What’s An Exciton?

If you read the scientific literature, you see the familiar subatomic particles you learned about in school: protons, neutrons, and electrons. If you are young enough, you see others you probably heard about, too, like quarks and gluons. But recently there has been a lot of buzz about excitons and even some transistor circuits demonstrated that use them. But what is an exciton?

It actually sounds like a subatomic particle, but it is a little more complicated than that. An exciton is a bound state of an electron and an electron hole and is technically a boson. You are probably familiar with the idea of an electron hole from semiconductor physics. Technically, it is a quasiparticle. The reason scientists are interested in the beast is that it can transport energy without transporting net electric charge. That is, the state itself is neutral, but also contains energy. Continue reading “What’s An Exciton?”

New Silicon Carbide Semiconductors Bring EV Efficiency Gains

After spending much of the 20th century languishing in development hell, electric cars have finally hit the roads in a big way. Automakers are working feverishly to improve range and recharge times to make vehicles more palatable to consumers.

With a strong base of sales and increased uncertainty about the future of fossil fuels, improvements are happening at a rapid pace. Oftentimes, change is gradual, but every so often, a brand new technology promises to bring a step change in performance. Silicon carbide (SiC) semiconductors are just such a technology, and have already begun to revolutionise the industry.

Mind The Bandgap

A graph showing the relationship between band gap and temperature for various phases of Silicon Carbide.

Traditionally, electric vehicles have relied on silicon power transistors in their construction. Having long been the most popular semiconductor material, new technological advances have opened it up to competition. Different semiconductor materials have varying properties that make them better suited for various applications, with silicon carbide being particularly attractive for high-power applications. It all comes down to the bandgap.

Electrons in a semiconductor can sit in one of two energy bands – the valence band, or the conducting band. To jump from the valence band to the conducting band, the electron needs to reach the energy level of the conducting band, jumping the band gap where no electrons can exist. In silicon, the bandgap is around 1-1.5 electron volts (eV), while in silicon carbide, the band gap of the material is on the order of 2.3-3.3 eV. This higher band gap makes the breakdown voltage of silicon carbide parts far higher, as a far stronger electric field is required to overcome the gap. Many contemporary electric cars operate with 400 V batteries, with Porsche equipping their Taycan with an 800 V system. The naturally high breakdown voltage of silicon carbide makes it highly suited to work in these applications.

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Perovskites: Not Just For Solar Cells Anymore

If you’ve been around long enough, you’ll know there’s a long history of advances in materials science that get blown far out of proportion by both the technical and the popular media. Most of the recent ones seem to center on the chemistry of carbon, particularly graphene and nanotubes. Head back a little in time and superconductors were all the rage, and before that it was advanced ceramics, semiconductors, and synthetic diamonds. There’s always some new miracle material to be breathlessly and endlessly reported on by the media, with hopeful tales of how one or the other will be our salvation from <insert catastrophe du jour here>.

While there’s no denying that each of these materials has led to huge advancements in science, industry, and the quality of life for billions, the development cycle from lab to commercialization is generally a tad slower than the press would have one believe. And so when a new material starts to gain traction in the headlines, as perovskites have recently, we feel like it’s a good opportunity to take a close look, to try to smooth out the ups and downs of the hype curve and manage expectations.

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In Praise Of The App Note

When I am at a loss for an explanation in the world of electronics, I reach for my well-thumbed Horowitz & Hill. When H&H fails me which is not that often, the chances are I’ll find myself looking in an application note from a semiconductor company who is in cut-throat competition with its rivals in a bid for my attention. These companies have an extensive sales and marketing effort, part of which comes in the dissemination of knowledge.

Razor blades may be sold to young men with images of jet fighters and a subtle suggestion that a clean-shaven guy gets his girl, but semiconductor brands are sold by piquing the engineer’s interest with information. To that end, companies become publishing houses in praise of their products. They produce not only data sheets that deal with individual device, but app notes documents which cover a wider topic and tell the story of why this manufacturer’s parts are naturally the best in the world.

