J. B. Roldán

Electron transport in strained Si inversion layers grown on SiGe-on-insulator substrates

We show by simulation that electron mobility and velocity overshoot are greater when strained inversion layers are grown on SiGe-On-insulator substrates (strained Si/SiGe-OI) than when unstrained silicon-on-insulator (SOI) devices are employed. In addition, mobility in these strained inversion layers is only slightly degraded compared with strained bulk Si/SiGe inversion layers, due to the phonon scattering increase produced by greater carrier confinement. Poisson and Schroedinger equations are self-consistently solved to evaluate the carrier distribution in this structure. A Monte Carlo simulator is used to solve the Boltzmann transport equation. Electron mobility in these devices is compared to that in SOI inversion layers and in bulk Si/SiGe inversion layers. The effect of the germanium mole fraction x, the strained-silicon layer thickness, TSi, and the total width of semiconductor (Si+SiGe) slab sandwiched between the two oxide layers, Tw were carefully analyzed. We observed strong dependence of the e...

Surface roughness at the Si–SiO2 interfaces in fully depleted silicon-on-insulator inversion layers

The effect of surface roughness scattering on electron transport properties in extremely thin silicon-on-insulator inversion layers is carefully analyzed. It is shown that if the silicon layer is thin enough (thinner than 10 nm) the presence of the buried interface plays a very important role, both by modifying the surface roughness scattering rate due to the gate interface, and by itself providing a non-negligible scattering rate. The usual surface roughness scattering model in bulk silicon inversion layers is shown to overestimate the effect of the surface-roughness scattering due to the gate interface as a consequence of the minimal thickness of the silicon layer. In order to account for this effect, an improved model is provided. The proposed model allows the evaluation of the surface roughness scattering rate due to both the gate interface and the buried interface. Once the scattering rates are evaluated, electron mobility is calculated by the Monte Carlo method. The effect of the buried interface ro...

Monte Carlo simulation of electron transport properties in extremely thin SOI MOSFET's

Electron mobility in extremely thin-film silicon-on-insulator (SOI) MOSFETs has been simulated. A quantum mechanical calculation is implemented to evaluate the spatial and energy distribution of the electrons. Once the electron distribution is known, the effect of a drift electric field parallel to the Si-SiO/sub 2/ interfaces is considered. The Boltzmann transport equation is solved by the Monte Carlo method. The contribution of phonon, surface-roughness at both interfaces, and Coulomb scattering has been considered. The mobility decrease that appears experimentally in devices with a silicon film thickness under 20 nm is satisfactorily explained by an increase in phonon scattering as a consequence of the greater confinement of the electrons in the silicon film.

Acoustic phonon confinement in silicon nanolayers: Effect on electron mobility

We demonstrate the confinement of acoustic phonons in ultrathin silicon layers and study its effect on electron mobility. We develop a model for confined acoustic phonons in an ideal single-layer structure and in a more realistic three-layer structure. Phonon quantization is recovered, and the dispersion relations for distinct phonon modes are computed. This allows us to obtain the confined phonon scattering rates and, using Monte Carlo simulations, to compute the electron mobility in ultrathin silicon on insulator inversion layers. Thus, comparing the results with those obtained using the bulk phonon model, we are able to conclude that it is very important to include confined acoustic phonon models in the electron transport simulations of ultrathin devices, if we want to reproduce the actual behavior of electron transport in silicon layers of nanometric thickness.

Modeling the Centroid and the Inversion Charge in Cylindrical Surrounding Gate MOSFETs, Including Quantum Effects

A semiempirical model was developed for calculating the inversion charge of cylindrical surrounding gate transistors (SGTs), including quantum effects. To achieve this goal, we used a simulator that self-consistently solves the 2-D Poisson and Schrodinger equations in a cross section of the SGT. By means of the proposed models, we correctly reproduced the simulation data for a wide range of the device radius and gate voltage values. Both the inversion charge and the centroid models consist of simple mathematical equations within an explicit calculation scheme suitable for use in circuit simulators.

