Return of the Vacuum Valve

Until the 1950s, all active electronic functions were performed by the vacuum valve. They were made up of metal electrodes arranged in a vacuum glass envelope. Their sizes varied, but even one of the latest valves had a volume of more than one cubic centimetre. When solid state devices were invented, one of their main attractions was their small size. As the technology developed, individual elements became smaller and smaller, until complete circuits could be designed on a single piece of silicon. This development resulted in the replacement of vacuum valves by transistors in receivers and low-power electronic systems. In high power transmitters vacuum valves continue to and thermionic emitters are still used where a free source of electrons is required as in cathode-ray tubes. But semiconductor devices proved to be poorly equipped to survive certain environments.

For example, when semiconductor devices are exposed to ionizing radiation in space and defence systems, they are bombarded by both neutral and charged particles, which cause fluctuations in current leading to failure of the device. Vacuum tubes are far more immune to such environments. Vacuum tubes work at much higher voltages than semiconductors and they have the potential to provide high frequency operation. Therefore some research centres have developed research programmes for producing micron-sized vacuum electronic devices. It is the semiconductor fabrication technology which now offers the opportunity of producing vacuum tubes as small as transistors.

There are many potential applications of vacuum microelectronics, but they all centre on the properties of field emitting devices. For many years a great deal of effort has been directed towards finding a cold electron source to replace the thermionic cathode in such devices as cathode ray tubes and traveling wave tubes. Most research programmes have concentrated on cold cathodes to take advantage of the small device size, low power consumption and high current densities which in rum will lead to high operating frequencies and fast switching.

17. Name the main problems of the text.

18. Find the paragraph in the text which describes one of the main attractions of solid state devices.

19. Explain why most research programmes have concentrated on cold cathodes as a source of electrons.

20. Make questions to the text.

21. Name advantages vacuum tubes have over solid state devices when they operate in certain environments.

22. Express your attitude to the facts given in the text. You may use the following phrases:

‑ It is full of interesting information…

‑ I find the text rather / very cognitive…

‑ I’ve learnt a lot…

‑ I don’t agree with it…

23. Give a short summary of the text.

Part C

24. Look through the following text, define the information presented in it and entitle the text.

Text C

The property of semiconductors that makes them most useful for constructing electronic devices is that their conductivity may easily be modified by introducing impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level of conductivity. Doped semiconductors are often referred to as extrinsic. By adding impurity to pure semiconductors, the electrical conductivity may be varied not only by the number of impurity atoms but also, by the type of impurity atom and the changes may be thousand folds and million folds.

The materials chosen as suitable dopants depend on the atomic properties of both the dopant and the material to be doped. In general, dopants that produce the desired controlled changes are classified as either electron acceptors or donors. A donor atom that activates (that is, becomes incorporated into the crystal lattice) donates weakly-bound valence electrons to the material, creating excess negative charge carriers. These weakly-bound electrons can move about in the crystal lattice relatively freely and can facilitate conduction in the presence of an electric field. Conversely, an activated acceptor produces a hole. Semiconductors doped with donor impurities are called n-type, while those doped with acceptor impurities are known as p-type. The n and p type designations indicate which charge carrier acts as the material’s majority carrier. The opposite carrier is called the minority carrier, which exists due to thermal excitation at a much lower concentration compared to the majority carrier.

25. Find the following information in the text:

– what makes semiconductors the most useful for constructing electronic devices;

– what doping is;

– what the difference between intrinsic and extrinsic semiconductors is;

– two types of doped semiconductors;

– what n and p type designations indicate.

26. Speak about the importance of doping for constructing electronic devices.