Device and circuit
⊖ ionized acceptor · ▬ inversion electrons (density ∝ |Q′n|) · + gate charge
Barrier along the surface (n+ → p)
Grey: the same φs with VSB = 0. Barrier for electrons = e(Vbi + VSB − φs).
Readout
| VGB | | VSB | |
| VGS = VGB − VSB | | φs | |
| φs − VSB | | nsurface | |
| Q′n | | Q′dep | |
| xd (gate) | | barrier | |
| VLB (φs = φfp+VSB) | | VMB (φs = 2φfp+VSB) | |
Mode: —
nsurface = Na exp{e[φs − (2φfp + VSB)]/kT}:
what counts is φs − VSB, not φs itself.
Energy bands vs. depth
EFp stays at its bulk value; EFn is set by the n+ region (lowered by eVSB). Depletion approximation, not to scale.
Plots vs. VGB
Solid: present VSB; dashed: VSB = 0 (two-terminal). ● = present bias.
Controls
Connect
Steps
Animate
What to notice
- (a)→(b): raising VGB lowers the barrier from n+; electrons flow in and the surface inverts.
- (b)→(c): VSB lowers Ec of n+ by eVSB; the barrier grows and the inversion layer thins.
- (c)→(d): raising φs by VSB restores the same barrier and the same nsurface.
- In the plots every curve moves right with VSB; on the deep-depletion line VSB has no effect.
- Common-S: holding VGS and raising VSB reduces |Q′n| (the body effect).
Charge-sheet model with the exact Q′s(φs, VSB); VFB = −0.70 V, γ = 0.53 V1/2, Vbi(n+p) = 0.99 V. Small reverse currents are neglected.