Chapter 3 · Animation 3 of 4 · Sections 3.3 and 3.4.2

The Field-Induced Junction and the Body Effect

Treat the inversion layer as the n side of a junction with the body. Reverse-bias it with VCB and watch electrons leave, the depletion region widen and the gate charge budget change.

electronhole⊖ acceptor ion (bold: uncovered)+ gate charge

What to look for

  • Electrons in the wire to terminal C: they leave when VCB rises and return when VGB rises.
  • The depletion width under the gate: it grows as √(ψS), and in strong inversion ψS ≈ φ0 + VCB.
  • The bar: for the same inversion charge, a larger VCB means more depletion charge, so more gate voltage.

The physics behind it

  • Strong inversion: the inversion layer is tied to terminal C, so ∂ψS/∂VCB ≈ 1 and VCB acts as the reverse bias of a field-induced junction (Section 3.3).
  • |Q′B| = γC′ox√(φ0 + VCB) at threshold, hence VT = VT0 + γ(√(φ0+VCB) − √φ0) and VTB = VT + VCB.
  • Weak inversion: ψS ≈ ψsa(VGB), set by the gate alone; Q′I ∝ e−VCB/φt (3.4.20).