Thursday, August 07, 2014

Semiconductors are also Semiresistors

By definition, a semiconductor is any material with a mediocre electrical conductivity. In other words, more conductive than glass but say less conductive than copper. So, everything that's not so great at insulating or conducting could be considered a semiconductor. This vagueness is not helpful to understanding Semiconductors. The broad dictionary definition includes half the periodic table.  The most common today are: silicon, germanium, and compounds of gallium, usually in the form of slices of artificially grown crystals. Diodes, LEDs, transistors and integrated circuits are all made from semiconductor material.

This is really important to understand because you are reading this on a device (mobile or otherwise) that contains a number of semiconductors it relies upon to function. But they are not quite so new or complicated. The Nobel prize was bestowed upon Marconi and Karl Braun in 1909 for the discovery of semiconductor rectification. [source] But yes, that was a semiconductor.

In 1874, Braun had discovered that current only passed freely in one direction through lead sulfide (galena). Sadly he didn't find much use for that until radio was invented two decades later. But all this led directly to the invention of the transistor. Greenleaf Pickard got his patent in 1906 on a silicon-based rectifier.
 In 1907, Physicist George W. Pierce demonstrated the rectification properties of carborundum diodes after experimenting with metals and semiconductors. This rectifier required the application of current to function but was more mechanically stable. In 19276 Julius Lilienfeld proposed a metal–semiconductor triode rectifier using copper-sulfide semiconductors. These were actually early semiconductor field effect transistors.there is some debate as to whether any of  his devices worked.. but that's not actually required for a US patent.

Bell labs and other groups were looking for a solid-state equivalent to the vacuum tube. When they found one, it was probably an accident, examining the diode point-contact. The first transistors were of that type. But strangely they didn't invent the point-contact transistor first. William Shockley produced a field-effect theory in 1939 and Bell Labs announced the invention of the junction field-effect transistor in 1951 and began licensing it. What happened to that point contact transistor, that simple semiconductor?

Pickar's point-contact rectifier was awfully close back in 1906. In other more patentable words, it had a lot of prior art. But lacking the vocabulary, crystal oscillation wasn't referred to as a semiconductor. But by 1920 related works with crystals had already been made by Oleg V. Lossev, William H. Eccles, Frank W. Jordan,Eugen Nesper, Julius Lilienfeld, Robert G. Adams, William T. Ditcham and many others. Hugo Gernsback predicted that crystals would someday replace valves in circuits. Lilienfeld actually patented it in 1923. But the semiconductor was "invented" 3 decades later. More here

Semiconductors have low resistance in one direction and high resistance in the other, the inverse of their conductive properties. the same is true of their insulation properties. Braun's rectifier was a diode, the simplest possible semiconductor device. The transistor is just two diodes back to back, and the integrated circuit is made of hundreds of these etched onto a silicon chip.