Operation:

Voltage V ... negative

• Electric field created across the dioxide layer
  attracts positive charges (holes) to the region 'R'

• Negative charges stored in the top metal electrode

• Structure behaves as a capacitor of magnitude

e_ox = e₀ K_ox  ... permittivity of the SiO₂ (~ 0.35 pF/cm)
K_ox               ... dielectric constant of the SiO₂ (K_ox ~ 3.9)
A                  ... area of the top electrode
l                    ... thickness of the SiO₂ layer

• p-type Si layer between R and the bottom electrode behaves as a resistor
  

• Overall structure behaves as a lossy capacitor

Accumulation region

Voltage V ... positive

• Electric field created across the dioxide layer
  repel positive charges (holes)
  Region 'R' (depletion layer) contains a net negative space charge

• Very small positive voltage (V<<1V)
  Structure behaves as a capacitor of the same magnitude as previously

• When the positive voltage is increased
  Depletion layer widens
  Charge in the depletion layer becomes greater
  Effective value of l increases ... C decreases
  
  Depletion region

• When the positive voltage is increased even further
  l becomes a SiO₂ thickness due to thermal electrons attracted to the depletion layer
  Structure behaves as a capacitor of the same magnitude as previously

Inversion region