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Transistors

 

These shortcomings in technology resulted in people trying to find an alternative way to valve. .
             Neils Bohr examined evidence by other scientists and recognised Planck's quanta and Einstein's idea that quanta applied to all electromagnetic radiations. He combined their ideas and predicted that Planck's oscillators were in fact electrons and that quanta were involved with electrons transferring from one energy state to another. Bohr's model only explained the hydrogen atom and couldn't explain some differences spectrum lines. Then in 1924 De Broglie released his model of the atom. One of the things he said was that electrons are arranged in various energy states. It is known that atoms have different energy bands that are filled with electrons and each level can hold a certain amount of electrons. In an atom there is a Valence band, a forbidden energy gap and the conduction band. The highest energy band is the valence band and the electrons occupy this. The conduction band is the uppermost energy band in the atom and it is empty. The forbidden energy gap lies between the valence and conduction band and no electrons are found here. From these energy levels they found that metals have very low electrical resistance because the conduction and valence bands overlap meaning there is more electrons available for conduction. Then during the Second World War they developed very pure germanium crystals to be used as rectifiers in radars. They used germanium because it has a smaller gap between the valence band and conduction band and the energy gap is 0.7eV this meant that with a bit of thermal energy the electrons could be freed into the conduction band and it would a conductor. Germanium was the semiconductor used up to about 1960 when it was replaced by silicon. Silicon too is a group 4 element like germanium; it has a 1.1eV energy gap. It is not found by itself, but it is a by-product of silica, and in the 1950's due to the difficulties in purifying silicon they went about to discover a suitable purification method.


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