SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

9.3. METAL-OXIDE-SEMICONDUCTOR CAPACITOR 437

eφ 1 eφ^2

e(φ 1 −φ 2 )

EVAC

EV(1)

EV(2)

EC(1)

EC(2)

−eVbi=EC(1)−EC(2)

Figure 9.5: Above: Band line-up before junction formation. Below: Band line-up after junction
formation

Following our convention
Vbi=−(φm−φs)=−φms

In the case shownφmsis negative and thereforeVbiis a positive number. Hence fromVfb=
−Vbiwe getVfb=φms. When applied to the case shown we see thatVfbis negative

9.3 METAL-OXIDE-SEMICONDUCTOR CAPACITOR...............

We have noted several times in this book that Si technology is so far unique in that a high-
quality oxide SiO 2 that can be formed on Si. The Si-SiO 2 interface perfection has been the reason
why field-effect devices are suitable for many applications. Their higher areal density, better
switching characteristics and lower power dissipation have made them the dominant device in
electronic systems and the engine driving Moore’s law.
To understand the operation of the MOSFET we first need to examine the MOS capacitor,
whose structure and band diagrams are shown in figure 9.6. An oxide layer is grown on top
of ap-type semiconductor and a metal contact is placed on the oxide. In general, the insulator
could be any large bandgap material. The main purpose of the oxide layer is to provide isolation
between the metal and the semiconductor.