New Transistor Packs a Punch With Reconfigurable Materials—Not Scaling

Traditional transistors have been subject to relentless miniaturization for decades, defined by the overarching trend of Moore's law. However, as we inch closer to the physical boundaries of this downsizing, the associated costs have soared, and quantum issues are presenting significant obstacles.

To continue obtaining the performance benefits of scaling without actually scaling devices any smaller, researchers are now pursuing a new concept: reconfigurable transistors. This week, researchers from Lund University announced a study that describes a so-called breakthrough in reconfigurable transistors using ferroelectric materials.

 

Reconfigurable transistors on a millimeter-sized chip. Image courtesy of Anton Persson/Lund University

 

In this piece, we’ll discuss the advantages of reconfigurable transistors, ferroelectric materials, and the new transistor prototype from Lund University. 

 

Reconfigurable Transistors

As Moore’s law slows to a near halt, researchers are now considering reconfigurable transistors as the new paradigm shift in technology.

In conventional transistors, the standard device properties, such as polarity (n- and p-type) and threshold voltage, are predefined during manufacturing and cannot be altered afterward. While this scheme has worked to this point, it ultimately limits the flexibility of the device and its adaptability to changing requirements or conditions.

 

A proposed symbol and IV curve for reconfigurable FETs. Image courtesy of Science Direct

 

Reconfigurable transistors, on the other hand, are a special form of transistor that can change these properties even after manufacturing. Scientists can reconfigure a transistor's properties using special materials and structures that control the device's characteristics. For instance, a single reconfigurable transistor could potentially perform the function of multiple traditional transistors, leading to more compact and efficient circuits. Additionally, reconfigurable transistors could be used to create more flexible and adaptable electronic systems capable of adjusting their behavior in response to changing conditions or requirements.

Unlike their conventional counterparts, reconfigurable transistors can alter their properties post-manufacturing, offering unprecedented flexibility and control. This adaptability is pivotal in the development of smaller, energy-efficient circuits that can enhance memory storage and computing power.

 

Ferroelectric Materials: The Key to Reconfigurable Transistors?

In the pursuit of commercially-feasible, reconfigurable transistors, one technology that is being heavily invested in is ferroelectric FETs (FeFETs).

Ferroelectric materials are a class of materials that exhibit an electric polarization influenced by the application of an external electric field. Notably, the polarization of a FeFET stays constant even after the applied voltage is removed. In the context of reconfigurable transistors, ferroelectric materials are used to create a "memory" function in which the material’s change in polarization can be used to alter the properties of the transistor. 

 

The device structure and schematic of a FeFET. Image courtesy of Purdue University

 

The ability to change and remember the state of the transistor opens up a wide range of possibilities for reconfigurability. For example, a single transistor could be reconfigured to perform different functions at different times, depending on the needs of the circuit. Alternatively, a reconfigurable FeFET could conceivably achieve a shift in threshold voltage by tuning the ferroelectric polarization and applying a voltage pulse to the gate electrode. 

Ultimately, this could lead to more compact and efficient circuits and even new types of electronic devices that can adapt to changing conditions or requirements.

 

Researchers Develop a Ferroelectric Tunnel-FET

In the new research study from Lund University, the team investigated new reconfigurable transistors based on ferroelectric materials.

The team created ferroelectric "grains" located directly adjacent to a junction that can control a tunnel junction in the transistor. These grains, which are about 10 nanometers in size, can be controlled on an individual level; previous research could only control entire groups of grains.

 

(A) The schematic symbol and (b) time domain waveforms in different configuration states of the ferro-TFET. Image courtesy of Nature Communications

 

This research culminated in the development of a novel transistor, termed the ferro-TFET (tunnel FET). The new device offers a significantly lower supply voltage (down to 50 mV) within a small footprint (~0.01 µm²) while retaining high output power concentration at a target frequency. According to the researchers, this is due to the vertical nanowire structure of the device and the parabolic shape of the transfer curve at various supply voltages, which substantially suppresses harmonics without filters.

The researchers also achieved a negative transconductance (NTC) in the ferro-TFETs by designing a gate/source overlap structure. The NTC behavior can be reconfigured by changing the polarization of the ferroelectric gate oxide, leading to a shift in the peak voltage in the IV curves and true reconfigurability.