Beyond 1 and 0: Increasing Feasibility of Multi-Value Logic Transistors
Rachel Maya Gallagher posted on June 21, 2019 |

As technology approaches the physical minimum for transistor size, researchers have begun looking for alternative ways to make smaller computers work smarter. As technology gets smaller, faster and more portable, the number of binary transistors that can fit onto a single computer chip is reaching its limit. In the late 90s, we reached the physical size limit for integrated circuits on silicon chips. By 2025, we will no longer be able to crowd more transistors into that space.

Why Downsize?

If a computer chip can do more on a smaller device, the resulting new technology will have widespread applications in industry and consumer electronics.

“The concept of multi-value logic transistors is not new, and there have been many attempts to make such devices,” said Dr. Kyeongjae Cho, professor of materials science and engineering at the University of Texas at Dallas. “We have done it.”

Using Zinc Oxide to Achieve Multi-Value Logic

The key to a multi-value logic transistor lies in alternating layers of zinc oxide (ZnO) with 4-mercaptophenol molecular layers with aluminum linkers (Al4MP).

Inorganic layers of ZnO alternate with organic Al4MP, creating a composite system of QDs and amorphous domains that can generate three distinct states within one transistor. An energy diagram of the third state is pictured to the right. (Image courtesy of Dr. Kyeongjae Cho et al.)
Inorganic layers of ZnO alternate with organic Al4MP, creating a composite system of QDs and amorphous domains that can generate three distinct states within one transistor. An energy diagram of the third state is pictured to the right. (Image courtesy of Dr. Kyeongjae Cho et al.)

This creates a composite system of quantum dots (QDs) formed from ZnO nanocrystals and amorphous domains of ZnO. Localized states of the amorphous domains and quantized discrete states of QDs can be used to induce selective hybridization of energy states through resonant energy matching, resulting in a third state that researchers termed “quantized extended.”

Left, zinc oxide crystals are embedded in amorphous zinc oxide. Right, a computer model of the “quantized extended state” identified by the researchers. (Image courtesy of UT Dallas.)
Left, zinc oxide crystals are embedded in amorphous zinc oxide. Right, a computer model of the “quantized extended state” identified by the researchers. (Image courtesy of UT Dallas.)

Ternary, and even quaternary and beyond, systems already exist in the form of negative differential resistance (NDR) devices with multiple threshold voltage values and quantum dot gate field effect transistors (QDGFETs). However, neither can be feasibly integrated with existing hardware in the near future. NDR devices are confined to materials that exhibit negative resistance or the property of current flow decreasing as voltage across terminals increases. They are not yet compatible with traditional semiconductors. QDGFETs, meanwhile, are synthesized in solution and aggregated into thin films that are not exactly factory ready.

Most importantly, both of these systems do not demonstrate a steady intermediate current state across a range of gate voltages. The UT Dallas system operates on a silicon-based chip, closing the gap between theoretical and applicable multi-state logic.

Mobility Edge Quantization

What exactly is the quantized extended state discovered by Cho’s team? It is a stable intermediate state with a current that can be modulated by varying the thickness of the ZnO layer closest to the voltage source. This is a unique property of this particular ZnO-silicon hybrid since it would be expected that two parallel, connected layers of ZnO would increase in current simultaneously.

The quantized extended state is so termed because it only occurs at the mobility edge, the energy separating the amorphous and crystalline domains of a layer of ZnO. Whereas the localized state of the transistor, just before the mobility edge, exhibits wavefunctions that do not overlap with one another and the extended state exhibits a distributed wave function, the quantized extended state demonstrates a delocalized wavefunction only along the boundaries between ZnO crystals and amorphous domains.

In supplementary image 8 of the paper, researchers included an energy diagram with corresponding computer models of the three types of states observed in the new chip. State I is the localized state, State II the quantized extended, and State III the extended state. (Image courtesy of Dr. Kyeongjae Cho et. al.)
In supplementary image 8 of the paper, researchers included an energy diagram with corresponding computer models of the three types of states observed in the new chip. State I is the localized state, State II the quantized extended, and State III the extended state. (Image courtesy of Dr. Kyeongjae Cho et. al.)

A Future in Quantum Devices?

“Moving forward, I also want to see how we might interface this technology with a quantum device,” Cho said.

Existing quantum computing systems rely on transistors that allow a qubit, or quantum bit, to choose between two states. The use of quantum-mechanical phenomena to choose between three or more states could open up a new world of possibilities in computing.

For more on multi-state logic, check out the Gunn diode oscillators in this tech review. Additionally, quantum dots aren’t just for computer chips. They’re being implemented to generate electricity in solar windows.


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