate confinement in the first place.

## 3. The Control Problem Fully-Depleted Operation Creates

Because threshold voltage in a fully-depleted fin is set almost entirely by gate work function and fin width rather than by channel doping, any variation in fin width from Step 5's uniformity concern now couples directly and sensitively into threshold-voltage variation, with little doping-based damping left to absorb it. Fin-width control and threshold-voltage control become, in effect, the same control problem rather than two separable ones.

## 4. What This Step Does and Does Not Cover

This step covers fin-body depletion as a distinct parasitic/device-physics effect from the source/drain capacitance terms in Step 12; it does not yet address SRAM-specific consequences of fin quantization, which Step 15 covers after Step 14 completes this series' look at standard non-SRAM design-library impact.

Fully-Depleted Fins Trade Doping-Based Damping for Tighter Width Coupling threshold-voltage sensitivity to channel-doping variation versus to fin-width variation, partially- versus fully-depleted operation partially-depleted (doped) channel: Vth sensitivity to doping variation dominates, width matters less fully-depleted (undoped) fin: Vth sensitivity to fin-width variation now dominates instead fin-width control and threshold-voltage control become effectively the same control problem process generation advances left to right across this chart there is no undepleted neutral body left to damp fin-width variation the way doping once did this is the same fin-width uniformity concern from Step 5, now seen through a device-physics lens
Partial Depletion Versus Full Depletion, in One Fin why going undoped for mismatch reasons changes how the channel turns off, not just how it mismatches DOPED, PARTIAL DEPLETION planar-style: a neutral body region remains even when the gate is off UNDOPED, FULL DEPLETION thin fin: depletion from both sidewalls meets in the middle, no neutral body left TIGHTER WIDTH COUPLING with doping-based damping gone, Vth now tracks fin width directly and sensitively this effect is a direct consequence of the undoped-channel choice this series made back in Step 5

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