A reconfigurable ferroelectric field effect transistor for frequency multiplication.
Frequency multipliers, circuits that can produce signals with multiple frequencies, are essential components for a number of technological tools, particularly wireless communication systems. Most existing multipliers, however, are built using filtering and amplifying circuits that are bulky and quickly drain a large amount of power.
Researchers at NaMLab in Germany have recently devised a single ferroelectric field-effect transistor that can serve as both a full-wave rectifier and a frequency multiplier. The device they developed, presented in an article published in Nature Electronics, is fully reconfigurable and energy efficient, since it can be used in isolation, without the need for additional circuits.
“Our institute (NaMLab) has been researching ferroelectric hafnium oxide (HfO_2 ) since the ferroelectric properties of this material were discovered in 2007,” Halid Mulaosmanovic, one of the researchers, told TechXplore. who carried out the study. “An attractive electronic device that can be made using this material is a ferroelectric field effect transistor (FeFET), which resembles conventional logic transistors, but has a ferroelectric layer on the gate stack.”
Hfo_2 can reversibly and rapidly switch between two stable crystal configurations. This unique property enables the development of FeFETs that can be used both as non-volatile memories and as neuromorphic devices. NaMLab researchers have been investigating these two applications of HfO2FeFETs based in collaboration with GLOBALFOUNDRIES, a Dresden-based company that manufactures high-quality FeFETs.
As part of his research, Mulaosmanovic and his colleagues studied the responses of FeFETs with different ferroelectric configurations in their gate stack. Interestingly, they noticed that by following a specific set of electrical and manufacturing parameters, the current voltage (I-V) characteristics of these transistors became increasingly symmetrical, taking on a parabolic shape. This symmetry was further enhanced by a specific type of leakage current dubbed Gate Induced Drain Leakage (GIDL). Interestingly, GIDL currents can also be found in classic transistors, but in this case, they are undesirable and can hamper the performance of the transistors.
“In particular, we found that a high degree of IV symmetry was only enabled by the ferroelectric properties of the device, which is probably the reason why this behavior has not been observed in conventional (non-ferroelectric) transistors until now,” said Mulaosmanovic. . “This surprising symmetry triggered the idea of frequency multiplication, because these parabolic I-V curves naturally promote the phenomenon of frequency doubling.”
The study by Mulaosmanovic and his colleagues draws on previous studies investigating frequency multiplication. For example, in the 1980s researchers predicted that symmetry in the I-V curves of resonant tunneling diodes could have several benefits for frequency multiplication; a prediction that later turned out to be true. In the past, some research teams also designed graphene RTAs with symmetric ambipolar I-V curves for similar purposes.
“By carefully adjusting the amount of the switched ferroelectric domains in the FeFET and at the same time allowing a sufficient level of the GIDL current under proper device bias, highly symmetric IV characteristics can be obtained, which closely resemble those of a parable”, explained Mulaosmanovic. “Later, when a sinusoidal signal with a certain input frequency is applied to said device, the output current will have twice the input frequency. This happens because the parabola always gives a positive output for an oscillation of the input signal negative as well as positive.”
In his recent article, Mulaosmanovic and his colleagues presented a single device with frequency doubling properties that works without additional filter circuits, which are instead required by conventional multipliers. Due to the lack of these circuits, the device is more compact than existing multipliers and uses less power, making it ideal for many practical applications.
The new FeFET relies on a field effect to complete read and write operations, which makes it very fast and leads to lower power consumption. Additionally, the HfO2-based transistor is electrically reconfigurable, which means it can act as both a frequency doubler and transmitter.
“The design strategy that we devised is completely reconfigurable, in the sense that you can activate the multiplication property or turn it off simply by electrically reprogramming the FeFET, that is, changing its ferroelectric to the other crystalline state,” said Mulaosmanovic. “This reconfigurability provides additional value and great design flexibility.”
The HfO2-based FeFET created by the researchers is highly scalable and the researchers demonstrated that it can be reduced to 20 nm of the channel length. It is also CMOS compliant, so it can be easily manufactured using existing industrial processes.
“Our FeFETs can be co-integrated with classic logic transistors, which will be extremely useful for building FeFET-based radio frequency circuits on a single chip,” said Mulaosmanovic. “Our work also provides an example of how a harmful device property, such as GIDL, can be turned into an asset. In fact, device engineers and circuit designers tend to avoid GIDL, but we were able to exploit it in a hurry. very useful way.”
In the future, the device developed by Mulaosmanovic and his colleagues could be used as a substitute for more conventional frequency multipliers, improving the energy efficiency of wireless communication systems and radio frequency circuits. Until now, the researchers were only able to use their FeFET to double frequencies within a 1MHz range, due to limitations associated with their experimental setup and the integration of non-optimal devices. In their future studies, however, they will explore ways this frequency range could be expanded.
“We now plan to further investigate different integration possibilities: for example, different semiconductor channel materials could greatly increase the frequency range (for example, strained silicon or germanium), as well as more advanced integration schemes, such as FinFET or silicon-on-insulation (SOI) technology might be a better option,” Mulaosmanovic said. “We also plan to improve the spectral purity of the output signal and one approach could be to further fine-tune the ferroelectric properties of the device. There is still a lot of room for improvement, but FeFET is a very promising device and we look forward to many more advancements for this exciting application.” .
