G. Albareda

Intrinsic noise in aggressively scaled field-effect transistors

According to roadmap projections, nanoscale field-effect transistors (FETs) with channel lengths below 30xa0nm and several gates (for improving their gate control over the source–drain conductance) will come to the market in the next few years. However, few studies deal with the noise performance of these aggressively scaled FETs. In this work, a study of the effect of the intrinsic (thermal and shot) noise of such FETs on the performance of an analog amplifier and a digital inverter is carried out by means of numerical simulations with a powerful Monte Carlo (quantum) simulator. The numerical data indicate important drawbacks in the noise performance of aggressively scaled FETs that could invalidate roadmap projections as regards analog and digital applications.

BITLLES: An approach to quantum time-dependent electron transport at the nanoscale

With the aim of manufacturing faster and smaller devices, the electronic industry is today approaching both the nano and the picosecond scales. In this particular scenario, electron dynamics becomes strongly correlated both in space and time. We have recently shown that Bohmian trajectories allow a direct treatment of the time-dependent many-particle interaction among electrons with accuracy comparable to Density Functional Theory techniques. In this work we present a general purpose time-dependent 3D quantum electron transport simulator based on Bohmian trajectories that we call BITLLES. As a numerical example of its capabilities, we compute the full electrical characteristics (DC, High frequency and fluctuations) of a Resonant Tunneling Diode.

Smaller are noisier: Signal‐to‐noise ratio and bit‐error degradation in bulk‐, quantum well‐ and quantum wire‐nanoscale FETs

For ballistic nanoscale field‐effect transistors (FET), at room temperature, the current and the noise are mainly determined by the injection process and by the maximum of the potential energy along the channel. The noise performance of nanoscale ballistic FETs for analog (signal‐to‐noise ratio) and digital (the bit‐error probability) applications are computed. It is shown that these parameters are affected by the electron confinement. The results (only shot and thermal noises are considered) imply important drawbacks for the noise performance of aggressively‐scaled 2D‐ and 1D‐ FETs.

Towards frequency performance improvement of emerging devices without length scaling

The improvement of the intrinsic high-frequency performance of emerging transistors is commonly based on reducing electron transit time and it is pursued by either reducing the channel length or employing novel high-electron-mobility materials. For gate-all-around transistors with lateral dimensions much shorter than their length, a careful analysis of the total time-dependent current shows that a time shorter than the electron transit time along the channel controls their high-frequency behavior. Both, the standard displacement current definition and the Ramo-Shockley-Pellegrini theorem are used to demonstrate this effect. Therefore, the high-frequency performance of such transistors, with a proper geometry design, can go beyond the intrinsic limits imposed by the electron transit time.

Accurate predictions of terahertz noise in ultra-small devices: A limiting factor for their practical application?

With the aim of manufacturing faster devices, the electronic industry scales down devices dimensions and is today entering into a nanoscale and Terahertz frequency regimes. In this work, we show how the reduction of the length L of the active region of two terminal devices implies (apart from the expected improvement of their dynamic behavior) a typically disregarded increment on the fluctuations of the current. In particular, the high-frequency current fluctuations are proportional to 1/L so that the noise grows unlimitedly when the devices dimensions are reduced. This important drawback, common to classical and quantum regimes, remains mainly unnoticed by the scientific community.

Multi-time measurement and displacement current in time-dependent quantum transport

With the aim of manufacturing faster and smaller devices, the electronic industry is today entering into the nanoscale and the high frequency regimes. In this particular scenario, the dynamics of the electron charge becomes affected by quantum mechanical laws, both, for its spatial or temporal description. We have recently shown that Bohmian trajectories allow a direct treatment of the time-dependent many-particle interaction among electrons with an accuracy comparable to Density Functional Theory techniques. In addition, Bohmian mechanics, by combining wave functions and trajectories, provides a very simple description on how to describe multi-time measurements in quantum scenarios. Using the previous formalism, in this work we present a general purpose time-dependent 3D quantum electron transport simulator named BITLLES (Bohmian Interacting Transport in large low-dimensional Electronic Structures) especially indicated for AC, transients and noise predictions. As a numerical example of its capabilities, we compute the full electrical characteristics (DC, High frequency and fluctuations) of a Resonant Tunneling Diode.

Geometry engineering for the RF behavior of low-dimensional gate-all-around transistors

The aim of this work is to show the dependence of the time dependent current of gate-all-around transistors on their geometries and thus to find out how to optimize their intrinsic AC behavior. The Ramo-Shockley-Pellegrini (RShP) theorem and many-particle Monte Carlo technique are used, through the recently developed BITLLES simulator which is devoted to simulate classical and quantum electronic devices, to tackle this problem. Analytical and Monte Carlo (MC) simulations show how the HF spectrum noticeably depends on the ratio between lateral (Ly, Lz) and longitudinal (Lx) dimensions of a gate-all-around transistor.

Towards the control of power dissipation through the use of many-body Coulomb correlations

Power dissipation constitutes a major constriction in modern and future nanoelectronic design [1]. In this context, predictive models elucidating new criterions to control Joule heating would be valuable. In this work we reveal how an accurate formulation of the many-body Coulomb correlations among carriers can lead to new perspectives on the design of power-optimized electron devices. In particular, we show that for a ballistic semi-classical system the rate at which carriers gain (or loose) kinetic energy is a function of carrier-carrier correlations and differs in general from the expected value (I)· (ΔV).

Study of the effect of device geometry on the AC behaviour of nanoelectronic devices

The main goal of this work is to study the effect of the structure geometries on the time dependent current in the nano electronic devices. The Ramo-Shockley-Pellegrini theorems are used together with many-particle Monte Carlo simulator to study this problem. In particular, it is shown that when the lateral surfaces (Ly, Lz) where the total current is collected are decreased, while keeping the longitudinal dimension (Lx) fixed, the high frequency range where the AC spectrum is still meaningful increases.

Bohmian formulation of Full Counting Statistics in mesoscopic systems

Predicting time-dependent current correlations in mesoscopic systems represents a challenge to theorists because it requires the ability to reproduce sequential measurements, i.e. unitary (Schrödinger-like) and non-unitary (collapse-like) evolutions of the quantum systems. On the contrary, Bohmian formulation of quantum theory, in terms of quantum trajectories guided by waves, by construction, exactly reproduces all quantum phenomena (even correlations) without needing to invoke the role of the operators in the systems measurement. Here, by simply highlighting the operatorless character of Bohmian mechanics, we show that the conceptual difficulties associated to sequential measurements, when electron transport is formulated within the orthodox quantum mechanics, can be substituted by a practical difficulty in the reduction of the degrees of freedom associated to open mesoscopic systems, when the problem is formulated within Bohmian mechanics.