A Review of Sharp-Switching Devices for Ultra-Low Power Applications

The reduction of the supply voltage is standard MOSFETs is impeded by the subthreshold slope, which cannot be lowered below 60 mV/decade, even in ideal fully-depleted devices. We review selected CMOS-compatible devices capable of switching more abruptly than MOSFETs, and discuss their merits and limitations. Tunneling FETs (TFETs) are reverse-biased gated PIN diodes where the gate controls the electric field in the interband tunneling junction. Technological solutions for improved performance will be described, including alternative channel materials and geometries, as well as a proposed paradigm shift of increasing the current drive by internal amplification in the bipolar-enhanced TFET. Other emerging sharp-switching mechanisms are reviewed, including the abrupt change in the polarization of ferroelectric materials, mechanical contact in nano-electro-mechanical systems, energy filtering of injected carriers, etc. Recently proposed band modulation feedback transistors are conceptually different from MOSFETs or TFETs. They have similar gated-diode configuration, but are operated in forward-bias mode. Electrostatic barriers are formed (via gate biasing) to prevent electron/hole injection into the channel until the gate or drain bias reaches a turn-on value. Due to bandgap modulation along the channel, these devices can switch abruptly (< 1 mV/decade) to a high current.