Sub-Nanosecond Switching of Si:HfO2 Ferroelectric Field-Effect Transistor.

The discovery of ferroelectric doped HfO2 enabled the emergence of scalable and CMOS-compatible ferroelectric field-effect transistor (FeFET) technology which has the potential to meet the growing need for fast, low-power, low-cost, and high-density nonvolatile memory, and neuromorphic devices. Although HfO2 FeFETs have been widely studied in the past few years, their fundamental switching speed is yet to be explored. Importantly, the shortest polarization time demonstrated to date in HfO2-based FeFET was ∼10 ns. Here, we report that a single subnanosecond pulse can fully switch HfO2-based FeFET. We also study the polarization switching kinetics across 11 orders of magnitude in time (300 ps to 8 s) and find a remarkably steep time-voltage relation, which is captured by the classical nucleation theory across this wide range of pulse widths. These results demonstrate the high-speed capabilities of FeFETs and help better understand their fundamental polarization switching speed limits and switching kinetics.

Ferroelectric transistors with asymmetric double gate for memory window exceeding 12 V and disturb-free read.

Ferroelectric field-effect transistors (FeFETs) with a single gate structure and using the newly discovered ferroelectric hafnium oxide as an active material are attracting considerable interest for nonvolatile memory devices. However, such FeFETs struggle to achieve a large separation between the two logic states (memory window, MW) because of the thickness limitations of the ferroelectric film. Moreover, they are affected by detrimental disturbs coming from the read operation because of the shared write and read paths. Therefore, significant performance improvements are needed for the device to compete with established memory technologies like flash. Here, we present an asymmetric double-gate FeFET structure, where only one gate stack comprises the ferroelectric layer. We propose a novel read operation at the non-ferroelectric gate and demonstrate an amplified MW exceeding 12 V thanks to the enhanced body effect factor and the increased sensitivity of the transfer characteristics to the ferroelectric polarization. As a result, the above physical limitation is circumvented, thus by far outperforming the MW values reported in the literature. Based on this, we implement the multi-level cell storage featuring 4 bits per cell and stable data retention. Finally, an essential benefit originating from the separated write and read paths in our structure is exploited to demonstrate the fully disturb-free read operation. Besides memory, this could be particularly favorable for those neuromorphic and in-memory computing concepts with an occasional update of the stored variable but a very frequent read.

Ferroelectric field-effect transistors based on HfO2: a review.

In this article, we review the recent progress of ferroelectric field-effect transistors (FeFETs) based on ferroelectric hafnium oxide (HfO2), ten years after the first report on such a device. With a focus on the use of FeFET for nonvolatile memory application, we discuss its basic operation principles, switching mechanisms, device types, material properties and array structures. Key device performance metrics such as cycling endurance, retention, memory window, multi-level operation and scaling capability are analyzed. We also briefly survey recent developments in alternative applications for FeFETs including neuromorphic and in-memory computing as well as radiofrequency devices.

Frequency Mixing with HfO2-Based Ferroelectric Transistors.

Second harmonic generation (SHG) and frequency mixing of electrical signals are fundamental for a wide range of radiofrequency applications. Recently, ferroelectric field-effect transistors (FeFETs), made from ferroelectric hafnium oxide (HfO2), have demonstrated promising SHG capabilities because of their unique symmetric transfer curves. In this paper, we illustrate how this symmetry is highly sensitive to material properties by varying the thickness of the ferroelectric layer and the doping of the silicon substrate. We show that the SHG conversion gain and the spectral purity are greatly increased (up to 96%) by precisely tuning the ferroelectric polarization reversal and the quantum tunneling currents. Based on this, we propose and experimentally demonstrate the generation of the difference and of the sum of two input frequencies (frequency mixing) with a single FeFET, which we attribute to the inherently strong quadratic component of the symmetric transfer characteristics. Because of the reversible and continuous ferroelectric switching in HfO2, our approach allows for an electrical control of the energy distribution of spectral components, thus opening up new and very promising paths for frequency manipulations with simple ferroelectric devices.