International

New Whys of G

Charles S< Jonest KZPBY 181 Pennsylvania Avenue Clairtonr Pennsylvania 15025

It is now more than a quarter of a century since a small group of physicists at the Bell Telephone Laboratory invented the transistor. The enormous technical progress that has been made in the art of semiconductor electronics during these years has been extraordinary, The field of solid state technology is moving so fast that one has difficulty in keeping up with current devel-opEnents. This is especially true in the area of microwave technology. Here, semi* conductors are making monumental strides that are causing a revolution in the design of microwave circuitry.

In this article I will discuss some of the most recent aspects of solid state application growth in the field of microwave power generation. Since the theory of operation of these devices is rather complex and involved, I have not devoted much attention to this area of thought. My main purpose is to shed light on what to expect from solid state microwave devices in the near future.

Today, one of the most versatile semiconductor materials is gallium arsenide. Its use has contributed greatly to the development of numerous solid state microwave devices. Transistors, varactors, microwave diodes and many other ultra-high devices are possible with gallium arsenide.

Just recently, another remarkable application for this multi-faceted semiconductor material was unearthed. It was discovered that a tiny chip of gallium arsenide can be made to emit microwaves simply by applying a steady dc voltage across it. This phenomenon, known as the Gunn Effect, is expected to revolutionize microwave technology. Ever since J. B, Gunn of the International Business Machine Corporation discovered this new solid state source of microwave radiation, engineers have been waiting for the potentially low-cost semiconductor to be made practical. All that would be required for a microwave source of power then would be a battery, a resonant cavity and the small chip of gallium arsenide. No longer would a microwave system require a klystron and its associated bulky power supply, or a radio-frequency oscillator with several stages of harmonic varactor multipliers j or a power limited microwave transistor.

Between the valence and conduction levels there is an energy level called the "forbidden" or depletion level, because no electron ever contains that exact energy level. The reason is not know, but the fact is that nature has prohibited certain energy levels.

Like silicon and germanium, gallium arsenide owes its semiconducting characteristics to the structure of its energy bands. These energy bands are shown in Fig, h Of course the depletion region in this illustration is amplified to a great extent. There is no sharp division between the crystal regions and the depletion region. As you move out from the junction between the P-and N-areas the charge density becomes less and less, and the number of charge carriers more and more.

At ordinary room temperature there will be very few electrons which will possess enough energy to cross the depletion region and therefore create a current flow. However, if the semiconductor material is doped with certain impurity atoms, electrons can be added at energy levels which are just below the conduction band. Very little energy is then required to boost these electrons across the depletion region and produce a flow of current.

These materials which are added to the semiconducto] material are called donar impurities because each atom donates an

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