According to the UOW research’s official coverage on FLEET (‘FLEET’ being the Australian Research Council’s ‘Arc Centre of Excellence in Future Low-Energy Electronics Technology’), Dirac spin-gapless semiconductors (SGSs) are altogether “a new class of zero-gap materials”. As covered below, SGSs can be defined as materials that—quite unconventionally—do not have a bandgap. The UOW scientists published their SGS analysis in the journal, Small.
We cover UOW’s research and ultimately consider why the concept of SGSs is so important to the field of spintronics (further news on spintronics can be read here and here on EP’s sister site, All About Circuits).
Understanding Both Spin Gapless and Conventional Semiconductor Efficiencies
The concept of spin gapless semiconductors was first put forth in a proposal (published 2008) by Professor Xiaolin Wang, the theme leader of FLEET and the chief investigator behind the very 2020 SGS study in question. The initial concept eventually led to Wang’s original (2017) journal on Dirac spin-gapless semiconductors, published in National Science Review.
Now, with the potential of Dirac SGSs becoming increasingly realised, the materials have been said (by FLEET’s aforementioned news article) to have an “electron mobility ... two to four orders of magnitude higher than in classical semiconductors”.
But to understand how such a high level of efficiency is achieved in spin-gapless semiconductors, it is first important to understand the relevance of typical semiconductors—i.e., those that do have a (narrow) bandgap. Accordingly, the concept of bandgaps (namely within conventional materials) is covered next.
A photograph of chief investigator Professor Xialoin Wang, alongside diagrams that reflect the electron flow of Dirac-type spin-gapless semiconductors. Image Credit: FLEET.
Defining the Bandgap in Semiconductors
The bandgap in a semiconductor is the gap in energy between its valence band’s highest occupied energy state and the lowest energy state of its conduction band. The former is the highest energy band occupied by electrons, whereas the latter band is defined as “the outermost shells of the atoms ... in which electrons are free to move and thereby carry an electric current”. Before the below conduction process takes place, the conduction band’s elecron orbitals are initially empty.
To facilitate the creation of an electric current in any relevant material, electrons present in the valence band (VB) must be excited (owing chiefly to thermal excitation), so that they may then travel through to the orbitals of the conduction band (CB). Once there, they can then move freely to achieve an electrical conduction within the semiconductor. (As covered below, bandgaps are also important—although in rather different ways—to both insulators and conductors.)
No electrons can stay active within a bangap (reflective of this, the term is also sometimes referred to as an ‘energy gap’), and accordingly, the bigger the bandgap is, the lower the conductivity of the material. Therefore, as per the below diagram and caption, the bandgaps of an insulator and a conductor (i.e. metal) are large and non-existent respectively. This is due to the low conductivity in insulators, and, opposingly, the very overlap of the VB and CB within a typical conductor. Meanwhile, a conventional semiconductor sits in the middle of the two extremes: it must of course have a bandgap—but only ever a small bandgap.
With the importance of bandgaps now introduced—particularly in terms of conventional semiconductors—let’s now move on to the significance of UOW’s research into their proposed, “new class of zero-gap” semiconductors.
A diagram that represents, left to right, a non-existent, small, and large bandgap. These apply respectively to a conductor, i.e. metal (labelled 'a'), a semiconductor (b), and an insulator (c).
Comparing Conventional Semiconductors to SGSs
To reiterate (and to further refer to the above bandgaps diagram), note that, in conventional semiconductors, a small-yet-definite bandgap is the status quo. As explained on ResearchGate, “The bandgap of a semiconductor is the minimum energy required to excite an electron that is stuck in its bound [valence] state into a free [conduction] state where it can participate in conduction”.
The breakthrough of SGSs, as referred to in the UOW journal paper, is that SGSs prevent the need for such “minimum energy required” altogether. This is in view of spin-gapless semiconductors’ ability to function without any bandgap at all—all while still thriving in a semi-conductive state. The benefits of this are explained in FLEET’s said news piece: “In a spin-gapless semiconductor”, the researchers write, “conduction and valence band edges touch in one spin channel, and no threshold energy is required to move electrons from valence states to conductive states”.
The UOW scientists attribute the resultant, high electron mobility in Dirac SGSs to a marked improvement in semiconductor energy efficiency. As UOW’s Small journal paper makes clear, this is very advantageous to spintronics—and in fact, conventional electronics too. “[The] SGS material,” write the Wollongong authors, “holds great potential for practical applications in ultrafast and low-energy consumption spintronic and electronic devices”.
This is in view of the fact that spin-gapless semiconductors circumvent the energy dissipation that is observed in typical semiconductors. (For more information, see Professor Wang’s aforementioned 2017 paper, in which the author originally introduced the importance that dissipationless materials have in spintronics.)
Further Practicalities of Dirac Spin-Gapless Semiconductors
To elaborate on a previously-mentioned quote from FLEET: “For Dirac-type SGSs, their electron mobility is two to four orders of magnitude higher than in classical semiconductors. Very little energy is needed to excite electrons in an SGS: charge concentrations are very easily ‘tunable’ ”.
The UOW researchers state that such a ‘tunable’ quality is reflected by the ease in which the SGSs’ electrons may be excited by the use of either doping or gating (the latter being when a magnetic or electric field is applied).
All in all, the University of Wollongong’s proposed concept of spin-gapless semiconductors is a promising sign in terms of both spintronics and electronics—but also, of course, semiconductors themselves.