The majority of electronics are built on transistors, semiconducting devices that control, amplify, and produce the flow of electrical current. Electronics experts have been working to create transistors that are smaller and smaller since doing so could enable the creation of more compact gadgets.
However, due to problems like short-channel effects and the leakage of current generated by the quantum mechanical process known as quantum tunnelling, shrinking transistors may have a negative impact on their energy consumption. By taking use of ferroelectric materials’ negative differential capacitance (NDC), smaller transistors’ energy consumption might be reduced.
As a result of a change in charge, the net voltage across a material shifts to the opposite direction as a result of the NDC phenomenon, whereby an increase in charge results in a drop in voltage. Ferroelectric hafnium dioxide, often known as hafnia, is one ferroelectric substance that might be utilised to achieve this.
“By incorporating a ferroelectric layer in the gate stack instead of a high-κ dielectric, a significant reduction in the operating voltage of transistors can be achieved,” Sanghyun Jo, one of the researchers who carried out the study, told Tech Xplore. “This is because the inclusion of a ferroelectric layer with NDC in the gate stack can result in an increased total capacitance compared to the case where the ferroelectric layer is not present. This stands in contrast to the conventional gate stack, where the addition of any layers in the stack always decreases the total capacitance.”
A rising number of research teams are attempting to take use of the NDC effect in transistors utilising techniques that can be easily integrated with current semiconductor fabrication processes as a result of the recent development of hafnia-based ferroelectrics. However, only a few number of research have been able to show how decreasing the energy consumption of transistors by including ultrathin ferroelectric hafnia into the gate stack is possible.
“So far, there is limited evidence of the electrical benefits of NDC on scaled silicon-based transistors,” Jo said. “Our primary objective was to demonstrate the observability of NDC in scaled silicon-based structures and investigate its potential for effective utilization in the development of advanced low-power logic devices.”
Jo and his colleagues first created ferroelectric hafnia (i.e., Zr-doped HfO2) films with a thickness similar to that used to make gate stacks in commercial logic devices in order to conduct their research. They eventually succeeded in creating ultrathin ferroelectric hafnia sheets with a thickness below 2 nm.
“The presence of ferroelectricity in these films was verified through various analyses,” Jo explained. “Subsequently, prior to incorporating the ferroelectric layer into the gate stack of FETs, we investigated the NDC phenomenon in Metal-Oxide-Semiconductor Capacitors (MOSCAPs) that included the ultrathin ferroelectrics with thicknesses reaching as low as 1 nm. The unambiguous proof of NDC was achieved by directly observing NDC through the S-shaped polarization-electric field relation.”
The NDC effect’s endurance when devices were working for a long period was also tested, as were the tunability of the NDC the researchers developed. Finally, they showed how the NDC of their ferroelectric sheets may improve field effect transistors’ (FETs’) performance. They did this by adding one of their 2nm films to a FinFET’s gate stack.
“This integration successfully generated NDC, as confirmed by the S-shaped polarization-electric field relation, within a voltage range suitable for FET operation, leading to a substantial enhancement in performance,” Jo said. “Therefore, our sequential approach involved the development of ultrathin ferroelectric hafnia films, verification of their ferroelectric properties, assessment of NDC observability, exploration of NDC tunability and endurance for long-term operation, and final application to FETs.”
Overall, the information acquired by Jo and his colleagues shows the potential for using ferroelectric materials’ NDC to build low-power logic devices. They particularly use ultrathin ferroelectric hafnia to showcase this promise through experimentation, but other ferroelectric materials could soon show the same benefits as well.
“The validation was achieved by directly observing NDC in scaled Si-based structures, and its confirmation was further supported by the observation of enhanced total capacitance,” Jo said. “These electrical measurements were further complemented by simulations. To ensure the robustness of our observations, we conducted in-depth analyses of the layers within the gate stack. This involved measuring their thicknesses and determining their precise atomic compositions, which was essential for verifying the electrical measurements and ruling out any potential high-k effects.”
In their research, Jo and his coworkers also presented a trustworthy methodology for tuning the ferroelectric hafnia’s NDC area, which is based on the manipulation of interface charges by doping methods. This technique, together with the resilient NDC phenomena they saw across 1015 voltage pulse cycles, may make it easier to successfully incorporate NDC in cutting-edge logic circuits.
It’s noteworthy that the team’s method for utilising NDC in tiny transistors doesn’t call for any structural changes to the devices and is equally appropriate for 3D architectures. This makes its widespread implementation simpler.
“In our recent study, we successfully demonstrated the potential of ferroelectric hafnia as a promising material for implementing NDC FETs,” Jo added. “Moving forward, our next objective will be to identify and optimize specific parameters or factors that can maximize the performance enhancement of NDC. This involves uncovering the precise mechanism of NDC in polycrystalline ferroelectrics, including ferroelectric hafnia, as the current NDC model is designed for single crystal ferroelectrics.”
