【International Papers】Cover Paper in Nano Letters: The First Semiconductor-free-Space Gate transistor

      Nano Letters recently featured a breakthrough study on its cover: the world's first Semiconductor-free-space Gate (SFGT) transistor. This achievement was accomplished by Professor Li Xiaohang's team from KAUST (King Abdullah University of Science and Technology) in Saudi Arabia. Based on β-Ga₂O₃ (Gallium Oxide), a brand-new gate control mechanism was constructed, opening up a new device paradigm for the next generation of power electronics, sensing and storage technologies.

 

I. Research Background: Breaking Through the Bottleneck of Traditional MOSFET Gate Media

      In traditional metal-oxide-semiconductor (MOS) transistors, the electric field needs to penetrate solid media such as SiO₂, Al₂O₃ to regulate the channel. However, trap charges within or at the interface of the medium often result in:

      • A decrease in gate control capability

      • Threshold voltage drift

      • The withstand voltage and reliability are limited

      • Performance deteriorates rapidly in high-temperature/irradiation environments

      • These problems are particularly prominent in wide band gap materials, especially Ga₂O₃.

      This work, for the first time, completely removes solid-state media and replaces traditional gate media with "free space (air/vacuum)" to construct a completely new transistor architecture - SFGT.

 

II. Core Innovation: A truly "Free-Space Gate-Controlled" Transistor

      The SFGT (Semiconductor-free-space Gate Transistor) presented in the paper has the following key features:

      A completely new gate control structure

      • The channel is a narrow fin structure of Ga₂O₃ <100 nm

      • Symmetrical side-gates

      There is no solid medium at all between the gate and the channel, only free space (air or vacuum).

      This structure realizes the first true "free-space gating" in the history of transistors and brings a series of advantages:

      • No dielectric traps → no oxide-related failures

      • Direct electric field coupling → Strong gate control capability

      • The gate area can be directly exposed → it can be used for sensing/environmental regulation

      • The threshold voltage can be regulated by environment, surface state, and functional material

 

III. Major Performance Breakthroughs (Based on β-Ga₂O₃ Devices)

      Experimental results demonstrate that the SFGT’s performance meets or even surpasses that of many Ga₂O₃ devices with oxide gates.:

      • Subthreshold swing (SS) is below 200 mV/dec

      • Maximum leakage current > 250 mA/mm

      • Switch ratio more than 10⁶

      • Threshold lag < 230 mV

      • withstand voltage > 500 V

      (corresponding to an internal electric field of Ga₂O₃ of ~4.8 MV/cm)

 

IV. Key Technological Breakthroughs

      1 High quality Ga₂O₃ narrow fin processing (<100 nm)

      The KAUST team used atomic layer etching (ALE) to significantly reduce sidewall damage, controlling the interfacial density of states to ~10¹² cm⁻²·eV⁻¹, ensuring excellent gate control performance even at the free-space interface.

      2 Free space controllability: Air vs. vacuum

      The device behaves differently in air and vacuum:

      • SS is significantly worse in vacuum

      • The threshold voltage shifts negatively

      • The lag increases from 100 mV to 1 V

      The free-space interface can regulate device characteristics through environmental molecule adsorption/desorption.
      This property makes SFGT naturally suitable for:

      • Gas sensing

      • Reconfigurable devices with surface functional layers

      • A new type of memory cell

 

V. Significance for the Gallium Oxide Industry

      The emergence of SFGT brings three major insights for oxide semiconductors:

      Completely break free from the "dielectric mass" constraint

      The oxide interface problem of Ga₂O₃ at high temperatures and large electric fields has always been a pain point in the industry. SFGT directly removes the gate medium, opening up a new direction.

      2 A potential mainstay in extreme environment electronics

      No solid medium → Radiation immunity, high-temperature stability and deep space, nuclear energy, extreme environment sensors, etc. are highly compatible.

      3 Scalable to GaN, AlN, Ga₂O₃ systems in the future

      The paper points out that SFGT shares the electric field regulation mechanism with self-switching diodes (SSDS), and thus can be extended to various semiconductors.