Elements of Modern Physics
292
convert part of the p-region back to n-type to form the emitter [Fig. 8.15(c).
The surface is again oxidized and two new windows are opened to expose the
emitter and base regions and a metal (usually aluminium) is evaporated into
those windows forming electrical contact with these regions. The contact with
the original epi-layer which forms the collector, can be made through the
substrate [Fig. 8.15(d)].
Other devices can be produced by the variations of the essential steps
involved in the development of the npn transistor described.
Amorphous Semiconductors
There are many amorphous substances which have significant electrical
conductivity. They are known as amorphous semiconductors. In these materials,
the conduction is by electrons. They differ from the crystalline semiconductors
in that while they have short-range order, long-range order is absent in them.
This can be illustrated by amorphous Ge. In this case, though each atom is
surrounded by four nearest neighbours, the location of the second-nearest
neighbour is not unique. In the amorphous semiconductors, the different possible
locations of farther-away neighbours are almost randomly filled leading to
disorder at long range. The effect of this long-range disorder is not significant
for energy levels deep inside an energy band but is important for those near the
edges, e.g., those near the top of the valence band and bottom of the conduction
band. It leads to narrowing of the energy gap as compared with the gap in
crystalline semiconductors. Some amorphous semiconductors are Ge, Si, Se,
As 2 Se 3 , etc.
Amorphous semiconductors are used in switching and memory components,
They are also used in xerographic processes. Here, typically a thin film of
amorphous selenium is deposited on a metallic substrate, usually Al. It is charged
electrically by means of a discharge. When a pattern of light to be copied falls
on this, the lighted areas become photoconductive and discharge their charge
whereas the dark areas retain their charge. A finely-powdered pigment is sprayed
on the surface. It is retained by the charged areas and then transferred to a sheet
of paper.
8.6 MAGNETIC PROPERTIES
In this section, we discuss the magnetic properties of materials. They are related
to the spin and orbital angular momentum of the electrons, and are observed in
the form of (i) diamagnetism (ii) free-electron paramagnetism
(iii) paramagnetism of atoms, ions or molecules, (iv) ferromagnetism, and
(v) antiferromagnetism and ferrimagnetism. Of these the first three are rather
weak effects while the last two are strong effects which are also of great
technological importance.
292
convert part of the p-region back to n-type to form the emitter [Fig. 8.15(c).
The surface is again oxidized and two new windows are opened to expose the
emitter and base regions and a metal (usually aluminium) is evaporated into
those windows forming electrical contact with these regions. The contact with
the original epi-layer which forms the collector, can be made through the
substrate [Fig. 8.15(d)].
Other devices can be produced by the variations of the essential steps
involved in the development of the npn transistor described.
Amorphous Semiconductors
There are many amorphous substances which have significant electrical
conductivity. They are known as amorphous semiconductors. In these materials,
the conduction is by electrons. They differ from the crystalline semiconductors
in that while they have short-range order, long-range order is absent in them.
This can be illustrated by amorphous Ge. In this case, though each atom is
surrounded by four nearest neighbours, the location of the second-nearest
neighbour is not unique. In the amorphous semiconductors, the different possible
locations of farther-away neighbours are almost randomly filled leading to
disorder at long range. The effect of this long-range disorder is not significant
for energy levels deep inside an energy band but is important for those near the
edges, e.g., those near the top of the valence band and bottom of the conduction
band. It leads to narrowing of the energy gap as compared with the gap in
crystalline semiconductors. Some amorphous semiconductors are Ge, Si, Se,
As 2 Se 3 , etc.
Amorphous semiconductors are used in switching and memory components,
They are also used in xerographic processes. Here, typically a thin film of
amorphous selenium is deposited on a metallic substrate, usually Al. It is charged
electrically by means of a discharge. When a pattern of light to be copied falls
on this, the lighted areas become photoconductive and discharge their charge
whereas the dark areas retain their charge. A finely-powdered pigment is sprayed
on the surface. It is retained by the charged areas and then transferred to a sheet
of paper.
8.6 MAGNETIC PROPERTIES
In this section, we discuss the magnetic properties of materials. They are related
to the spin and orbital angular momentum of the electrons, and are observed in
the form of (i) diamagnetism (ii) free-electron paramagnetism
(iii) paramagnetism of atoms, ions or molecules, (iv) ferromagnetism, and
(v) antiferromagnetism and ferrimagnetism. Of these the first three are rather
weak effects while the last two are strong effects which are also of great
technological importance.