Semiconductor Junction

Modern display and solar cell technologies are built with a material called Indium Tin Oxide (ITO). ITO has excellent optical transparency and electrical conductivity, and the material properties needed for integration in large-scale manufacturing. However, we’re not content with just merely “good enough” nowadays, and need better materials to build ever better devices. Graphene and carbon nanotubes have been considered as suitable replacements, but new research has identified a different possibility: nanowires.

Researchers from the Indian Association for the Cultivation of Science (IACS) and the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) in Ireland have demonstrated a seamless silicon nanowire junction that can be used for photodetector and display technology.

Before you get lost in the jargon, let’s take a step back. A nanowire is just a very narrow length of wire, on the order of 1 nanometer across. When silicon is used at this scale, electrical charges can become stuck (called “charge trapping”), which means that the holes and electrons are separated, allowing for transistors and photovoltaics. By controlling where these holes form in the nanowire, you can create a “seamless” junction without using any dopant materials to create impurities, as is done in modern CMOS transistors

These material properties allow the functionality of a junction, but it still needs to be easily and repeatably manufactured. To solve this problem, the team put the nanowire transistors on a flexible polymer, which should enable flexible nanowire applications, such as a roll-up screen.

The first step towards a display is a simple photodetector, just consisting of a basic P-N junction, but they hope this technology will eventually be useful in “smart windows” due to the junctions’ applicability to photodetectors and cameras. Moving to emitting light for displays or creating a solar cell using this technology will probably take some time.

Do you have any experience with different materials for creating junctions? What would you do with a small, transparent photodetector? We’ve featured homebrew solar cells before, as well as creating DIY semiconductors. We’ve also seen silver nanowires for wearable circuits.

[Via IEEE Spectrum]

The semiconductor devices were put to the test under different atmospheres in this chamber.

One of the humbling things about writing for Hackaday comes when we encounter our readership and learn the breadth of our community and the huge variety of skills and professions you represent. Among your number are a significant representation among scientists, and as a result we often receive fascinating previews of and insights into their work. Sometimes they deserve a little bit more attention than one of our normal short daily pieces, and such a moment has come our way this week.

We’ve been fortunate enough to have an early look at a paper which makes detailed observations of a hitherto barely characterised property of semiconductor junctions that might have some interest for Hackaday readers in their work. In their paper, [Mellie], [Bacon] et al at Fulchester University in northeast England take a look at incandescent luminescence, a fleeting and curious effect exhibited by all semiconductor junctions in which they emit short-duration high-intensity infra-red and visible light with an extremely fast rise time when presented with high levels of current. This is a property which has been rarely exploited in commercial devices due to the large current densities required to reproduce it.

Incandescent Luminescence Explained

If you’ve never heard of incandescent luminescence before then you’re in good company, for neither had we until it was explained to us. It appears that there are a set of higher energy state conductivity bands in a semiconductor junction that can only be reached once the current passing through it breaches a threshold governed by the available quantum plasma dipole moment of the semiconductor material in question. At this point the junction assumes a plasma condition resulting in the abrupt emission of infra-red and visible radiation, the incandescent luminescence phase has been triggered.

A near-infra-red spectrum of incandescent luminescence in a halogen-doped silicon semiconductor junction. — A near-infra-red spectrum of incandescent luminescence in a silicon semiconductor junction.

Though it has been known to science since first being observed in the early 20th century by the earliest experimenters in the field of semiconductor junctions, the transitory nature of the phenomenon has traditionally been a barrier to its proper examination. The British team took a selection of commercial semiconductor devices very similar to the types that might be used by Hackaday readers, placed them in a chamber, and used an array of photoelectric sensors coupled with ionising detectors using americium-241 alpha radiation sources to measure their emissions.

The resulting data was then harvested for processing through a stack of custom high-speed ADC cards. Current densities from as low as a few milliamps to hundreds of amps were tested across forward-biased PN diode junctions using a computer-controlled DC power supply, resulting in a variety of spectra and showing the resulting thermionic photon emission at higher currents to have a preponderance in the infra-red region.

Incandescent luminescence in action, through an infra-red pyrometer.

A series of experiments were conducted to investigate a related effect first described by those early scientists in the field: that the atmosphere in which the semiconductor junction sits has a significant effect on the way it exhibits incandescent luminescence. Bathing it in gaseous CO₂ or nitrogen was found to reduce the phenomenon by as much as 95%, while immersing it in liquid nitrogen resulted in it becoming completely unobservable. Oxygen-rich atmospheres by comparison served to enhance the luminescence observed, to the point that in one of pure oxygen it reached an efficiency level of 100%.

The high conversion efficiencies and rapid onset of incandescent luminescence once it has been triggered compares favourably to those of existing devices such as LEDs or wire-wound resistors used where either infra-red or visible light is required. The researchers expect the effect to be exploited in such product families as photographic flash generators, electronic igniters, and other short-duration high-intensity applications. Given their obvious advantages, we’d expect their effects on those particular markets to be nothing short of incendiary.

Thanks Ellie D. Martin-Eberhardt for some invaluable inspiration and technical help with covering this story.

Near-IR spectrum: Deglr6328 [CC-BY-SA-3.0].

Test chamber: David Monniaux [CC-BY-SA-3.0].

Infra-red image: National Institute of Standards and Technology [Public domain].