What was the first semiconductor in use?
Think a minute.
Was it Silicon? Germanium? The Transistor from Bell Labs?
Nope. It was the crystal diode in old crystal radio sets.
A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used.
But the first ones were made of Galena
1874: Semiconductor Point-Contact Rectifier Effect is Discovered
In the first written description of a semiconductor diode, Ferdinand Braun notes that current flows freely in only one direction at the contact between a metal point and a galena crystal.
Ferdinand Braun shared the 1909 Nobel Prize in Physics with Guglielmo Marconi
German physicist Ferdinand Braun, a 24-year old graduate of the University of Berlin, studied the characteristics of electrolytes and crystals that conduct electricity at Würzburg University in 1874. When he probed a galena crystal (lead sulfide) with the point of a thin metal wire, Braun noted that current flowed freely in one direction only. He had discovered the rectification effect at the point of contact between metals and certain crystal materials.
Braun demonstrated this semiconductor device to an audience at Leipzig on November 14, 1876, but it found no useful application until the advent of radio in the early 1900s when it was used as the signal detector in a “crystal radio” set. (1901 Milestone) The common descriptive name “cat’s-whisker” detector is derived from the fine metallic probe used to make electrical contact with the crystal surface. Braun is better known for his development of the cathode ray tube (CRT) oscilloscope in 1897, known as the “Braun tube” (Braunsche Röhre) in German. He shared the 1909 Nobel Prize with Guglielmo Marconi for his “contributions to the development of wireless telegraphy,” mainly the development of tunable circuits for radio receivers.
So lead sulfide was first. Close behind it was another material. Silicon carbide.
Electronic circuit elements
Silicon carbide was the first commercially important semiconductor material. A crystal radio “carborundum” (synthetic silicon carbide) detector diode was patented by Henry Harrison Chase Dunwoody in 1906. It found much early use in shipboard receivers.
While not as sensitive as the Lead Sulfide detector, the Silicon Carbide “carborundum” detector was much more sturdy. It was used on ships as it stood up better to rough physical and electrical conditions. Still, ships carried spares just in case something like a lightning strike fried the primary one.
Then there’s another odd quirk of history. The LED has been around since before the transistor.
The phenomenon of electroluminescence was discovered in 1907 using silicon carbide and the first commercial LEDs were based on SiC. Yellow LEDs made from 3C-SiC were manufactured in the Soviet Union in the 1970s and blue LEDs (6H-SiC) worldwide in the 1980s. The production was soon stopped because gallium nitride showed 10–100 times brighter emission. This difference in efficiency is due to the unfavorable indirect bandgap of SiC, whereas GaN has a direct bandgap which favors light emission. However, SiC is still one of the important LED components – it is a popular substrate for growing GaN devices, and it also serves as a heat spreader in high-power LEDs
Seems that back when they were using those carborundum detectors, some of them glowed, and that lead to the discovery of the LED.
SiC LEDs came before transistors
Early in the 20th century, experimenters were finding that crystals of various substances such as germanium could give ‘unsymmetrical passage of current’ or rectification as we would know it, which found use in ‘crystal’ radios. When silicon carbide was tried, a strange phenomenon occurred; the crystal glowed yellow, sometimes green, orange or even blue. The first LED had been discovered, forty years before the transistor.
As an LED, SiC was soon superseded by gallium arsenide and gallium nitride with 10-100 times better emission but, as a material, SiC still generated interest in the electronics world; it has a thermal conductivity 3.5 times better than silicon and can be heavily doped for high conductivity while still maintaining high electric field breakdown. Mechanically, it is very hard, inert and has a very low coefficient of thermal expansion and high temperature rating. SiC does not even melt – it sublimates at about 2700⁰C.
Sturdy stuff indeed. Present at the start of the radio era. First LEDs, then set aside for more efficient materials. We’ll come back to those sturdy qualities a bit later.
If the walk was over a meteor impact crater you might find a few specks – the only naturally occurring SiC is in the form of Moissonite, debris from a supernova or ejecta from carbon-rich, red giant stars picked up in space and ending up as micron-sized particles in meteorites. Stardust indeed.
We might have never noticed SiC’s existence but in 1891 American inventor Edward G Acheson was trying to find a way to produce artificial diamonds, by heating clay (aluminium silicate) and carbon. He noticed shiny hexagonal crystals attached to the carbon arc light used for heating and called the compound carborundum, thinking it was a form of crystallized alumina like corundum. He might have thought he’d hit second best, as rubies and sapphires are types of corundum, but he realized he had something new, a compound nearly as hard as diamond which could be made as chips or powder on an industrial scale with application as an abrasive.
From that time forward we’ve used a lot of silicon carbide as industrial abrasives. Semiconductors not so much. Yet is is a semiconductor. (So I’m fighting back this urge to go get some Ultra Coarse carborundum paper and try making a diode with it ;-)
If we look at the current trend in semiconductors, the hot item is something called “Wide Band Gap” semiconductors. Why? I wondered. With a forward voltage of 0.2 VDC for Germanium, you get very little resistive heating. Germanium has been used for big power transistors for a long time and was the common material back when transitors were just replacing tubes. Silicon has a 0.7 VDC forward voltage. If you had a 2 VDC forward voltage, wouldn’t that make more heat and more problems? Well, it turns out there are other considerations.
