The diode is one of the oldest and most important electronic devices, although it is not as famous as its cousin, the transistor. Used in all sorts of electrical and electronic systems, the diode functions as a one-way valve for electric current—it only allows current to flow in one direction. This is useful in converting AC to DC, processing high frequency signals, regulating voltages, and in other applications. There are two basic types of diodes. One is an electron tube similar to the triode. The other type uses semiconductors, like the transistor. Both were invented early in the 20th century.
The first diode was a modified light bulb. Thomas Edison discovered that including an extra electrode in a light bulb and connecting it to the positive side of a battery resulted in a current flowing from the filament through the empty space. He was not sure what to do with this discovery, and moved on to other projects. Others, however, found a use for this device. In the early 1900s, for example, English engineer John Ambrose Fleming used this one-way electrical “valve,” to convert radio waves into a flow of current that could be measured by a galvanometer. The Fleming valve is remembered as the first true electronic device. It came into use for radio transmission and soon became the basis of Lee De Forest’s Audion electron tube, which he invented in 1906.
Also around 1906, American engineer Greenleaf W. Pickard invented a new type of diode. Pickard based his design on the earlier discovery that electricity can flow in only one direction through certain types of mineral crystals, such as silicon. By placing a silicon crystal between a metal base and a carefully placed fine wire, Pickard created a valve that could also be used to detect radio waves. This type of “cat’s whisker” diode (so-named because of the fine wire used in it) became more popular after American H. C. Dunwoody patented a version of it that used a material called carborundum.