Understanding a Ferroelectric Hysteresis Loop in Electronics

October 15, 2020 Cadence PCB Solutions

Key Takeaways

  • Ferroelectric materials exhibit a hysteresis loop, just like ferromagnetic materials.

  • Ferroelectrics also retain their polarization state after being exposed to an electric field, making them useful as memories.

  • The current class of ferroelectric components is limited to memories from a few manufacturers, but newer areas of electro-optics and electronics can take advantage of ferroelectrics to create a broad range of devices.

 Ferroelectric RAM (FRAM) component, which takes advantage of ferroelectric hysteresis loops to store data

Ferroelectric RAM takes advantage of a ferroelectric hysteresis loop to store data.

When you think of hysteresis, you probably think of ferromagnetic components like iron-core inductors and ferrite transformers. Hysteresis also occurs in Schmitt triggers to provide some level of noise immunity, which provides simple electrical switching once the input voltage exceeds some predefined threshold. There is another class of materials that exhibits hysteresis and a type of switching behavior: ferroelectrics.

A ferroelectric material is the electrical analog of ferromagnetic materials, but the physical mechanism that drives hysteresis in these materials is different. Ferromagnetics have enjoyed many applications involving power conversion, filtering, and isolation, but ferroelectrics have not enjoyed the same level of adoption. Here are some of the potential applications of this unique class of materials and how they might behave in your circuits.

What Is a Ferroelectric Hysteresis Loop?

Unique devices in electronics can take advantage of hysteresis in ferroelectric materials, which occurs through interaction between the electric field and matter. The difference between ferroelectric materials and other dielectrics is that ferroelectrics retain their polarization after the field is removed, while other dielectrics return to a neutral state. Furthermore, ferroelectrics can be returned to a neutral state as long as a sufficiently strong opposite-pointing field is applied to the material.

This is similar to ferromagnetism in that a ferromagnet can retain its magnetization after being exposed to a magnetic field. This need to overcome a threshold to change the magnitude and direction of remnant polarization means these materials exhibit hysteresis, just like a magnet. The graph below shows a ferroelectric hysteresis loop, which has the same basic structure as a magnetic hysteresis loop. Some important points are marked on the figure.

Ferroelectric hysteresis loop and polarization graph

Ferroelectric hysteresis loop showing total polarization in a ferroelectric material.

Important Points on a Ferroelectric Hysteresis Loop

There are three important points on a ferroelectric hysteresis loop: 

  • Electric coercivity (EC): This is the electric field required to switch the polarization between positive and negative values. Note that a positive field can induce negative polarization, giving rise to negative capacitance.

  • Remnant polarization (PR): The amount of polarization that remains in the material after the electric field is removed

  • Saturation polarization (PS): This is the maximum amount of polarization that can be induced in the material at high electric field strength.

Note that the same points can be extracted from a ferromagnetic hysteresis loop. These important points on the hysteresis loop depend on the physical mechanism that drives polarization hysteresis in ferroelectric materials.

Drivers of Ferroelectricity

At the macroscopic level, an incident electric field creates a shift in the spatial distribution of bound charges, which is quantified as polarization in Maxwell’s equations. The structure of these materials allows this change in the bound charge distribution to be locked in, where bound charge remains in the new distribution even after the incident electric field is removed. The physical mechanisms for this phenomena at the microscopic level include ion migration and trap state filling, to name a few. 

Mathematically, polarization in the material is piecewise nonlinear, depending on whether the field is increasing or decreasing. The same techniques that are used to model magnetic hysteresis can also be used to model a ferroelectric hysteresis loop. This becomes important when using ferroelectrics to construct new components and for running circuit simulations with these components. Some interesting devices can take advantage of ferroelectric hysteresis in a number of areas.

Applications of Ferroelectric Materials

There are many applications of ferroelectric materials for use in electronics, ranging from tunable nonlinear components to energy generation. Some examples include:

  • Dynamic capacitors with larger dynamic range and sensitivity than varactors

  • Capacitors with negative capacitance

  • Nonlinear waveguides for photonics applications

  • Perovskite solar cells, where efficiency could be maintained if there is a field drop

  • High-sensitivity pyroelectric sensors

  • Modulators

  • Non-volatile memories

Among these potential applications, perovskite solar cells have received plenty of research attention and may be the next commercially available ferroelectric products. However, the ferroelectric hysteresis loop in these devices is seen as a roadblock to successful power conversion, as the definition of conversion efficiency becomes ambiguous. Dynamic capacitors and negative capacitors could also be commercialized if manufacturing processes for ferroelectric semiconductors continue to advance.

Perhaps the most popular ferroelectric device that is currently available is ferroelectric RAM  (FRAM). These devices have a relatively simple structure that uses a ferroelectric semiconductor in the CMOS process. In this type of device, a ferroelectric material is placed as a dielectric spacer layer on the base in an NPN transistor structure. A bit can be stored as polarization in the ferroelectric. Furthermore, hysteresis in the ferroelectric ensures that the bit is not lost unless a sufficiently large field is applied at the word line to reverse or clear the polarization.

Graphic for ferroelectric hysteresis loop and polarization in FRAM

Ferroelectric hysteresis loop showing total polarization in a ferroelectric material.

The current range of FRAM products only provides up to 8 MB of storage per module, which is spread across multiple banks. However, the remnant polarization in this material has unlimited read-write cycling capacity, and the remnant polarization ensures the material functions as non-volatile memory. If ferroelectric semiconductor processes are improved, more FRAM devices and other ferroelectric products could be commercialized.

When you need to account for a ferroelectric hysteresis loop in your new designs, you can use the front-end design features from Cadence to build an electrical schematic for your design, and you can use the modeling applications in the PSpice Simulator to create ferroelectric components for simulation. Once you’ve designed your circuits, you can run standard simulations with your ferroelectric components and perfect your ferroelectric designs.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.

 

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