Has an interesting chart with the band gap voltage for various materials. It is interesting to note that Diamond is listed with a band gap of 5.47 V. In theory one ought to be able to make a radio detector out of a diamond ring (details left as an exercise for the reader ;-) Silicon is shown as 1.12 V. This is different from the forward voltage of a junction device ( 0.7 V) due to doping and construction specifics. Germanium is 0.67 V.
Various SiC compounds with different ratios of C and Si give band gaps from 2.0 to 3.0 to 3.3 V. Relatively high. (Then there is Grey Tin at 0.0 V to 0.8 V – which gives me ideas but who knows what problems lurk…)
There’s a lot of other choices, from things like Aluminum Nitride or Boron Nitride both over 6 V to Zink Oxide or Zinc Sulfide at near 3.5 V. or Copper Sulfide at 1.2 V (first effective thin film solar cell). The point? While it would be necessary to take into account all the physical and chemical interactions of a given material, there are a lot of them to choose from and band gaps vary all over the place.
After a flush of activity in the Silicon technologies, folks have started to explore all these other system. Turns out that there are special properties for wide band gaps.
In the last century, silicon semiconductor-based power electronics – which control or convert electrical energy into usable power – transformed the computing, communication electric vehicle, and energy industries and gave consumers and businesses more powerful laptops, cell phones and motors. Over the coming decade, that era will begin to come to an end.
Today, Wide Band Gap (WBG) semiconductors offer new opportunities to achieve unprecedented performance while using less electricity. WBG semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) can operate at higher temperatures, have greater durability and reliability at higher voltages and frequencies and be smaller, more efficient and cost less. A WBG semiconductor-based inverter, which switches electricity from direct current to alternating current, could be four times more powerful, half the cost and one-fourth the size and weight of a traditional inverter.
Indeed, adoption of WBG semiconductors is already happening today, notably in photovoltaic power generation – solar power replacing power stations – followed by more general power grid applications. Next, WBG semiconductors will transform the plug-in EV industry – making it easier and cheaper to own an EV and/or give it longer range. They may reduce the size of a vehicle cooling system by about 60% and cut the size of a fast DC charging station (eg 60kW) to the size of a kitchen microwave.
Heady stuff… The band gap determines the color of light emitted in LEDs as well as that absorbed in solar cells. This bit of physics matters as does the temperature limits of a given material and what voltage will break down the gap.
Wide-bandgap semiconductors (WBG or WBGS) are semiconductor materials which have a relatively large band gap compared to typical semiconductors. Typical semiconductors like silicon have a bandgap in the range of 1 – 1.5 electronvolt (eV), whereas wide-bandgap materials have bandgaps in the range of 2 – 4 eV. Generally, wide-bandgap semiconductors have electronic properties which fall in between those of typical semiconductors and insulators.
Wide-bandgap semiconductors permit devices to operate at much higher voltages, frequencies and temperatures than conventional semiconductor materials like silicon and gallium arsenide. They are the key component used to make green and blue LEDs and lasers, and are also used in certain radio frequency applications, notably military radars. Their inherent qualities make them suitable for a wide range of roles, and they are one of the leading contenders for next-generation devices for general semiconductor use.
But why? There’s clearly something other than resistive loss at a junction going on here. Seems that it is thermalized electrons and how much they can leak across a junction.
The wider bandgap is particularly important for allowing wide bandgap devices to operate at much higher temperatures, on the order of 300 °C. This makes them highly attractive for military applications, where they have seen a fair amount of use. The high temperature tolerance also means that these devices can be operated at much higher power levels under normal conditions. Additionally, most wide bandgap materials also have a much higher critical electrical field density, on the order of ten times that of conventional semiconductors. Combined, these properties allow them to operate at much higher voltages and currents, which makes them highly valuable in military, radio and energy conversion settings. The US Department of Energy believes they will be a foundational technology in new electrical grid and alternative energy devices, as well as the robust and efficient power components used in high energy vehicles from electric trains to plug-in electric vehicles. Most wide-bandgap materials also have high free-electron velocities, which allows them to work at higher switching speeds, which adds to their value in radio applications. A single WBG device can be used to make a complete radio system, eliminating the need for separate signal and radio frequency components, while operating at higher frequencies and power levels.
Currently, WBG materials are far behind the development level of conventional semiconductors, which has seen massive investment since the 1970s. However, their clear inherent advantages in many roles, combined with some unique features of the materials themselves, has led to increasing interest as an everyday material, replacing silicon in even more commonplace roles. Their higher energy density handling is particularly interesting in attempts to extend Moore’s law, where conventional technologies appear to be reaching a density plateau.
The band gap also determines the color of light made, so if you want out of the IR range, you must go wide band gap.
