Researchers recently discovered that transistors kept at ultra-low temperatures can retain bit information with a retention time of a few minutes. What challenges do current memory technologies face, what did the researchers demonstrate, and is the new discovery practical for commercial memory?

What challenges do current memory technologies face?

Transistors can create complex computational devices thanks to their ability to switch electrical current on and off. However, a computational device that cannot store its results is nothing more than a calculator. As such, computing devices use memory circuits to store information for later retrieval, which allows for complex program flow.

Implementing memory circuits presents challenges as there are many different types of memory, all with advantages and disadvantages. Generally speaking, when choosing a memory technology, there are three categories to consider where only two can be picked; memory complexity, memory price, and memory speed.

SRAM is a memory technology that provides high speed and is simple to implement as it is entirely made of transistors. Furthermore, it is a static memory technology meaning it does not require to be refreshed. Still, its large physical dimensions mean that it offers low memory density and thus is extremely expensive.

DRAM is a memory technology that uses a single transistor and capacitor to store bits. The minimal footprint of the DRAM memory cell means that DRAM has far greater memory density than SRAM, making it a lower-cost option. DRAM is also a high-speed memory technology allowing for CPUs to run code stored in DRAM. However, DRAM is complex to interface with and requires regular refresh cycles, which, if not done, will cause DRAM to forget its contents even if power remains.

FLASH is a memory technology that stores information by trapping electrons in a floating gate. Unlike SRAM and DRAM, FLASH does not forget its data when power is removed, meaning that FLASH memory is non-volatile. FLASH is ideal for storing large amounts of data for long periods, but FLASH suffers from degradation from each write (i.e. it can only be written so many times before failing).

Researchers discover ultra-cold transistors can store information

Recently, researchers from the National Institute of Standards and Technology (NIST) discovered that transistors can operate as memory devices when kept at ultra-low temperatures close to absolute zero (3K). After further experimentation, it was determined that a transistor can retain its state due to an effect called impact ionisation.

Simply put, the transistor produces extra charges in its conductive channel when the gate voltage exceeds 1.3V, and these additional charges remain trapped in the channel when the transistor is switched off. A typical transistor conducts no electricity when turned off, but the researchers noticed that a small current flows when charges are trapped. As such, information can be stored onto transistors by using larger gate voltages to represent binary data. This data can be read out without the need for an additional capacitor as in DRAM.

The memory technology presented by the research team has three significant advantages over DRAM. The first is that the memory can persist for several minutes before requiring a refresh cycle. This would allow computers to fully retain their state during brown-outs and power cycles.

Secondly, the memory technology is only a single transistor, meaning that it can be made far more compact than current DRAM technology. Thirdly, the single-transistor memory cell uses standard off-the-shelf transistor technology, which can already be produced in semiconductor foundries.

Is this new memory discovery practical?

While memory technology appears to have many benefits, the very fact that it requires to be kept at almost absolute zero means it will unlikely ever be used in the commercial market. Producing such temperatures can require costly and advanced cryogenic systems, and the advantages of the memory are hardly significant compared to DRAM.

However, the researchers did mention that the transistor may be helpful for use in quantum computer systems that require such low temperatures and the ability to trap bits. As such, the new memory technology may be able to help with storing qubits. Furthermore, the researchers are looking to create the same effect at higher temperatures to overcome the disadvantages of working with such low temperatures.