The history of the diode is a fun one as it’s rife with accidental discoveries, sometimes having to wait decades for a use for what was found. Two examples of that are our first two topics: thermionic emission and semiconductor diodes. So let’s dive in.

Vacuum Tubes/Thermionic Diodes

Our first accidental discovery was of thermionic emission, which many years later lead to the vacuum tube. Thermionic emission is basically heating a metal, or a coated metal, causing the emission of electrons from its surface.

In 1873 Frederick Guthrie had charged his electroscope positively and then brought a piece of white-hot metal near the electroscope’s terminal. The white-hot metal emitted electrons to the terminal, which of course neutralized the electroscope’s positive charge, causing the leafs to come together. A negatively charged electroscope can’t be discharged this way though, since the hot metal emits electrons only, i.e. negative charge. Thus the direction of electron flow was one-way and the earliest diode was born.

Thomas Edison independently discovered this effect in 1880 when trying to work out why the carbon-filaments in his light bulbs were often burning out at their positive-connected ends. In exploring the problem, he created a special evacuated bulb wherein he had a piece of metal connected to the positive end of the circuit and held near the filament. He found that an invisible current flowed from the filament to the metal. For this reason, thermionic emission is sometimes referred to as the Edison effect.

But it took until 1904 for the first practical use of the effect to appear. John Ambrose Fleming had actually consulted for the Edison Electric Light Company from 1881-1891 but was now working for the Marconi Wireless Telegraph Company. In 1901 the company demonstrated the first radio transmission across the Atlantic, the letter “S” in the form or three dots in Morse code. But there was so much difficulty in telling the received signal apart from the background noise, that the result was disputed (and still is). This made Fleming realize that a more sensitive detector than the coherer they’d been using was needed. And so in 1904 he tried an Edison effect bulb. It worked well, rectifying the high frequency oscillations and passing the signals on to a galvanometer. He filed for a patent and the Fleming valve, the two element vacuum tube or thermionic diode, came into being, heralding decades of technological developments in many subsequent types of vacuum tubes.

Vacuum tubes began to be replaced in power supplies in the 1940s by selenium diodes and in the 1960s by semiconductor diodes but are still used today in high power applications. There’s also been a resurgence in their use by audiophiles and recording studios. But that’s only the start of our history.

Solid-State/Semiconductor Diodes

At almost the same time that Frederick Guthrie was discovering thermionic emission with his electroscope, in 1874, Karl Ferdinand Braun was investigating conductivity of metal salts in solution. He realized that some of the salts, such as galena (aka lead-sulfide), conducted when not dissolved. He subsequently discovered that its resistance varied depending on the magnitude and polarity of the voltage and that this effect worked best if the electrode was a pointed wire. And thus he invented the galena (aka lead-sulfide) point-contact rectifier. Galena is a semiconductor and so this was a semiconductor diode.

This is what became known as the cat’s whisker detector and was used in 1894 for experiments with microwaves. In 1906, G.W. Pickard patented a silicon detector while Henry Harrison Chase Dunwoody patented the carborundum detector. And so began the widespread use of the cat’s whisker in crystal radios which were made in the millions.

But by the 1920s vacuum tubes largely replaced the use of the cat’s whisker detector. However, during World War II, point-contact semiconductor detectors, both silicon and germanium, were revived for microwave radar detectors since the vacuum tube detectors couldn’t work at those frequencies.

After World War II, germanium diodes that didn’t have a point-contact that needed adjusting were manufactured in large quantities and proved to be as sensitive as galena. Since they didn’t need the adjusting that the cat’s whisker detector used, that began the age of crystal radios with modern semiconductor diodes.

Mercury-Arc Rectifier

A scary sounding and eerie looking type of diode was the mercury-arc rectifier. This was invented in 1902 by Peter Cooper Hewitt and developed in the 1920s and 1930s. These were used until the 1970s for converting high voltage AC and high current AC into direct current. They consisted of a container of mercury vapor provided by a pool of mercury at the bottom. The pool of mercury also acted as the cathode. Also in the container were carbon anodes. The mercury emitted electrons freely whereas the anode emitted very few. An arc was struck at the pool, which ionized the mercury vapor between the cathode and the anode, creating the conducting path. Applications included battery charging, arc lighting systems, trolleys, subways and electroplating.

Mercury-arc rectifiers were replaced in the 1970s by thyristors. But since thyristors have a gate contact in addition to the anode and cathode, we won’t cover them here.

Copper-Oxide And Selenium Diodes

Selenium diodes are another example of an early discovery followed by a delayed practical use. The first selenium diode was constructed in 1886 by C.E. Fitts but was not made practical until the 1930s. It eventually found use in radios, high current battery chargers, televisions and photocopiers. They were made of a steel plate with a layer of selenium and then a cadmium-tin layer between which a layer of cadmium-selenide formed. This selenium and cadmium-selenide formed a semiconductor-semiconductor junction. They could be easily stacked indefinitely to withstand high voltages. They were replaced in the 1960s by silicon rectifiers which have a lower voltage drop. In 1961 IBM tried to develop computer logic using selenium diodes due to their low cost but they proved to be not reliable and were replaced with silicon diodes.

Copper-oxide diodes were invented around the same time as selenium diodes and had similar uses. In their case the copper-oxide layer on copper metal formed the semiconductor layer. Like selenium diodes, they could be stacked to withstand high voltages. These also were replaced by silicon diodes.

Schottky Diode

It’s perhaps hard to find who invented the Schottky diode because even the cat’s whisker detector is a point-contact Schottky diode. A Schottky diode is formed of a metal in contact with a moderately doped n-type semiconductor and cat’s whiskers fits that description. The diode is named after German physicist Walter H. Schottky who came up with physics dealing with the metal-semiconductor junction.

DNA nano-diode

And why not end this history of the diode with some fun recent history? Published in April 4, 2016 in the journal Nature Chemistry, researchers from the University of Georgia and Ben-Gurion University reported that they have made a diode from DNA. They did this by inserting two small coralyne molecules at specific locations in a custom-designed 11-base-pair DNA duplex. When 1.1V was applied across the structure, fifteen times higher current flowed in one direction versus the other depending on the polarity. This may have an impact on the development of molecular electronic devices, but as we saw above, there’s sometimes a delay before the practical application. But as we also know, it’s worth the wait.

Conclusion

Where does word diode come from? William Henry Eccles, an English physicist, came up with it in 1919 by combining the Greek roots di, meaning ‘two’, and ode, meaning ‘path’, though some sources say the ode was borrowed from ‘electrode’ which was coined by Michael Faraday.

There are many other types of diodes that can be covered but space and the difficulty in finding the history of some demands we stop here. However, if you know of any other interesting steps in the history of the diode we’d love to hear about them in the comments below.