Shunsuke Koba

The impact of increased deformation potential at MOS interface on quasi-ballistic transport in ultrathin channel MOSFETs scaled down to sub-10 nm channel length

It is a common view that ballistic transport is enhanced due to channel length scaling because of decreased scattering number. In this study, based on Monte Carlo (MC) simulation technique, we have successfully extracted quasi-ballistic transport parameters such as backscattering coefficient, by carefully monitoring particle trajectories around the potential bottleneck point. We have found that contrary to expectations, ballistic transport in ultra-scaled double-gate (DG) MOSFETs is not enhanced mainly due to intensified surface roughness (SR) scattering if the channel length reduces less than 10 nm.

Channel length scaling limits of III–V channel MOSFETs governed by source–drain direct tunneling

The difference in the impact of source–drain (SD) direct tunneling in In0.53Ga0.47As and InP metal–oxide–semiconductor field-effect transistors (MOSFETs) was investigated by a quantum Wigner Monte Carlo simulation. It was found that the subthreshold current increase due to SD direct tunneling is more marked in In0.53Ga0.47As MOSFETs owing to their lower effective mass. In addition, the critical channel length at which a drastic increase in subthreshold current occurs owing to SD direct tunneling was found to be about 20 nm for both In0.53Ga0.47As and InP MOSFETs. Since this value is significantly larger than that for Si MOSFETs, SD direct tunneling can be a major obstacle in downscaling III–V MOSFETs into Lch < 20 nm. Hence, to go beyond the end of the roadmap, we will need a selection of materials to suppress SD direct tunneling.

Performance Analysis of Junctionless Transistors Based on Monte Carlo Simulation

A junctionless (JL) transistor has no pn junctions and has a number of advantages to fabricate ultrashort-channel metal–oxide–semiconductor field-effect transistors. In this paper, we study the electron transport in JL transistors based on a Monte Carlo simulation. We demonstrate that high channel doping will not degrade the drive current seriously, because ionized impurities scatter electrons mostly forward, and thus there is less chance for scattered electrons to return back to the source. We also find that smaller parasitic resistance in the source of a JL transistor also contributes to achieve high drive current.

Quantum transport simulation of nanoscale semiconductor devices based on Wigner Monte Carlo approach

In this paper, we present quantum transport simulation of nanoscale semiconductor devices based on Wigner Monte Carlo (WMC) approach. We have found that the WMC approach can accurately handle higher-order quantized subbands, tunneling, quantum reflection, and decoherence processes occurring in nanoscale semiconductor devices. Furthermore, we have demonstrated that carrier quantum transport in source electrode plays an important role in devices extremely downscaled into the nanometer regime.

Effects of increased acoustic phonon deformation potential and surface roughness scattering on quasi-ballistic transport in ultrascaled Si-MOSFETs

It is a common view that ballistic transport is enhanced by channel length scaling because of a decreased scattering number. On the other hand, the acoustic phonon (AP) scattering rate is higher in silicon-on-insulator (SOI) MOSFETs than in bulk Si-MOSFETs; moreover, surface roughness (SR) scattering caused by spatial fluctuation of quantized subbands emerges in extremely scaled SOI channels. Therefore, the influences of these scattering mechanisms on ballistic transport in ultrathin-body Si-MOSFETs are examined in this paper using a Monte Carlo simulation technique. First of all, the effect of increased AP scattering rate on the drain current and ballistic efficiency is found to be negligible. Furthermore, contrary to the common view, ballistic transport in double-gate MOSFETs is shown to be degraded when the channel length decreases to less than 10 nm, mainly owing to SR scattering intensified by the spatial fluctuation of quantized subbands. The gate and drain bias voltage dependencies of ballistic efficiency are also discussed.

Influence of Source/Drain Parasitic Resistance on Device Performance of Ultrathin Body III–V Channel Metal–Oxide–Semiconductor Field-Effect Transistors

The influence of source/drain (S/D) parasitic resistance in ultrathin body (UTB) III–V channel metal–oxide–semiconductor field-effect transistor (MOSFET) was investigated based on Monte Carlo simulation. We found that heavily doped S/D improves source starvation and suppresses carriers backscattering from drain to channel, owing to increased electron–electron scattering. As a result, the heavily doped S/D was shown to be effective to enhance the current drive and transconductance of UTB III–V channel MOSFET. In addition, we demonstrated that the heavily doped S/D has the advantage to provide unsaturated drain current characteristics.

Increased Subthreshold Current due to Source–Drain Direct Tunneling in Ultrashort-Channel III–V Metal–Oxide–Semiconductor Field-Effect Transistors

The influence of quantum transport effects in ultrashort-channel InP metal–oxide–semiconductor field-effect transistors (MOSFETs) was investigated using a Wigner Monte Carlo method, in which both quantum transport and carrier scattering effects can be fully incorporated. It was found that source–drain (SD) direct tunneling becomes evident for channel lengths of less than about 20 nm. Since this critical channel length is approximately three times larger than that in Si-MOSFETs, countermeasures should be taken to suppress SD direct tunneling in order to aggressively downscale III–V channel MOSFETs. In contrast, quantum reflection effects were found to have a negligible influence on the on-state drain current.

Wigner Monte Carlo approach to quantum and dissipative transport in Si-MOSFETs

We investigate the influences of quantum transport and scattering effects in Si double-gate MOSFETs based on Wigner Monte Carlo (WMC) approach. It is shown that quantum reflection effect makes significant differences in microscopic features of electron transport between classical and quantum approaches and can even reduce drain current at on-state, but it does not necessarily produce drastic change in macroscopic properties including the drain current. On the other hand, source-drain direct tunneling crucially degrades the subthreshold properties in scaled MOSFETs with sub-10 nm gate length. Furthermore, the ability of the WMC method to describe quantum-classical transition of carrier transport is demonstrated.

Coupled Monte Carlo simulation of transient electron-phonon transport in small FETs

Using a coupled Monte Carlo technique for solving both electron and phonon Boltzmann transport equations, the transient electrothermal simulation of nanoscale FETs is performed. It is shown that the time constants for the electron and phonon transport are different in order of magnitude, and the self-heating has little impact on digital circuit delay, while it would affect the bias temperature instability because of the long decay time of the created hot spot. The effectiveness of introducing the lightly doped drain structure is also discussed to reduce the hot spot temperature.

Extraction of quasi-ballistic transport parameters in Si double-gate MOSFETs based on Monte Carlo method

In this study, we have developed an evaluation tool of quasi-ballistic transport parameters in realistic devices, to clarify practical benefits of downscaling MOSFETs into ultimate physical scaling limit. It is found that ballistic transport in double-gate (DG) MOSFETs is enhanced due to the channel length (Lch) scaling until Lch = 10 nm, but when Lch is further scaled to less than 10 nm using TSi = Lch/3 scaling rule, where TSi is the channel thickness, surface roughness scattering intensified by spatial fluctuation of quantized subbands drastically degrades ballistic transport. Furthermore, on-current increase or decrease of ultra-scaled DG MOSFETs is found to be basically determined by a backscattering coefficient R. Gate and drain bias voltage dependencies of ballisticity are also evaluated.