UltraRAM is an emergingstorage device technology and brand name that aims to "combine thenon-volatility of a data storage memory, like flash, with the speed, energy-efficiency, and endurance of a working memory, likeDRAM," which means it could retain data like a hard drive.[1] Silicon-based UltraRAM devices have demonstrated extrapolated data storage times (estimated lifespan based on testing) of at least 1,000 years and endurance up to 1,000 times greater than flash, offering significant potential improvements over existing memory technologies.[2]
It is being developed by researchers atLancaster University’s Physics and Engineering Department, in collaboration with theUniversity of Warwick’s Physics Department, who described it in a paper in 2022.[3]
Though initial experiments at Lancaster have demonstrated its principles, UltraRAM remains largely theoretical,[4] with ongoing efforts to improve quality, refine fabrication, and scale the technology for practical use.[5]
In 2023 the company Quinas was formed to further develop UltraRAM.[6]
ULTRARAM is a charge-based memory where thelogic state is determined by the presence or absence ofelectrons in an FG (Front Gate). The FG is electrically isolated from the control gate (CG) byAl2O3 dielectric, and from the underlying channel by theInAs/AlSb TBRTheterostructure. The presence of electrons in the FG (defining a logic 0 state) depletes carriers in the underlying n-type InAs channel, reducing its conductance. Thus, the charge state of the FG and, therefore, the logic state of the memory, is read nondestructively by measuring the current through the channel when a voltage is applied between the source (S) and drain (D) contacts. The final component of the memory is the InAs back-gate (BG), which allows voltages to be applied vertically across the gate stack for various operations.
The novelty underpinning the memory is the TBRT (Triple BarrierResonant Tunneling) structure, which, unlike single layer barriers, can be switched from a highly electrically resistive state to a highly conductive state by the application of just ±2.5 V. This is achieved by careful design of the thicknesses of the AlSb barriers and InAs QW (Quantum Well) layers. When the memory is in the retention state, i.e., when no voltage is applied to the device, the electron ground states in the TBRT QWs are misaligned with each other and are energetically well above the 300 K electron populations of the InAs FG and channel layers. Indeed, nonvolatility is strengthened by the QW ground states residing at an unusually high energy for a resonant-tunneling structure. This is due to a combination of the ultrathin QWs and the extraordinarily low electron effective mass in InAs. In this state, the TBRT provides a large barrier that prevents electron transfer into or out of the FG. However, the application of a suitable bias across the device tilts theconduction band such that the TBRT QW ground states align with occupied electron states in the channel (during the program operation) or the FG (during the erase operation). This allows electrons to move rapidly across the TBRT region in the intended direction by the inherently fast quantum-mechanical process of resonant tunneling. Due to the low voltages required and the lowcapacitance per unit area of the device compared toDRAM, ultralow logic state switching energies of 10−17 J are predicted for 20 nm feature size ULTRARAM memories, which is two and three orders of magnitude lower thanDRAM andflash respectively. However, before this ultralow switching energy can be realized by fabricating nm-scale devices, the fundamental properties of μm-scale devices must first be understood and optimized. ULTRARAM prototype devices grown on GaAs substrates have previously exhibited experiment-limited, not device-limited, nonvolatile retention of 105 s and an endurance of 106program-erase cycles.[3]
A charged FG is defined aslogic '0', and the absence of charge as logic '1'. Program and erase cycles, to charge and discharge the FG respectively, usevoltage pulses of ≤±2.55 V on the CG.
InAs channeltransistors with submicrometer feature sizes and a subthreshold swing of <100 mV/dec have previously been demonstrated.[7] Consequently, due to the threshold voltage window of 350 mV in the devices designed by the Lancaster team, one can expect the 0/1 current contrast of ULTRARAM to improve to three decades with the implementation of a normally-off channel. Such an improvement of the 0/1 contrast through careful modification of the channel will allow memory arrays to be built with a novel high-densityRAM architecture.[3]
The ULTRARAM on silicon devices actually outperform previous incarnations of the technology onGaAs compound semiconductor wafers, demonstrating (extrapolated) data storage times of at least 1000 years, fast switching speed (for device size) and program-erase cycling endurance of at least 10 million, which is one hundred to one thousand times better than flash. Professor Manus Hayne of the Department of Physics at Lancaster, who leads the work said, "ULTRARAM on silicon is a huge advance for our research, overcoming very significant materials challenges of largecrystalline lattice mismatch, the change from elemental to compound semiconductor and differences in thermal contraction."[1]
On August 11, 2023, it won the "Most Innovative Flash Memory Startup" award at the 17th Flash Memory Summit (FMS 2023).[8]
