Lesson 46 - Semiconductors - Diodes and TransistorsBut we've already discussed diodes. They are a simple form of vacuum tube aren't they? Well - yes and no. While diodes existed in tube form for many years vacuum tube diodes had their problems, and the electronics industry would try to find a way around those problems. Vacuum tube diodes did a fine job of rectifying (turning ac into dc), but they wasted a lot of electrical energy in the process, which made them inefficient and costly to operate. Quite a bit of power was lost just keeping the filament warm! They also had the problem of being physically fragile, and tended to be the main cause of an electrical equipment failure. As early as 1874, researchers noted that a metal-lead sulfide junction had rectifying properties. They found that it would conduct electrical current in one direction, but if they reversed the current, it would not flow in the opposite direction. This "junction" was "semi-conductive" in nature. They had, without knowing it, invented the semi-conductor. The problem was, they had no practical application for it, and to be honest, didn't understand how it worked. However, In 1926, P.H. Geiger and L.O. Grondahl discovered the rectifying properties in a semiconducting copper oxide-copper junction. Armed with W. Schottky's theoretical explanation of how it worked, this was the first practical diode that didn't involve a vacuum tube. Other materials that involved semi-conductive junctions included silicon, germanium, and selenium.  On Tuesday, December 16, 1947, Physicists John Bardeen and Walter Brattain, while working for William B. Shockley at Bell Laboratories, invented the semiconductor transistor. With the single statement "This thing's got gain!", Brattain announced the discovery of a SOLID-STATE device that could actually amplify electrical current!. By 1956 Bardeen, Shockley, and Brattain, shared the Nobel Prize for jointly inventing the transistor. It was a grand time for the electronics industry. The invention of the transistor would mean the disappearance of the tube... or would it?Still to this day, there are applications where the transistor isn't practical. Tubes tend to work better for high power or high frequency applications. It is a tube that you look at on your computer monitor (CRT = Cathode Ray Tube). Your TV set and microwave ovens employ tube technology. Most Television Transmitters use a tube as their final output stage. "Nanotubes" are the latest craze in electronic innovation. Audiophiles swear by the rich, full, warm, reproductive sound that can only tubes can make in audio circuits. It is my belief that we need to further understand both tube and transistor technologies, and use them in tandem. It is for this reason that I taught tubes first, and that I will emphasize them throughout the rest of the course. But now let's take a look at semiconductors: |
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