Role of surface-roughness scattering in double gate silicon-on-insulator inversion layers

The effect of surface-roughness scattering on electron transport properties in extremely thin double gate silicon-on-insulator inversion layers has been analyzed. It is shown that if the silicon layer is thin enough the presence of two Si–SiO2 interfaces plays a key role, even for a very low transverse effective field, where surface-roughness scattering is already noticeable, contrary to what happens in bulk silicon inversion layers. We have studied the electron transport properties in these devices, solving the Boltzmann transport equation by the Monte Carlo method, and analyzed the influence of the surface-roughness parameters and of the silicon layer thickness. For low transverse effective fields, μSR decreases as the silicon layer decreases. However, at higher transverse effective fields, there is a different behavior pattern of μSR with Tw since it begins to increase as Tw decreases until a maximum is reached; for lower silicon layer thicknesses, surface-roughness mobility abruptly falls. Finally we ...

A Monte Carlo study on the electron‐transport properties of high‐performance strained‐Si on relaxed Si1−xGex channel MOSFETs

We have studied the electron‐transport properties of strained‐Si on relaxed Si1−xGex channel MOSFETs using a Monte Carlo simulator adapted to account for this new heterostructure. The low‐longitudinal field as well as the steady‐ and nonsteady‐state high‐longitudinal field transport regimes have been described in depth to better understand the basic transport mechanisms that give rise to the performance enhancement experimentally observed. The different contributions of the conductivity‐effective mass and the intervalley scattering rate reduction to the mobility enhancement as the Ge mole fraction rises have been discussed for several temperature, effective, and longitudinal‐electric field conditions. Electron‐velocity overshoot effects are also studied in deep‐submicron strained‐Si MOSFETs, where they show an improvement over the performance of their normal silicon counterparts.

Electron mobility in extremely thin single-gate silicon-on-insulator inversion layers

Inversion-layer mobility has been investigated in extremely thin silicon-on-insulator metal–oxide–semiconductor field-effect transistors with a silicon film thickness as low as 5 nm. The Poisson and Schrœdinger equations have been self-consistently solved to take into account inversion layer quantization. To evaluate the electron mobility, the Boltzmann transport equation has been solved by the Monte Carlo method, simultaneously taking into account phonon, surface-roughness, and Coulomb scattering. We show that the reduction of the silicon layer has several effects on the electron mobility: (i) a greater confinement of the electrons in the thin silicon film, which implies an increase in the phonon-scattering rate and therefore a mobility decrease; (ii) a reduction in the conduction effective mass and the intervalley-scattering rate due to the redistribution of carriers in the two subband ladders as a consequence of size quantization resulting in a mobility increase; and (iii) an increase in Coulomb scatte...

An in-depth simulation study of thermal reset transitions in resistive switching memories

An in-depth characterization of the thermal reset transition in RRAM has been performed based on coupling self-consistent simulations to experimental results. A complete self-consistent simulator accounting for the electrical and thermal descriptions of the conductive filaments (CFs) has been developed for the numerical study of the temporal evolution of the reset transition in RRAM. The CFs series resistance, including the contributions of the setup and Maxwell components, has been included in the calculations. Using this simulation tool, we have been able to reproduce many experimental details of the experimental reset data obtained in Cu/HfO2/Pt devices. In doing so, we explained the current steps observed in some reset cycles by considering CFs with several coupled branches that break down at different times. The reset voltage dependence on the initial resistance of the CF has been analyzed and the relevant role played by the CF shape has also been demonstrated. In this respect, devices with a same in...

Modeling effects of electron-velocity overshoot in a MOSFET

A simple analytical expression to account for electron-velocity overshoot effects on the performance of very short-channel MOSFETs has been obtained. This new model can be easily included in circuit simulators of systems with a huge number of components. The influence of temperature and low-field mobility on the increase of MOSFET transconductance produced by electron-velocity overshoot as channel lengths are reduced can be easily taken into account in our model. The accuracy of this model has been verified by reproducing experimental and simulated data reported by other authors.