Page 224 - Electrical Properties of Materials

206 Principles of semiconductor devices

gas has the opposite effect. These changes can be detected by measuring the
conductivity of a thin film. For high sensitivity, an extremely large surface area
is required and as a result the films are generally highly porous or nanostruc-
tured. Gas sensors have therefore been major beneficiaries of new materials
developments in nanotechnology. One problem is to make the response spe-
cific to a single gas species, rather than to many, and to avoid confusing a
change in temperature with the presence of the gas. The solution is generally
to use several sensors, each having different responses to temperature and to
various common gases, and to derive the concentrations of the different species
by processing the signals from all the sensors together. Important applications
include the detection of combustible and toxic gases, and oxygen depletion.
Forms of the varistor can be targetted on the gases associated with explos-
ives or drugs. Hence, these ‘electronic noses’ are replacing the ‘sniffer dogs’
often featured in news items. It is a pity that dogs as well as people are being
made redundant by semiconductors, but at least there is now less chance that a
cat crossing the road will cause a dog to misbehave and misdiagnose.

9.22 Microelectronic circuits
We shall conclude the discussion of semiconductor devices by saying a few
words about the latest techniques for producing them. Since the techniques
are suitable for producing very small electronic circuits, they are called micro-
electronic circuits; and because these circuits can be interconnected, they are
Invented by Jack Kilby in 1959, often referred to as integrated circuits. The material most often used is silicon;
working at Texas Instruments. He the small piece of material upon which one unit of the manufactured device
received the Nobel Prize in 2000. is presented is known as a chip. Once the exclusive preserve of a few engin-
eers, today even sociologists and politicians know about the silicon chip. They
learned to love it or hate it; indifference is no longer possible.
The crucial property of silicon that made this technology possible is its abil-
ity to acquire a tenacious ‘masking’ layer of silicon dioxide. SiO 2 is familiar
in an impure state as sand on the beach; it has a ceramic form used for furnace
tubes and a crystalline form (quartz) that has good acoustic and optical prop-
erties. It is very hard, chemically resistant, an insulator, and has a high melting
◦
point (≈1700 C). An oxide layer can simply be grown, by heating the silicon
◦
to 1200 C in an oxygen atmosphere. The growth rate is very slow, about 1 μm
per hour, and the thickness can thus easily be controlled. To reduce the process
temperature (which generally leads to a reduction in the defect density and
an increase in device lifetime), oxide layers may alternatively be deposited in
a process known as plasma enhanced chemical vapour deposition (PECVD).
The sources are gases (silane or dichlorosilane and oxygen), and much thicker
◦
layers can be deposited at temperatures of around 350 C.
What else needs to be done besides growing an oxide layer? Quite a lot,
there are a number of other operations to be performed. Instead of discuss-
ing them separately, we shall give here a brief description of the production
of a single n–p–n transistor, and shall introduce the various techniques as we
go along.
We shall start with a p-type substrate which has an n-type epitaxial layer
of about 4–8 μm on the top as shown in Fig. 9.53(a). Next, we grow an oxide
layer, then cover it by a thin film of a material called a photoresist, place a