SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

452 CHAPTER 9. FIELD EFFECT TRANSISTORS: MOSFET

Qss = 0

Vfb(2) Vfb(0)

Cmos(min)

Cox

C

VG

Gate bias

Positive charge at

interface

Negative charge at interface

No interface states

With interface states

EF = EFi at the interface

Cox

C

(^0) V
G
Cmos(min)
Gate bias
(a)
(b)
Vfb = –Qss
Cox
EC(ox)
EC(Si)
EV
EV(ox)
Interface states
Figure 9.13: (a) A schematic plot of the high-frequency capacitance voltage of MOS capacitors
with different values of the fixed oxide charge. (b) Interface states cause a smearing out the C-V
curves.
When the interface charge is positive, the C-V curve shifts toward negative voltages, while
when it is negative, the curve shifts toward positive voltages. This is shown schematically in
figure 9.13b. The C-V curve is thus “smeared out” due to the presence of interface states. In
modern high-quality MOS structures, the interface state density is maintained below 1010 cm−^2 ,
so that the effect is negligible.