J. A. López-Villanueva

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.

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.

Universality of electron mobility curves in MOSFETs: a Monte Carlo study

The universal behavior of electron mobility when plotted versus the effective field is physically studied. Due to charged centers in the silicon bulk, the oxide, and the interface, Coulomb scattering is shown to be responsible for the deviation of mobility curves. Silicon bulk-impurities have a double effect: (a) Coulomb scattering due to the charge of these impurities themselves, and (b) reduction of screening caused by the loss of inversion charge when the depletion charge is increased. The electric-field region in which mobility curves behave universally regardless of bulk-impurity concentration, substrate bias, or interface charge has been determined for state-of-the-art MOSFETs. Finally, this study shows that electron mobility must be a function of the inversion and the depletion charges rather than a simple function of the effective field. >

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.

Direct and trap-assisted elastic tunneling through ultrathin gate oxides

The direct and assisted-by-trap elastic tunnel current in metal–oxide–semiconductor capacitors with ultrathin gate oxide (1.5–3.6 nm) has been studied. Bardeen’s method has been adapted to obtain the assisted tunnel current, in addition to the direct tunnel current. The dependence of the assisted current on the trap distribution in energy has also been analyzed. This allows us to obtain the trap distribution in energy from experimental current curves. Finally, we have analyzed the role of the image force, the inclusion of which can avoid a barrier height dependence on the oxide thickness.