Semiconductors

The electrical qualities of semiconductors lie half-way between those of insulators and conductors. Semiconductor material is not only characterized by its resistivity factor but also by the great influence that various factors, such as heat and light, have upon this value.

The semiconductor group is made up either of elements or of chemical compounds. These have four valence electrons in there outer shell. Two of these elements, germanium and silicon, are especially important.

All the elements of interest in semiconductor physics are given from periodic table of elements. The group number is the number of valence electrons. With each element appears its chemical symbol and atomic number, which is the number of electrons within the atom.

Some intermetallic command pentavalent elements behave like the tetravalent elements. These are the alloys of two metals, such as gallium arsenide, indium antimonide, and so on.

Finally, various oxides, notably copper and titanium oxides, behave like semiconductors. We shall also mention the element selenium, which does not belong to the fourth group of elements.

A germanium atom has 32 electrons. Of these, 28 are in the inner shells and 4are in the outer shell. These four electrons in the valence band take part in any chemical reactions or electrical conductivity.

Transistors and Vacuum Tubes

The transistor is often better understood as a part of an electronic circuit with the help of analogy. It is now desirable to make a final comparison between two amplifying devices.

In vacuum tubes, the control of the electron flow is performed in a nearly perfect vacuum, whereas in the transistors the flow of charged carries (either electrons or holes) is performed in a nearly perfect crystal lattice of a semiconductor. The vacuum tube can be represented as an equivalent resistor whose magnitude is changing with the variations of the control-grid bias; the result is the changing current and voltage across the load resistor in the output. Similarly, a transistor can be conceived as a variable resistor whose magnitude changes with variations of the base current. In fact, the name transistor, implying this concept, is a combination of the words transfer and resistor.

An analogy can be drawn between the functions of the electrodes of a transistor and the function of a triode tube. The emitter can be compared to the cathode, the base to the grid, and the collector to the anode. A further can be drawn with the basic configurations; the CE configuration corresponds to the grounded-cathode circuit, the CB configuration to the grounded-grid circuit, and the CC configuration to the cathode-follow. Moreover, the electrical characteristics of the transistor and the vacuum tube are very similar.

The Atomic Pile

Every achievement of science can bless or curse mankind. Atomic energy may kill or cure. Many peacetime applications use artificially radioactive nuclei manufactured in the atomic pile.

In order to understand how a pile operates, you must know how atoms of low atomic weight slow down neutrons. Suppose uranium-235 to be embedded in paraffin. Each neutron that strikes a hydrogen nucleus head on will stop like a billiard ball hitting another squarely. The neutron will lose all of its kinetic energy. In a slanting impact, the neutron will lose a considerable fraction of the energy, a substance of low atomic weight that slows down neutrons being called a moderator.

Graphite is used as a moderator in the atomic pile. It is cheap, can be highly purified, and can stand high temperatures. Some of the neutrons transform U-238 into neptunium, whish becomes plutonium. Other neutrons bumping into the graphite nuclei get slowed down so that they split U-235 nuclei to liberate more neutrons and keep the “fire burning”.

The beryllium rods absorb neutrons. Inserting these rods farther into the pile decreases the activity and dampens the “fire”.