WHY THE INSULATOR MATTERS

The oxide's four gifts

No DC gate current The oxide blocks charge flow, so the gate only sets a field

High input resistance The gate looks like a capacitor, ideal for sensing a voltage

Sensitive to charge Trapped or nearby charge changes how the device behaves

A sensing surface If the gate is exposed to a liquid, the oxide surface becomes the sensor

Keep the last one in mind: it's the bridge from transistor to biosensor.

METAL–OXIDE–SEMICONDUCTOR

The capacitor at the heart

gate | oxide | Si

Under the gate, the stack forms a capacitor: gate on top, oxide in the middle, silicon below. The oxide capacitance per area is C_ox = ε_ox / t_ox, with ε_ox = 3.9 ε₀ for SiO₂. A 10 nm oxide gives about 3.45 fF per µm².

Thinner oxide means more capacitance, so the gate grips the channel harder.

THREE STATES

Accumulation, depletion, inversion

Take a p-type body. A negative gate voltage pulls holes to the surface (accumulation). A small positive voltage pushes holes away, exposing fixed negative ions (depletion). A larger voltage pulls electrons to the surface, forming an n-type layer (inversion).

Inversion is the channel: an electron layer in p-type silicon.

WALKING THE GATE VOLTAGE

What each range does

VG < 0 · accumulation Holes gather at the surface, so it looks even more p-type

0 < VG < Vth · depletion Holes are repelled, uncovering fixed negative acceptor ions

VG > Vth · inversion Electrons gather at the surface and form a conducting channel

The boundaries shift slightly with the flat-band voltage, which we'll meet next.

STRONG INVERSION

How much bending is enough?

φs = 2φF

The surface is strongly inverted once the band bending φs reaches twice the Fermi potential, φF = (kT/q) ln(N_A/nᵢ). For N_A = 10¹⁶ cm⁻³, φF is about 0.36 V, so 2φF is about 0.71 V.

The channel's electron density at the surface matches the body's hole density.

UNIT 4 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered the gate-oxide-silicon capacitor and its three surface states.