Page 210 - Electrical Properties of Materials

192 Principles of semiconductor devices

I I
D D

U decreasing
GS

Fig. 9.36
U 0 0 U
The characteristics of a MOSFET. GS DS

we may claim that at sufficiently large negative gate voltage inversion will oc-
cur, that is the material in the vicinity of the insulator will turn into a p-type
semiconductor. Holes may then flow unimpeded from source to drain. The rest
of the story is the same as for an ordinary FET and the characteristics are fairly
similar, as shown in Fig. 9.36, though in the present case there is no proper
current saturation, only a knee in the I D versus U DS characteristics.
The devices in which conduction occurs by inversion are said to operate
in the enhancement mode. There is also a depletion mode device in which
one starts with a p-channel [Fig. 9.35(b)] and depletes the holes by applying a
positive bias to the gate. This is more similar to the traditional FETs.
Naturally, both the enhancement and depletion devices described have their
+
counterparts with n drains and sources and p-type substrate. In principle there
is no difference between them. In practice there is some difference, because the
surface potential at the Si - SiO 2 interface tends to be positive, thus it is easier
to achieve inversion in an n-type material.
Having so many different types of FETs has tested the ingenuity of those
whose job it is to think up symbols for new devices. The solutions they came
up with are shown in Fig. 9.37.
Applications for performing logic functions are obvious. Depending on the
gate voltage the FET of Fig. 9.35 is either ON (U DS low) or OFF (U DS high).
There are naturally many varieties on the basic theme; I want to mention only
one of them, known as CMOS (complementary MOS), which rose to fame

Junction Enhancement Depletion

n-Channel

p-Channel
Fig. 9.37
Symbols for FETs.