SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

9.2. MOSFET: DEVICES AND IMPACT 435

n-sourceGate L

Polysilicon

or metal

Gate width,

Z

Oxide

n-drain

p-substrate

n-type

semiconductor

(a)

STRUCTURE

G

S

B

D

Schematic symbol

(b)

CROSS-SECTIONAL VIEW

p-type body, B

Channel length L

Channel

region

Source Drain

Field oxide

Silicon dioxide

SiO 2 SiO 2

Metal source

contact

S

Metal source

drain

D

Gate, G

n-type

polysilicon

Deposited

insulator

n+ n+

p+

}

dox

L

Figure 9.2: (a) A schematic of an NMOS device along with a symbol for the device. The contact
Bdenotes the body or substrate of the device. (b) A cross-section of the NMOS. Modern devices
involve considerably more complexities.

voltage required to align the two Fermi levels, is therefore equal to

Vbi=−(φ 1 −φ 2 ) (9.2.1)
The applied voltage necessary to create flat bands in the junction isVfb=−Vbi.
Now let us consider an MOS capacitor. Figure 9.6c shows the device band diagrams with
zero bias across an MOS structure andV=Vfbapplied to material 1 with respect to material 2.