05-11-2020 | | By Sam Brown
Recently, researchers have developed transistors out of quantum dots to create functional blocks. What are quantum dots, what is the significance of the research, and what end applications can it help?
A quantum dot is a nano-sized particle that exhibits electrical and optical properties as a result of quantum effects as opposed to macroscopic effects. Generally speaking, nanoparticles (which can be made up of multiple atoms), have their electrons at specific energy levels, thus acting as a single atom collectively (artificial atom). One noticeable effect of quantum dots is their photoluminescent properties whereby they emit specific wavelengths of light when exposed to UV light. The light re-emitted by quantum dots is of a specific wavelength as a result of the discrete energy levels which electrons can be in, and this makes quantum dots useful for a wide range of applications including displays. Semiconductor quantum dots are those who can exhibit semiconductive properties (such as a change in conductance), and these are crucial to the creation of quantum dot transistors.
What challenges do modern semiconductor technologies face?
Modern semiconductors face many challenges that those of the past did not. To start, the size of features in modern semiconductors is now reaching individual nanometres in size, meaning that quantum effects, such as tunnelling, are becoming more apparent. Such effects can lead to a rise in power consumption, and as such, an increase in heat production. Quantum effects also make it difficult for low voltage logic levels as quantum tunnelling can result in invalid logical states. Modern semiconductors also face the challenge that creating small feature sizes is a very complex task that requires advanced engineering techniques such as Deep-UV which is only achievable by a handful of companies. Such technologies are also costly, and continuing the development of smaller devices will only further increase this cost. As feature sizes become smaller, and the number of transistors on a single die increases, then the chance of failure increases, thus potentially decreasing yield rates.
The ability for semiconductor quantum dots to change their charge blocking capabilities depending on a nearby electric field (known as Coulomb Blocking), allows for the creation of single-electron transistors and single-hole transistors. These devices operate similarly to traditional FET devices whereby an electric field from a gate controls whether electrons can move from the source to the drain. However, in a quantum transistor, the electron flow is controlled as a result of changing the energy levels of the quantum dot instead of its conductive ability.
One challenge that researchers have faced in the field of quantum dot transistors is the development of P and N-type devices on the same substrate. This is important as it allows for the implementation of CMOS logic which is essential for low-power, high-speed devices as well as highly efficient proven logic circuits. Recently, researchers from Los Alamos National Laboratory with partners from the University of California have achieved just this with the development of quantum dot-based logic gates and functional blocks. The logic gates utilise gold as the connecting medium with the PFET quantum transistor (Single Hole Transistor), and Indium as the connecting medium with the NFET quantum transistor (Single Electron Transistor). The gate to both devices sits underneath as the substrate layer (made of p++ Si), and the insulation layer between the gate and the quantum dots is made of Al2O3 with a thickness of 70nm.
The ability to put both N and P-type materials onto the same substrate is only half the announcement; the method of manufacture is simpler than that used by current semiconductor technologies and allows for chemical solution-based fabrication techniques. The use of solution-based manufacturing methods also brings the possibility of using quantum dot technologies in flexible applications which are often made using similar processes. Instead of requiring high-grade silicon in large foundries, circuits could be printed onto flexible substrates, and then quantum dots can be deposited using laboratory-like techniques. Such technology could realise flexible displays, wearable electronics, and implantable electronics that move organically with the body. The use of Indium and Gold also helps to produce a device that has low toxicity (when compared to others based on Cadmium), thus making the quantum dot transistors ideal for many biological applications.