F. Gámiz

On the enhanced electron mobility in strained-silicon inversion layers

The recently reported large enhancement of the electron mobility in strained-Si inversion layers at large carrier concentrations cannot be easily explained: The strong carrier confinement in inversion layers removes the sixfold degeneracy of the conduction-band minima, much as tensile in-plane strain does, so that the effect of strain should become irrelevant at large sheet carrier densities. The problem is studied by calculating the electron mobility accounting for scattering with phonons and interface roughness. Surprisingly, the latter process is found to be significantly stronger in strained layers for a given interface roughness. Only the ad hoc assumption of increasingly smoother interfaces with increasing strain seems to explain the data.

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...

Monte Carlo simulation of double-gate silicon-on-insulator inversion layers: The role of volume inversion

The electron mobility in a double-gate silicon-on-insulator (DGSOI) device is studied as a function of the transverse effective field and silicon layer thickness. The contributions of the main scattering mechanisms (phonon scattering, surface roughness scattering due to both Si–SiO2 interfaces, and Coulomb interaction with the interface traps of both interfaces) are taken into account and carefully analyzed. We demonstrate that the contribution of surface scattering mechanisms is by no means negligible; on the contrary, it plays a very important role which must be taken into account when calculating the mobility in these structures. The electron mobility in DGSOI devices as Tw decreases is compared with the mobility in single-gate silicon-on-insulator structures (i) when only phonon scattering is considered, (ii) when the effect of surface-roughness scattering is taken into account, and (iii) when the contribution of Coulomb interaction with charges trapped at both interfaces is taken into consideration (...

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...

A comprehensive model for Coulomb scattering in inversion layers

A comprehensive model for Coulomb scattering in inversion layers is presented. This model simultaneously takes into account the effects of: (i) the screening of charged centers by mobile carriers, (ii) the distribution of charged centers inside the structure, (iii) the actual electron distribution in the inversion layer, (iv) the charged‐center correlation, and (v) the effect of image charges. A Monte Carlo calculation to obtain the effective mobility of electrons in an n‐Si(100) inversion layer by using the model proposed for Coulomb scattering has been developed. The importance of correctly taking into account the effects above to study Coulomb scattering in inversion layers is pointed out.

A Comprehensive Study of the Corner Effects in Pi-Gate MOSFETs Including Quantum Effects

In this paper, simulation-based research on the electrostatics of Pi-gate silicon-on-insulator (SOI) MOSFETs is carried out. To do so, a 2-D self-consistent Schrodinger-Poisson solver has been implemented. The inclusion of the quantum effects has been demonstrated to be necessary for the accurate simulation of these devices in the nanometer range. Specifically, this paper is focused on the corner effects in multiple-gate SOI MOSFETs, defined as the formation of independent channels with different threshold voltages. Corner effects are studied as a function of different parameters, such as the doping density, silicon-fin dimensions, corner rounding, and gate oxide thickness. Finally, the relation between corner effects and the transition from a fully to a partially depleted body is analyzed.

Physical model for trap-assisted inelastic tunneling in metal-oxide- semiconductor structures

A physical model for trap-assisted inelastic tunnel current through potential barriers in semiconductor structures has been developed. The model is based on the theory of multiphonon transitions between detrapped and trapped states and the only fitting parameters are those of the traps (energy level and concentration) and the Huang–Rhys factor. Therefore, dependences of the trapping and detrapping processes on the bias, position, and temperature can be obtained with this model. The results of the model are compared with experimental data of stress induced leakage current in metal-oxide-semiconductor devices. The average energy loss has been obtained and an interpretation is given of the curves of average energy loss versus oxide voltage. This allows us to identify the entrance of the assisted tunnel current in the Fowler–Nordheim regime. In addition, the dependence of the tunnel current and average energy loss on the model parameters has been studied.

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.

Effects of the inversion-layer centroid on the performance of double-gate MOSFETs

The role of the inversion-layer centroid in a double-gate metal-oxide-semiconductor field-effect-transistor (DGMOSFET) has been investigated. The expression obtained for the inversion charge is similar to that found in conventional MOSFETs, with the inversion-charge centroid playing an identical role. The quantitative value of this magnitude has been analyzed in volume-inversion transistors and compared with the value obtained in conventional MOSFETs. The minority-carrier distribution has been found to be even closer to the interfaces in volume-inversion transistors with very thin films, and therefore, some of the advantages assumed for these devices are ungrounded. Finally, the overall advantages and disadvantages of double-gate MOSFETs over their conventional counterparts are discussed.

Effects of the inversion layer centroid on MOSFET behavior

The effects of the average inversion-layer penetration, the inversion-layer centroid, on the inversion-charge density and the gate-to-channel capacitance have been analyzed. The quantum model has been used, and a variety of data have been obtained by self-consistently solving the Poisson and Schrodinger equations. An empirical expression for the centroid position that is valid for a wide range of electrical and technological variables has been obtained and has been applied to accurately model the inversion-layer density and capacitance.