Negative capacitance (NC) is primarily used to reduce power dissipation in electronic devices and enable ultra-low power nanoelectronics. By exploiting unique properties in ferroelectric materials, NC can be used to overcome the limitations of conventional transistors and potentially lead to more energy-efficient electronic devices ranging from sensors to high-frequency, high-power gallium nitride (GaN) HEMTs.
Normal capacitance is the ability to store charge proportional to an applied voltage. When the change in charge occurs in the opposite direction to the change in applied voltage, that’s NC.
NC is primarily found in certain ferroelectric materials, and is sometimes called ferronic negative capacitance. Those ferroelectrics exhibit a double free energy characteristic below their transition temperature (Figure 1a). That results in an ‘S’ shaped curve when plotting polarization (P) versus the electric field (E) in the material (Figure 1b).
NC ferroelectrics
Examples of ferrielectric materials that can exhibit NC characteristics include:
- Hafnium zirconium oxide (HfO2-ZrO2), already used as a high-K dielectric in computer chips, has shown promise for NC applications.
- Lead zirconate (PbZrO3) has antiferroelectric properties and exhibits NC during non-polar-to-polar phase transitions.
- Hafnium dioxide (HfO2), or hafnia, is a high-K dielectric material that is sometimes used in place of silicon dioxide (SiO2) in MOSFETs. When combined, hafnia and SiO2 can create an NC effect.
- Lead titanate (PbTiO3), especially in nanoscale layers, is being developed for NC applications.
Overcoming the Boltzmann limit
Combining a ferroelectric layer with a dielectric layer, like SiO2, can create a structure that exhibits NC. Those heterostructures are being investigated as a possible tool for overcoming the Boltzmann limit.
The Boltzmann limit refers to the theoretical minimum achievable subthreshold swing (SS), which is a key measure of FET switching efficiency. Equivalent oxide thickness (EOT) scaling is a tool that aims to reduce the effective thickness of the gate dielectric, thereby increasing the capacitance and improving the performance of the device.
EOT scaling runs into limits with very thin dielectric layers. The use of high-K metal gates (HKMG) is the usual way to support continued scaling of FETs in ICs. However, the use of HKMG solutions is hitting a limit.
As noted above, hafnia is a high-K dielectric material that can be used in place of SiO2 in FETs. When combined, hafnia and SiO2 can create an NC effect and are one of the materials being explored as a possible solution to breaking through the Boltzmann limit.
When NC is integrated into the gate stack of a FET, it can effectively amplify the internal potential within the transistor, allowing a smaller change in applied gate voltage to achieve the same change in the channel’s potential, leading to a steeper subthreshold swing and exceeding the Boltzmann limit (Figure 2).
Using NC in GaN HEMTs
Like the FETs in ICs, GaN HEMTs, used in 5G and power conversion, have a limit called the HEMT limit that refers to the operational limits in terms of frequency, power, and temperature. Typical GaN HEMTs rely on Schottky gates for the high current “on” state. However, this can lead to high gate leakage in the “off” state, compromising efficiency.
A common solution is to add a conventional dielectric layer to reduce leakage. That also reduces the “on” current, creating a trade-off. One way to minimize the trade-off is to add a thin layer of dielectric material between the gate metal and the AlGaN barrier layer, forming a metal-insulator-semiconductor HEMT (MIS-HEMT), reducing leakage currents and enhancing switching performance.
By incorporating a ferronic NC layer, researchers have demonstrated that it may be possible to eliminate the trade-off. The NC enhances the gate’s control over the channel, boosting the “on” current, while the ferroelectric layer also helps reduce leakage (Figure 3).
Summary
In NC, the store charge changes in the opposite direction to changes in the applied voltage, for example, increasing when the voltage decreases. NC is being applied and explored for both digital and power semiconductor devices. For digital devices, it can help overcome the Boltzmann limit, and for GaN HEMTs, it may be able to simultaneously enhance gate control and reduce leakage.
References
Ferroelectric Materials On Radar Screen for Negative Capacitance, European Passive Components Institute
In-Memory Sensing and Logic Processing in Negative Capacitance Phototransistors, Advanced Functional Materials
Negative capacitance appears in ferroelectric materials, physicsworld
Negative Capacitance Devices, Nanoelectronic Materials Laboratory
Negative capacitance overcomes Schottky-gate limits in GaN high-electron-mobility transistors, arXiv
Progress and future prospects of negative capacitance electronics, APL Materials
Reconfigurable signal modulation in a ferroelectric tunnel field-effect transistor, Nature Communications
The ferroelectric field-effect transistor with negative capacitance, Nature NPJ Computational Materials
The future of electronics: harnessing negative capacitance FETs, Number Analytics
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