When the proper impurities are added in trace amounts, semiconductors display interesting and useful properties. This process is usually referred to as doping. The oldest ancestor of semiconductor devices was the crystal detector that was used in early wireless radios. This device was composed of a single metal wire (often called a "cat's whisker") that would touch against a semiconductor crystal. The result was a rectifying diode that allows current to flow easily in one direction but hinders the flow in the other direction. The rectifying diode had two terminals, but by 1930 vacuum-tube diodes had pretty much replaced the smaller crystal detector. The crystal and "cat's whisker" were abandoned, and would eventually emerge as popular children's toys known as crystal radios.
The development of radar during World War II did a lot to revive the fate of crystal detectors. Though they were temperamental, crystals were better than vacuum-tube diodes at rectifying the high frequencies used by radar. During the war, a lot of effort was invested in improving semiconductors, particularly the silicon and geranium used in crystal detectors. ...
Most of the "magic" surrounding semiconductor devices occurs at the barrier between P-type and N-type semiconductor material. This type of barrier is called a P-N junction. Ohl and his peers found that a P-N junction created an effective diode.
Like many other components, diodes possess a positive side and a negative side. The positive side is known as an anode, and the negative side is known as a cathode. When the voltage on the anode is higher than on the cathode, the current flows through the diode with very low resistance. When the voltage is lower on the anode than on the cathode, the current is prevented from flowing due to a very high resistance. An easy way to commit this to memory is by examining the symbol for a diode. The arrow in the diode symbol points in the direction in which it allows current to flow. The cathode of a diode is usually marked with a line next to it. A similar line can be observed in the schematic symbols above the arrow. Diodes are also sometimes marked with an identifying color code that is similar but not identical to those used for resistors.
It is important to note that when current is flowing through a diode, the voltage on the positive leg is higher than that on the negative leg. This phenomenon is generally referred to as the diode's "forward voltage drop." The magnitude of the voltage drop is a function of the semiconductor material that the diode is made from. Silicon diodes are the most common and the cheapest. They have a forward voltage drop of about 0.65 volts. Geranium diodes have a forward voltage drop of approximately 0.1 volt. Geranium diodes, however, are typically much more expensive that silicon diodes. But, they are salvageable and can be collected from old