It is well-known that conventional field effect transistors (FETs) require a change in the channel potential of at least 60 mV at 300 K to effect a change in the current by a factor of 10, and this ...minimum subthreshold slope S puts a fundamental lower limit on the operating voltage and hence the power dissipation in standard FET-based switches. Here, we suggest that by replacing the standard insulator with a ferroelectric insulator of the right thickness it should be possible to implement a step-up voltage transformer that will amplify the gate voltage thus leading to values of S lower than 60 mV/decade and enabling low voltage/low power operation. The voltage transformer action can be understood intuitively as the result of an effective negative capacitance provided by the ferroelectric capacitor that arises from an internal positive feedback that in principle could be obtained from other microscopic mechanisms as well. Unlike other proposals to reduce S, this involves no change in the basic physics of the FET and thus does not affect its current drive or impose other restrictions.
Spin-based computing schemes could enable new functionalities beyond those of charge-based approaches. Examples include nanomagnetic logic, where information can be processed using dipole coupled ...nanomagnets, as demonstrated by multi-bit computing gates. One fundamental benefit of using magnets is the possibility of a significant reduction in the energy per bit compared with conventional transistors. However, so far, practical implementations of nanomagnetic logic have been limited by the necessity to apply a magnetic field for clocking. Although the energy associated with magnetic switching itself could be very small, the energy necessary to generate the magnetic field renders the overall logic scheme uncompetitive when compared with complementary metal-oxide-semiconductor (CMOS) counterparts. Here, we demonstrate a nanomagnetic logic scheme at room temperature where the necessity for using a magnetic field clock can be completely removed by using spin-orbit torques. We construct a chain of three perpendicularly polarized CoFeB nanomagnets on top of a tantalum wire and show that an unpolarized current flowing through the wire can 'clock' the perpendicular magnetization to a metastable state. An input magnet can then drive the nanomagnetic chain deterministically to one of two dipole-coupled states, '2 up 1 down' or '2 down 1 up', depending on its own polarization. Thus, information can flow along the chain, dictated by the input magnet and clocked solely by a charge current in tantalum, without any magnetic field. A three to four order of magnitude reduction in energy dissipation is expected for our scheme when compared with state-of-the-art nanomagnetic logic.
The emergence of negative capacitance as a way to limit power dissipation in CMOS logic transistors has raised the question of response delay of ferroelectric negative capacitance. Latency ...requirements for digital logic require a response time on the order of 10 ps or less. In this letter, we establish a coherent theoretical framework to analyze the delay between the clock edge at the gate and the response of the semiconductor channel in a ferroelectric negative capacitance transistor. The standard Landau-Khalatnikov equation approximates the slow, diffusive limit of the classical equation of motion. Therefore, using it to predict the response speed is unphysical. After extracting the damping and kinetic inductance from THz spectroscopy data, we simulate the full classical equation of motion and analyze the delay. We find that for doped hafnium oxides, the intrinsic delay is around 270 fs, far less than what is required for digital logic.
To further reduce the power dissipation in nanoscale transistors, the fundamental limit posed by the Boltzmann distribution of electrons has to be overcome. Stabilization of negative capacitance in a ...ferroelectric gate insulator can be used to achieve this by boosting the transistor gate voltage. Up to now, negative capacitance is only directly observed in polymer and perovskite ferroelectrics, which are incompatible with semiconductor manufacturing. Recently discovered HfO2‐based ferroelectrics, on the other hand, are ideally suited for this application because of their high scalability and semiconductor process compatibility. Here, for the first time, a direct measurement of negative capacitance in polycrystalline HfO2‐based thin films is reported. Decreasing voltage with increasing charge transients are observed in 18 and 27 nm thin Gd:HfO2 capacitors in series with an external resistor. Furthermore, a multigrain Landau–Khalatnikov model is developed to successfully simulate this transient behavior in polycrystalline ferroelectrics with nucleation limited switching dynamics. Structural requirements for negative capacitance in such materials are discussed. These results demonstrate that negative capacitance effects are not limited to epitaxial ferroelectrics, thus significantly extending the range of potential applications.
Negative capacitance in ferroelectric materials can be directly measured by applying bipolar voltage pulses to a series connection of a resistor and a ferroelectric capacitor. This measurement is used to show negative capacitance in ferroelectric HfO2, which is a promising material for energy‐efficient nanoelectronic devices. Additionally, this proves that even polycrystalline ferroelectrics can exhibit negative capacitance phenomena.
Spin orbit torque (SOT) provides an efficient way to significantly reduce the current required for switching nanomagnets. However, SOT generated by an in-plane current cannot deterministically switch ...a perpendicularly polarized magnet due to symmetry reasons. On the other hand, perpendicularly polarized magnets are preferred over in-plane magnets for high-density data storage applications due to their significantly larger thermal stability in ultrascaled dimensions. Here, we show that it is possible to switch a perpendicularly polarized magnet by SOT without needing an external magnetic field. This is accomplished by engineering an anisotropy in the magnets such that the magnetic easy axis slightly tilts away from the direction, normal to the film plane. Such a tilted anisotropy breaks the symmetry of the problem and makes it possible to switch themagnet deterministically. Using a simple Ta/CoFeB/MgO/Ta heterostructure, we demonstrate reversible switching of the magnetization by reversing the polarity of the applied current. This demonstration presents a previously unidentified approach for controlling nanomagnets with SOT.
One important use of layered semiconductors such as molybdenum disulfide (MoS2) could be in making novel heterojunction devices leading to functionalities unachievable using conventional ...semiconductors. Here we demonstrate a metal-semiconductor-metal heterojunction photodetector, made of MoS2 and amorphous silicon (a-Si), with rise and fall times of about 0.3 ms. The transient response does not show persistent (residual) photoconductivity, unlike conventional a-Si devices where it may last 3-5 ms, thus making this heterojunction roughly 10X faster. A photoresponsivity of 210 mA/W is measured at green light, the wavelength used in commercial imaging systems, which is 2-4X larger than that of a-Si and best reported MoS2 devices. The device could find applications in large area electronics, such as biomedical imaging, where a fast response is critical.
We analyze the effects of ferroelectric leakage on the performance of a negative capacitance field-effect transistor (NCFET), which has an intermediate metallic layer between the ferroelectric and ...the high-K dielectric. We show that, when designed without taking the dielectric leakage into account, the NCFET performance can actually degrade significantlywith respect to that of the baseline FET. To overcome these detrimental effects of leakage, we propose the concept of work-function engineering, where metals of dissimilar work-functions are used for the external gate electrode and the intermediate metallic layer. Using this approach, the ferroelectric charge-voltage characteristic is shifted along the voltage axis, which results in superior performance of the NCFET.
PDF
