SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

440 CHAPTER 9. FIELD EFFECT TRANSISTORS: MOSFET

p

p 0

n 0

Z

n

W

p

Na p^0

Z

n

W

p

p 0

n 0

Z

n

W

Na

Inversion

Depletion

Accumulation

Ec

EFi

EF

Ev

MO S

EF

eVG

VG < 0

Electric field E

Ec

EFi

EF

Ev

MO S

EF

eVG

VG > 0

Electric field E

Ec

EFi

EF

Ev

EF MO S

eVG

VG >> 0

Electric field E

+++

(a)

(b)

(c)

n(interface) > po

CARRIER DENSITY

CARRIER DENSITY

CARRIER DENSITY

• –

Figure 9.8: Effects of applied voltage on interface charge density in the ideal MOS capaci-
tor: (a) negative voltage causes hole accumulation in thep-type semiconductor; (b) positive
voltage depletes holes from the semiconductor surface; and (c) a larger positive voltage causes
inversion—an “n-type” layer at the semiconductor surface.