What gets used in in future designs will depend on power requirements.
As discussed in Reducing Subthreshold Swing With TFETs, papers at December’s IEEE Electron Device Meeting examined a variety of potential designs for tunneling transistors (TFETs). That focus continued at the recent CS International Conference. In particular, Nadine Collaert discussed IMEC’s work on InGaAs homo-junction devices.
Many compound semiconductor devices depend on heterojunctions between different compounds: InGaAs/GaAsSb junctions are common in TFET designs. Unfortunately, interfaces between dissimilar materials tend to have high levels of traps and other defects. Trap-assisted tunneling is a significant contributor to leakage in TFETs. Homo-junctions can potentially achieve better interface quality because both sides of the junction use the same material with different doping levels: CMOS devices are probably the most familiar example, with n-doped and p-doped silicon forming the junction.
IMEC’s InGaAs homo-junction devices use zinc doping for the source, delivered by either solid or gas-phase diffusion, with between 53% and 70% indium. Their best devices so far, with 53% indium and gas-phase doping, demonstrated a subthreshold swing as low as 54 mV/decade. They believe the very thin 1nm Al2O3/2nm HfO2 gate dielectric is a key enabler for this result.
Based on the results available so far, it appears that TFETs will offer different power/performance tradeoffs from conventional MOSFETs. Many methods for reducing leakage also reduce tunneling current and lead to slower devices. Thus, TFETs may offer better performance when power consumption requirements are very tight, but CMOS will likely deliver superior absolute speed in less power-constrained situations. One approach, suggested by researchers at Penn State, depends on heterogeneous circuits, using TFETs for “slow-always-on” functions, but CMOS for “fast-then-sleep” functions.
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