Computer Engineering and Nanotechnology - Molecular Computing: Origins and Promises - page 38

Fig. 2.10 Bipolar (a) and
field-effect (b) transistor
As a result of continued rapid development of the theory and semiconductor
technology, various versions of two basic types of transistors—bipolar and field-
effect transistors—were created (Fig. 2.10 ). They employ a combination of p-n and
n-p semiconductor junctions.
A bipolar transistor comprises two semiconductor areas of the same type (emit-
ter and collector), separated by a thin layer of semiconductor of the other type
(base). In simple terms it can be thought of as two p-n junctions joined back-to-
back. If voltage is applied only between the emitter and the collector, then for any
polarity of the voltage current will not flow. One of the two p-n junctions will be
closed. But when the voltage is applied between the emitter and the base, current
flows in the chain emitter-collector, and the strength of this current can be con-
trolled by much weaker emitter-base current.
The field-effect transistor introduced somewhat later is based on the idea
expressed as early as 1925 by the American researcher Julius Lilienfeld. He
proposed to control the resistance of a semiconductor layer in a system which is
essentially a capacitor, with one plate made of metal and the second one from doped
semiconductor, using voltage applied between the metal and the semiconductor. If
negative potential is applied to the metallic plate, the field will displace the
electrons from the surface layer of the semiconductor, leading to a lack of current
carriers and an increase of resistance. When the polarity is reversed the number of
carriers in this area will go up and the resistance will increase. The mechanism of
action of the field-effect transistor is shown schematically in Figure.
A very important factor in creating the field-effect transistor was the availability
of suitable materials: silicon (semiconductor) and silicon dioxide (insulator).