P. Cartujo

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.

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 ...

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.

The dependence of the electron mobility on the longitudinal electric field in MOSFETs

The dependence of the electron mobility on the longitudinal electric field in MOSFETs has been studied in detail. To do so, a Monte Carlo simulation of the electron dynamics in the channel, coupled with a solution of the two-dimensional Poisson equation including inversion-layer quantization and drift-diffusion equations, has been developed. A simplified description of the silicon band structure in the effective-mass approximation including non-parabolicity has been considered. Different-channel-length MOSFETs and different biases have been taken into account. It has been shown that in order to accurately describe electron-mobility behaviour in short-channel MOSFETs it is necessary to take into account the electron-velocity overshoot. An analytical expression, easy to include in device simulators, is provided to account for the dependence of the electron mobility on the high values of the longitudinal electric field and of its gradient found in state-of-the-art MOSFETs.

Monte Carlo simulation of remote-Coulomb-scattering-limited mobility in metal-oxide-semiconductor transistors

An improved theory for remote-charge-scattering-limited mobility in silicon inversion layers is developed. The model takes into account the effects of image charges, screening, inversion layer quantization, the contribution of different subbands, oxide thickness, the actual distribution of charged centers inside the structure, the actual distribution of carriers in the inversion layer, the correlation of charged centers, and the charged centers sign. The model is implemented in a Monte Carlo simulator, where the effects of the ionized impurities charge, the interface trapped charge, and the contribution of other scattering mechanisms are taken into account simultaneously. Our results show that remote Coulomb scattering cannot be neglected for oxide thicknesses below 2 nm, but that its effects for tox>5 nm are negligible. Good agreement with experimental results has been obtained.

Analysis of the effects of constant‐current Fowler–Nordheim‐tunneling injection with charge trapping inside the potential barrier

Charge trapping and the generation of interface traps in thermally grown SiO2 and its interface with silicon, produced by Fowler–Nordheim tunneling injection at low temperatures from highly doped Si substrates, have been investigated. The results that can be obtained with the constant‐current‐injection method, when a moderate amount of charge is trapped inside the potential barrier, have been analyzed. This has afforded information about the position of the charge trapped in the oxide. No increase in the interface‐trap density has been produced immediately after injection at 77 K, but, as the temperature is raised after injection, the growing of a peak of interface states has been observed. This phenomenon had been reported to be produced as a consequence of a previous hole trapping but, in this case, this intermediate stage of positive‐charge building has not been observed. This effect is discussed, taking into account published models.

COULOMB SCATTERING IN STRAINED-SILICON INVERSION LAYERS ON SI1-XGEX SUBSTRATES

Results of electron mobility in strained‐Si inversion layers grown on Si1−xGex substrates are reported. Drift velocities are calculated by Monte Carlo simulations including electron quantization and Coulomb scattering, in addition to phonon and surface roughness scattering. The strain is shown to contribute as well to the enhancement of the Coulomb‐limited mobility due to better screening of the interface centers by the mobile carriers. Even in the case of high‐doped substrates, Coulomb scattering does not cancel the mobility enhancement provided by the reduction of both intervalley scattering and conduction effective mass.

Study of the effects of a stepped doping profile in short-channel MOSFETs

The performance of a stepped doping profile for improving the short-channel behavior of a submicrometer MOSFET has been analyzed in detail by using a quasi-two-dimensional (quasi-2-D) MOSFET simulator including inversion-layer quantization coupled with a one-electron Monte Carlo simulation. Several second-order effects, such as mobility degradation both by bulk-impurity and interface traps, carrier-velocity saturation, and channel-length modulation, have been included in the simulator. Very good agreement between experimental and simulated results is obtained for short-channel transistors. It has been shown that including a low-doped zone of convenient thickness next to the interface over a high doping substrate improves both the electron mobility and the threshold voltage of the device, while avoiding short-channel effects. The use of simulation has allowed us to study certain kinds of devices without needing to make them.

Effects of oxide-charge space correlation on electron mobility in inversion layers

The effect of the space correlation of oxide charges on the effective mobility of electrons in the channel of an NMOS transistor has been studied. Mobility has been obtained by a single-electron Monte Carlo simulation in which space correlation has been included simultaneously with other effects such as screening of point charges by mobile carriers, image charges, and phonon and surface-roughness scattering. Results have been obtained by assuming different degrees of space correlation for several temperature values and diverse oxide-charge concentrations and positions. The charged-centre space correlation is shown to have a noticeable effect on the electron mobility.