J. E. Carceller

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...

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. >

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...

Hole confinement and energy subbands in a silicon inversion layer using the effective mass theory

We present a study of the main features of a two-dimensional hole gas confined near a Si–SiO2 heterointerface. Starting from the framework of the effective mass theory, we were able to separate the Luttinger Hamiltonian into two 3×3 matrices using a semiaxial approximation and still retaining the warped shape of the isoenergetic surfaces in the kx−ky plane and the coupling of heavy, light, and split-off holes. This allows us to solve iteratively and simultaneously the Schrodinger and Poisson equations in the case of an inversion layer of holes in a P-channel metal–oxide–semiconductor structure for different applied gate biases. We have obtained the energy subbands and the main characteristics of the inversion layer. The form of the energy subbands suggests that the use of parabolic bands should be seriously questioned, and that even the use of a unique effective mass in each subband is not a realistic assumption. Furthermore, our results show that the character of the subbands becomes mixed as k∥ separate...

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.

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.

Electron mobility in double gate silicon on insulator transistors: Symmetric-gate versus asymmetric-gate configuration

We have studied electron mobility behavior in asymmetric double-gate silicon on insulator (DGSOI) inversion layers, and compared it to the mobility in symmetric double-gate silicon on insulator devices, where volume inversion has previously been shown to play a very important role, being responsible for the enhancement of the electron mobility. Poisson’s and Schroedinger’s equations have been self-consistently solved in these structures to study and compare the distribution of the electrons. We show that the lack of symmetry in the asymmetric DGSOI structure produces the loss of the volume inversion effect. In addition, we show that as the silicon thickness is reduced the conduction effective mass of electrons in asymmetric devices is lower than that in the symmetric case, but that the greater confinement of electrons in the former case produces a stronger increase in the phonon scattering rate, and in the surface roughness scattering rate. We have solved the Boltzmann transport equation by the Monte Carl...

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.