“Unlocking the Future: How a Simple Discovery Revolutionized Technology and Changed Our Lives Forever”
The triode or Audion, invented by de Forest in 1906, was similar to the diode but with an extra component: a metal grid between the anode and the cathode. Applying an electric charge to the grid repelled electrons coming from the anode, allowing the number that made it through to the cathode to be adjusted. This meant that a weaker current could be used to control a stronger one, allowing weak signals – like those from a radio receiver or telephone – to be effectively amplified. de Forest’s invention launched the modern era of electronics, making possible such breakthroughs as long-distance telephone and radio communications. Triodes were also widely used as electronic switches, being more reliable and less prone to wear than electromechanical relays. Indeed, the earliest electronic computers like the British Colossus – used to break the German Lorentz cipher – and the American ENIAC – used to generate ballistics tables for naval guns – used thousands of networked vacuum tubes to perform high-speed calculations.
However, vacuum tubes had a number of serious shortcomings. For one thing, their filaments needed to heat up in order to work, such that old electronic equipment like radios and television sets often took anywhere from a few seconds to a few minutes to fully power up. They were also fragile, consumed large amounts of power, and generated large amounts of heat, meaning early electronic computers required massive air conditioning plants to keep their processors cool. And while subminiature vacuum tubes just a few centimetres long were developed, these power and heat issues placed a lower limit on the size of electronic circuits. For such devices to be made truly compact and portable, a new, more compact and energy-efficient type of electronic switch was needed.
Ironically, the solution to this problem would ultimately be found in an older technology. As mentioned at the start of the video, early commercial radio sets used a device called a crystal detector to pick up radio signals. Also known as a cat’s whisker detector, this device comprised a crystal of lead sulphide or galena and a small spring called the cat’s whisker mounted on a pivoted handle. To use this type of radio, the user touched the cat’s whisker to various parts of the galena crystal until they found a spot that rectified the radio signal and allowed it to be heard over headphones.
As can be imagined, this device was finicky to use and took a great deal of practice to master. The crystal detector worked by forming a temporary metal-semiconductor junction, also known as a Schottky diode after its discover, German physicist Walter H. Schottky.
Galena, along with iron pyrite, carborundum, silicon, germanium, and several other substances, belongs to a class of materials known as semiconductors. Neither excellent conductors like most metals nor full-blown electrical insulators, semiconductors can have their electrical properties modified by treating or doping them with various impurities such as arsenic or phosphorus. Such doping produces either an N-type semiconductor, which has an excess of electrons in the outer shells of its atoms; or a P-type semiconductor, which has an excess of missing electrons – known as electron holes. Sandwiching a P and N semiconductor together produces a PN-junction. At the interface between the two semiconductors, the difference in electric charges causes a so-called diffusion current to flow, with electrons flowing from the N side to the P side and electron holes flowing from the P side to the N side. This in turn results in the formation of two adjacent layers of positive and negative change – known as the depletion region.
When an external current is applied from the N to the P side – that is, in the direction of the internal diffusion current – it will flow freely through the diode. If, however, the current is applied in the opposite direction, it will cause the depletion region to grow, forming a barrier through which the current cannot flow. A PN junction thus performs the same function as a vacuum tube diode, allowing current to only flow in one direction.
In a metal-semiconductor junction like a crystal detector, the semiconductor is N-type while the metal acts as the P-type semiconductor, with the interface between the two forming a depletion region or Schottky barrier like in a PN junction.
The PN junction diode was discovered in 1939 by Bell Labs researcher Russel Ohl when he accidentally cut a section of a silicon ingot across the PN junction and noted its rectifying qualities. During the Second World War, self-contained Schottky and PN diodes developed for use in military radars, as vacuum tubes could not operate on the required frequencies. These devices were the first truly solid-state miniaturized electronic components, and pointed the way toward the use of semiconductors to create a new, efficient analogue to the triode vacuum tube.
Interestingly, a design for a type of semiconductor-based electronic switch now known as a Field-Effect Transistor or FET was patented as early as 1925 by Austrian-American inventor Julius Lilienfeld. However, as sufficiently pure semiconductors were not available at the time, Lilienfeld was unable to construct a working prototype, and his design remained little more than a footnote in the history of electronics. It would not be until after the Second World War that his ideas would finally become a reality.