Use in devices
Wide bandgap materials have several characteristics that make them useful compared to lower bandgap materials. The higher energy gap gives devices the ability to operate at higher temperatures, and for some applications, allows devices to switch larger voltages. The wide bandgap also brings the electronic transition energy into the range of the energy of visible light, and hence light-emitting devices such as light-emitting diodes (LEDs) and semiconductor lasers can be made that emit in the visible spectrum, or even produce ultraviolet emission.
There’s a bunch more about the QM effects and electron behaviours in the gap in that article. Worth a read if you want to know the hows and whys.
Silicon and other common materials have a bandgap on the order of 1 to 1.5 electronvolt (eV), which implies that such semiconductor devices can be controlled by relatively low voltages. However, it also implies that they are more readily activated by thermal energy, which interferes with their proper operation. This limits silicon based devices to operational temperatures below about 100 °C, beyond which the uncontrolled thermal activation of the devices makes it difficult for them to operate correctly. Wide-bandgap materials typically have bandgaps on the order of 2 to 4 eV, allowing them to operate at much higher temperatures on the order of 300 °C. This makes them highly attractive in military applications, where they have seen a fair amount of use.
Melting temperatures, thermal expansion coefficients, and thermal conductivity can be considered to be secondary properties that are essential in processing, and these properties are related to the bonding in wide bandgap materials. Strong bonds result in higher melting temperatures and lower thermal expansion coefficients. A high Debye temperature results in a high thermal conductivity. With such thermal properties, heat is easily removed.
So there’s the answer. It’s not about the creation of resistive heat, it is about the energetic electrons in a hot device not jumping the gap… This lets them work at higher voltages too. This is where are old friend Silicon Carbide returns to the stage:
High power applications
The high breakdown voltage of wide bandgap semiconductors is a useful property in high-power applications that require large electric fields.
Devices for high power and high temperature applications have been developed. Both gallium nitride and silicon carbide are robust materials well suited for such applications. Due to its robustness and ease of manufacture, semiconductors using silicon carbide are expected to be used widely, create simpler and higher efficiency charging for hybrid and all-electric vehicles, reduced energy loss and longer life solar and wind energy power converters, and elimination of bulky grid substation transformers. Cubic boron nitride is used as well. Most of these are for specialist applications in space programmes and military systems. They have not begun to displace silicon from its leading place in the general power semiconductor market.
Then there’s the cutting edge where folks have figured out how to make organic semiconductors. Who knows what that will lead to.
A Wide Band Gap Naphthalene Semiconductor for Thin-Film Transistors
Organic field transistors attracted considerable attention due to their excellent compatibility with flexible and biological substrates, low cost, easy processing and chemical tunability of properties that enable their wide application in electronics, including: large area flexible displays, smart cards and sensors, biomedical devices and radio frequency identification tags among many more applications. Many high mobility organic semiconductors have been developed based on π-extended acenes, oligothiophene and their derivatives. A majority of organic semiconductors have relatively low band gaps, typically in the visible region of the spectrum (~2-3 eV). On the other hand, large band-gap semiconductors would be beneficial due to their intrinsically higher stability and illumination-independent transport. They could likely find applications in transparent field-effect transistors that are currently fabricated with oxide semiconductors and carbon nanotubes.
Researchers led by professor Dmitrii Perepichka at McGill University in collaboration with professor Hong Meng at Nanjing Tech University in China developed a novel organic semiconductor 2,6-bis(4-methoxyphenyl)naphthalene (BOPNA) with unprecedentedly large band gap of 3.35 eV and an apparent hole mobility measured in thin-film organic field-effect transistors in a saturation regime. Their work, now published in Advanced Electronic Materials, presents a detailed analysis of photo-physical, electrical, and structural properties of BOPNA, showing its high thermal stability, facile morphology reorganization upon annealing at low temperatures, and band-like transport established through the temperature-dependent mobility measurements..
The researchers reported a multi-gram scale high-yield synthesis of BOPNA by a simple Suzuki coupling of commercially available 2,6-dibromonaphthalene with 4-methoxyphenylboronic acid. Photo-physical studies revealed the optical band gap of 372 nm demonstrating complete optical transparency of the film in the visible region. An excellent photo-stability of current response of BOPNA was also observed in the output characteristics of the organic field transistors.
In totality, a novel naphthalene semiconductor BOPNA with a wide band gap of 3.35 eV, high thermal, high photo-stability and complete optical transparency has been synthesized herein. A distinctive characteristic of BOPNA-based transistors is their excellent photo-stability and independence of the electrical characteristics of the illumination conditions; this is important for a wide-range practical application of low-cost organic electronics. Quoting Dr. Perepichka, “it is now clear that a notion of the “semiconducting region” in organic semiconductors being limited by 3 eV band-gap is just a misconception and that it is possible to design efficient field-effect transistors with large band-gap organic materials”.
So there you have it. A quick tour of the Wide Band Gap semiconductor and why you will be seeing more of them in the future. Literally, as they are the LED Light Bulbs in all the stores…