The Problem with Scaling Down Transistors
Our modern transistors are typically made of semiconductor-type materials, most consisting of a type of silicon. As the semiconductor industry currently approaches nanometer node scales, it becomes much more difficult to fabricate such transistors without design complications.
Our current field effect transistors are approaching material limitations as the scalability of silicon and similar materials experience quantum effects that can cause inconsistency at nanometer scales. These issues have given rise to research invested in alternative materials such as graphene and wideband gap semiconductors.
State of the art fabrication methods for transistors have already reached the nanometer scale. There have been various distinct ways to build nanostructures 5nm or smaller using high fidelity fabrication methods; however, complications with transistor interference and manufacturing cost arise when we reach these scales.
There has been a large focus on the reduction of transistor size as we find new applications in our electronic and computer technologies. However, once the transistor drops below micrometer size, complications arise as the likeliness of the tunneling effects causing electron flow between adjacent transistors increases. In our current design approaches, the nanoparticle structures are not built individually but instead as complex systems with unreliable components. These components are also tailored to specific technologies, resulting in low compatibility between devices and making their integration difficult due to the cost of producing many different variations.
Coulomb Transistors via Organic Materials
A research team from the University of Hamburg has now found a way to produce inexpensive and potentially industrially suitable Coulomb transistors. The group, led by Dr. Christian Klinke, utilized a method known as the Langmuir-Blodgett method to produce self-assembled metal nanoparticles.
The Langmuir-Blodgett method is used to apply monolayers of organic materials by immersing a solid substrate into a liquid and then applying the liquid to a thin layer of hydrophobic film. Once on the hydrophobic film, the liquid will spread as far away from itself as possible but never leave the surface or allow the solvent to evaporate. Then, using controlled barriers the surface was pushed together and pressurized for two hours. This method leaves a densely packed monolayer without overlap or holes which can be extracted from the substrate.
Afterwards, the resulting monolayer can then be transferred to devices.
A single-electron transistor. Image by Fbianco
The Coulomb transistor consists of drain and source electrodes that are adjoined by tunnel junctions and a low self-capacitance electrode in-between. The low capacitance electrode is known as the island, which changes electrical potential as the voltage to the gate capacitor is changed. If enough positive voltage is applied to the gate, then the electrical potential of the island decreases, allowing the electron to tunnel through the island and into the drain
Since the monolayers are deposited individually to each area, the process provides versatility in transistor design, as the process can be tuned to create transistors of different scale and material. With this versatility, device properties such as the threshold voltage, Coulomb gap, and oscillation can be tuned to fit the needs of various electrical circuits.
The newly fabricated transistors exhibited an on/off ratio over 90% and had stable Coulomb oscillations in addition to their lower power consumption and potential scalability.
“By applying a voltage, we can shift the energy of this gap, which means that the current in the devices can be switched on and off as desired,” said Dr. Klinke. “The transport mechanism in these devices is based on percolative tunneling and hopping of electrons governed by Coulomb blockade instead of classical band transport as in silicon.
The team is currently working on developing an even smaller nanoparticle transistor, in addition to investigating chemical sensor applications.
The original article can be found in the Journal of Science Advances.