These app notes frequently make for fascinating reading, and if you haven’t found them yet you should head for the documentation sections of semiconductor biz websites and seek some of them out.

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Julius Lilienfeld And The First Transistor

Here’s a fun exercise: take a list of the 20th century’s inventions and innovations in electronics, communications, and computing. Make sure you include everything, especially the stuff we take for granted. Now, cross off everything that can’t trace its roots back to the AT&T Corporation’s research arm, the Bell Laboratories. We’d wager heavily that the list would still contain almost everything that built the electronics age: microwave communications, data networks, cellular telephone, solar cells, Unix, and, of course, the transistor.

But is that last one really true? We all know the story of Bardeen, Brattain, and Shockley, the brilliant team laboring through a blizzard in 1947 to breathe life into a scrap of germanium and wires, finally unleashing the transistor upon the world for Christmas, a gift to usher us into the age of solid state electronics. It’s not so simple, though. The quest for a replacement for the vacuum tube for switching and amplification goes back to the lab of  Julius Lilienfeld, the man who conceived the first field-effect transistor in the mid-1920s.

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New Transistor Uses Metal And Air Instead Of Semiconductors

The more things change, the more things stay the same. Early electronic devices used a spark gap. These have been almost completely replaced with tubes and then semiconductor devices such as transistors. However, transistors will soon reach a theoretical limit on how small they can be which is causing researchers to find the next thing. If the  Royal Melbourne Institute of Technology has its way, we’ll go back to something that has more in common with a spark gap than a conventional transistor. You can find the source paper on the Nano Papers website although the text is behind a paywall.

The transistor uses metal, but instead of a semiconductor channel — which is packed with atoms that cause collisions as electrons flow through the channel — the new device uses an air gap. You might well think that if fewer atoms in the channel are better, why not use a vacuum?

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What Good Are Counterfeit Parts? Believe It Or Not, Maybe A Refund

[Charles Ouweland] purchased some parts off Aliexpress and noticed that the Texas Instruments logo on some of his parts wasn’t the Texas Instruments logo at all, it was just some kind of abstract shape that vaguely resembled the logo. Suspicious and a little curious, he decided to take a closer look at the MCP1702 3.3v LDO regulators he ordered as well. Testing revealed that they were counterfeits with poor performance.

Left: counterfeit part. Right: genuine Microchip MCP1702-3302

Looking at the packages, there were some superficial differences in the markings of the counterfeit MCP1702 versus genuine parts from Microchip, but nothing obviously out of place. To conclusively test the devices, [Charles] referred to Microchip’s datasheet. It stated that the dropout voltage of the part should be measured by having the regulator supply the maximum rated 250 mA in short pulses to avoid any complications from the part heating up. After setting up an appropriate test circuit with a 555 timer to generate the pulses for low duty cycle activation, [Charles] discovered that the counterfeit parts did not meet Microchip specifications. While the suspect unit did output 3.3 V, the output oscillated badly after activation and the dropout voltage was 1.2 V, considerably higher than the typical dropout voltage of 525 mV for the part, and higher even than the maximum of 725 mV. His conclusion? The parts would be usable in the right conditions, but they were clearly fakes.

The usual recourse when one has received counterfeit parts is to dump them into the parts bin (or the trash) and perhaps strive to be less unlucky in the future, but [Charles] decided to submit a refund request and to his mild surprise, Aliexpress swiftly approved a refund for the substandard parts.

While a refund is appropriate, [Charles] seems to interpret the swift refund as a sort of admission of guilt on the part of the reseller. Is getting a refund for counterfeit parts a best-case outcome, evidence of wrongdoing, or simply an indication that low value refund requests get more easily approved? You be the judge of that, but if nothing else, [Charles] reminds us that fake parts may be useful for something perhaps unexpected: a refund.