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ULTRARAM™ is a flash-like floating-gate memory. However, unlike flash, which uses a highly-resistive oxide barrier to retain charge in the floating gate, ULTRARAM™ uses atomically-thin layers of InAs/AlSb to create a triple-barrier resonant-tunnelling (TBRT) charge-confining structure. The TBRT switches between a highly-resistive (locked) state (with no bias, ‘store’), to a highly-conductive (unlocked) state on application of just 2.5 V across the gate stack (program/erase). It is this mechanism that gives ULTRARAM™ its remarkable properties.

Unlike DRAM and flash, which are silicon based, ULTRARAM™ uses III-V compound semiconductors. Specifically, the so-called 6.1-angstrom family of semiconductors (GaSb, InAs and AlSb). This allows engineering of the memories’ electrical properties to exploit the underlying physics to full effect, whilst also being capable of volume manufacture using established processes in the compound semiconductor and silicon industries. The extremely low electron-effective-mass in InAs also opens the possibility for a new high-speed embedded III-V logic to address arrays.

The factors determining energy efficiency vary widely for different memory technologies. For example, many emerging memories function by making/breaking interatomic bonds or by switching atomic magnetic moments. These processes are energy intensive, resulting in high program/erase energies. Charge based memories such as flash and DRAM are superior in this respect as they manipulate electrons rather than atoms, which requires less energy. However, there is still room for even greater efficiency! With its combination of low capacitance and low voltage program/erase, ULTRARAM™ has a switching energy per unit area that is 100x lower than DRAM, 1,000x lower than flash and over 10,000x lower than other emerging memories. ULTRARAM™’s ultra-low energy credentials are further enhanced by its non-destructive read and non-volatility, which removes the need for refresh.

Endurance refers to the number of times a memory cell can be programmed/erased before it is worn-out. This is a weakness of non-volatile memories such as flash, which typically only withstand 10,000 program/erase cycles. In contrast, ULTRARAM™ has demonstrated degradation free operation in excess of 10 million program/erase cycles. This is due to the low-voltage, ultra-low-energy program/erase process enabled by quantum resonant tunnelling. A memory is non-volatile if it is able to retain data when unpowered, requiring a robust logic state that is difficult to change, e.g. flash. In contrast, a fast memory seemingly requires the logic state to be frail so that it can be changed quickly and easily, e.g. DRAM. Therefore, a memory that is fast and non-volatile seemingly requires contradictory physical properties and has long been dismissed as unachievable.

ULTRARAM™ breaks this paradigm through the use of quantum mechanics and resonant tunnelling. Extrapolated retention times in excess of 1,000 years have been demonstrated and scaling of devices down to state-of-the-art feature sizes is predicted to achieve speeds that match or exceed DRAM.