SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

9.4. CAPACITANCE-VOLTAGE CHARACTERISTICS OF THE MOS 451

Low frequency

(~1Hz)

High frequency

(10^3 Hz)

Inversion

Accumulation

Cmos = Cox C

Cmos(fb)

Vfb 0 VT

C = Cox

Cmos(min)

(i)

(ii)

SURFACE VOLTAGE,Vs(volt)

A

REAL

C

HARGE

D

ENSITY

, |

Qs

| (C/cm

2 )

–0.2 –1.0 0 1.0 0.2 0.3 0.4

10 –5

10 –6

10 –7

10 –8

10 –9

2 φF

Accumulation

Region

Depletion Weak

Inversion

Strong

Inversion

Flat Band

VGS

ns ~10^12 cm–2

Figure 9.12: (a) A typical dependence of MOS capacitance on voltage. Curve (i) is for low
frequencies and curve (ii) is for high frequencies. Also shown are the various important regions
in the capacitance-voltage relations. (b) The charge density|Qs|is shown schematically as a
function of the surface potentialVs.