Low ON Current Research Articles

(Invited) Boosting the On-Current of Si-Based Tunnel Field-Effect Transistors

Tunnel-FETs (TFETs) have the potential for a sub-60 mV/dec subthreshold swing and therefore allow for scaling the supply voltage beyond the 1 V plateau of metal-oxide-semiconductor FETs (MOSFETs). The latter scaling is a necessary condition for a reduction of the power consumption per transistor. Silicon-based TFETs are the most attractive because they allow for a full re-use of the existing expertise in fabricating silicon MOSFETs. However, the large bandgap of silicon results in low on-currents. Therefore, the incorporation of heterostructures is proposed. In particular, a germanium-source silicon-channel n-TFET and, as complementary p-TFET, an indium(gallium)arsenide-source silicon-channel TFET reach on-currents comparable to MOSFETs. To beat the MOSFET performance and allow ultra-low voltage operation, additional performance boosters are required. We analyze the impact of the device configuration and of heterostructure strain. The former is illustrated with experimental data of all-silicon FinFET-based TFETs.

Tunnel Field-Effect Transistors for Future Low-Power Nano-Electronics

Tunnel-FETs (TFETs) have the potential for a subthreshold swing below 60 mV/dec such that supply voltage scaling beyond the 1 V plateau of metal-oxide-semiconductor FETs (MOSFETs) is possible. The latter scaling is a necessary condition for a reduction of the power consumption per transistor. Silicon-based TFETs are the most attractive because they allow for a full re-use of the existing expertise in fabricating silicon MOSFETs. However, the large bandgap of silicon results in low on-currents. Therefore, the incorporation of heterostructures is proposed. In particular, a germanium-source silicon-channel n-TFET and, as complementary p-TFET, an indium(gallium)arsenide-source silicon-channel TFET reach on-currents comparable to MOSFETs. TFETs offer new opportunities, like the short-gate configuration, due to their different operating principle. At the same time, the latter implies that models need to be newly developed. The agreement of models with experimental data is still qualitative only.

Complementary Silicon-Based Heterostructure Tunnel-FETs With High Tunnel Rates

As a solution to the low on-current of silicon-based tunnel-FETs (TFETs), the source material of the n-channel TFET is replaced with the small-bandgap material germanium, which results in a current boost up to the same level as the current of MOSFETs. However, no solution has been reported to boost the on-current of the all-silicon p-TFET, a necessity for making an inverter and competing with the MOSFET. We have investigated the heterostructure TFET with respect to complementarity based on our semi-analytical model, and we propose the In <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sub> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</i> As-source silicon-TFET as p-TFET. This design is particularly applicable to nanowire-based transistor architectures. We discuss the complementarity of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> curves, and we analyze the threshold voltage behavior of the complementary TFETs.

Boosting the on-current of a n-channel nanowire tunnel field-effect transistor by source material optimization

Tunnel field-effect transistors (TFETs) are potential successors of metal-oxide-semiconductor FETs because of the absence of short-channel effects and of a subthreshold-slope limit. As a solution to the low on-currents of silicon-based TFETs, the incorporation of silicon-germanium at the source-channel interface has been considered. However, the understanding of the band-to-band-tunneling mechanism at heterojunctions is incomplete. We have investigated through device simulations and modeling the impact of source-material modifications on the tunnel current in n-channel nanowire TFETs. Our modeling work includes the development of a semi-analytical model, which determines the tunnel probability along the dominant tunnel path in two-dimensional TFETs. In particular, we have analyzed the impact of the bandgap, electron affinity, effective mass, dielectric constant, and density of states of both source and channel material. We show that a small-bandgap source material and a large positive electron-affinity offset at the source-channel interface boost the tunnel current of n-channel TFETs.

Tunnel field-effect transistor without gate-drain overlap

Tunnel field-effect transistors are promising successors of metal-oxide-semiconductor field-effect transistors because of the absence of short-channel effects and of a subthreshold-slope limit. However, the tunnel devices are ambipolar and, depending on device material properties, they may have low on-currents resulting in low switching speed. The authors have generalized the tunnel field-effect transistor configuration by allowing a shorter gate structure. The proposed device is especially attractive for vertical nanowire-based transistors. As illustrated with device simulations, the authors’ more flexible configuration allows of the reduction of ambipolar behavior, the increase of switching speed, and the decrease of processing complexity.

P-Channel Tunnel Field-Effect Transistors down to Sub-50 nm Channel Lengths

Experimental results of p-channel silicon vertical tunnel field-effect transistors down to sub-50 nm channel length are shown. As predicted by two-dimensional simulations, we show that the device on-current is nearly independent of channel length scaling. As the drain current is determined by electrons tunneling from the valence band to the conduction band, we show that mobility does not play any role in determining the device characteristics. Low temperature measurements reveal weak positive temperature coefficient in the transfer characteristics due to the dependence of bandgap on temperature. However, as expected for the silicon devices, low on-current is observed. Thus, we propose a lateral tunnel FET on SiGe-on-insulator with high on-currents and symmetric performance in n-channel as well as p-channel operating modes.

Threshold switching in niobate glass — thick-film devices

Thick-film, niobate-glass threshold switches have been made in reducing gas atmospheres at high temperatures (1100–1400°C) using SiO 2 and B 2O 3 as glass formers. Their temperature characteristics are far superior to those of similar vanadate switches previously reported [1,2], and they are stable at low ON currents (< mA) if they are protected from atmospheric oxidation. They do not require forming. The process consists in pre-firing the printed, powdered glass in air and then reducing in argon/hydrogen gas. While the reducing conditions are not critical and virtually full reduction to tetragonal NbO 2 occurs, the pre-firing conditions in air are critical if high-resistance devices are to be obtained. Many devices can be made on a single alumina chip and the results of a statistical analysis are given. The electrical characteristics — measured, chiefly, using a double-pulse technique to examine the change in delay time with pulse separation, ON current, pulse width, etc. — are similar to those previously reported for the vanadate switches [2]. These measurements, together with the results of a microscope and electron-microprobe examination, indicate that a thermal mechanism of switching with filament or channel formation is the most likely. A steady-state computer simulation based on the thermal model indicates the temperatures to be expected during switching, and these are calculated to be less than the NbO 2 semiconductor-metal transition temperature of roughly 800°C, for all the cases examined.