Frans Widdershoven

Modeling statistical dopant fluctuations in MOS transistors

The impact of statistical dopant fluctuations on the threshold voltage V/sub T/ and device performance of silicon MOSFETs is investigated by means of analytical and numerical modeling. A new analytical model describing dopant fluctuations in the active device area enables the derivation of the standard deviation, /spl sigma/V/sub T/, of the threshold voltage distribution for arbitrary channel doping profiles. Using the MINIMOS device simulator to extend the analytical approach, it is found that /spl sigma/V/sub T/, can be properly derived from two-dimensional (2-D) or three-dimensional (3-D) simulations using a relatively coarse simulation grid. Evaluating the threshold voltage shift arising from dopant fluctuations, on the other hand, calls for full 3-D simulations with a numerical grid that is sufficiently refined to represent the discrete nature of the dopant distribution. The average V/sub T/-shift is found to be positive for long, narrow devices, and negative for short, wide devices. The fast 2-D MINIMOS modeling of dopant fluctuations enables an extensive statistical analysis of the intrinsic spreading in a large set of compact model parameters for state-of-the-art CMOS technology. It is predicted that V/sub T/-variations due to dopant fluctuations become unacceptably large in CMOS generations of 0.18 /spl mu/m and beyond when the present scaling scenarios are pursued. Parameter variations can be drastically reduced by using alternative device designs with ground-plane channel profiles.

Closed- and open-boundary models for gate-current calculation in n-MOSFETs

The gate current of different submicron MOS structures has been calculated using two different approaches to evaluate the eigenvalue energy and the escape-time of the quasi-bound states of the potential energy well at the Si/SiO/sub 2/ interface. The numerical issues involved in the implementation of these approaches (one semi-classical, the other quantum-mechanical) inside a device simulator are presented. Simulations performed on different thin-oxide MOS structures show that, compared to the quantum-mechanical treatment, the semi-classical approach is faster, numerically less demanding, and surprisingly accurate in estimating the escape-times. Nevertheless, differences in the eigenvalue energy computed assuming open or closed boundary-conditions at the system boundaries sensibly affect the predicted gate current values.

Cathode Hot Electrons and Anode Hot Holes in Tunneling MOS Capacitors

This paper presents simulations of electron and hole gate currents in thin oxide tunneling MOS capacitors, based on a newly developed Monte Carlo code for Si-SiO Si stacks. Fully bipolar simulations with state of the art Si and SiO transport models predict a previously neglected population of cathode hot electrons proportional to that of the anode hot holes, often regarded as responsible of oxide degradation. The bias dependence of this population is discussed in view of recently reported results on the role of hole injection and transport in device degradation.

Numerical simulation of the position and orientation effects on the impedance response of nanoelectrode array biosensors to DNA and PNA strands

This paper investigates by simulation the response of a nanoelectrode capacitive biosensor to single and double strands of DNA/PNA and up to frequencies well above the electrolyte dielectric relaxation limit. The expected change in capacitance both for an idealized two electrodes system with 2D cylindrical symmetry and a complex nanoelectrode array, where the biomolecule is represented by a dielectric and charged rod, is calculated for different positions and orientations of the strand. DNA and PNA hybridization is also considered. The results provide indications on optimum detection conditions for admittance based DNA biosensors.

A comparative analysis of substrate current generation mechanisms in tunneling MOS capacitors

This paper presents a critical analysis of the origin of majority and minority carrier substrate currents in tunneling MOS capacitors. For this purpose, a novel, physically-based model, which is comprehensive in terms of impact ionization and hot carrier photon emission and re-absorption in the substrate, is presented. The model provides a better quantitative understanding of the relative importance of different physical mechanisms on the origin of substrate currents in tunneling MOS capacitors featuring different oxide thickness. The results indicate that for thick oxides, the majority carrier substrate current is dominated by anode, hole injection, while the minority carrier current is consistent with a photon emission-absorption mechanism, at least in the range of oxide voltage and oxide thickness covered by the considered experiments. These two currents appear to be strictly correlated because of the relatively flat ratio between impact ionization and photon emission scattering rates and because of the weak dependence of hole transmission probability on oxide thickness and gate bias. Simulations also suggest that, for thinner oxides and smaller oxide voltage drop, the photon emission mechanism might become dominant in the generation of substrate holes.

A comparison between semi-classical and quantum-mechanical escape-times for gate current calculations

In this paper the semi-classical and quantum-mechanical definitions of escapetime from quasi-bound states have been compared in the frame of MOSFET gate leakage-current calculations. The theoretical background and the numerical issues involved in the implementation of these approaches inside device simulators have been compared. Results on many different thin gate-oxide capacitors, and on a special purpose test structure with mercury-probe contact, point out that the semi-classical approach is faster, less demanding from the numerical point of view, and surprisingly accurate compared to the fully quantum-mechanical treatment of more physically-sound models.

Gate-stack analysis for 45-nm CMOS devices from an RF perspective

Three gate stacks for the 45-nm node are analyzed from an RF perspective. The authors present an expression of the gate resistance valid for all three stacks, quantify the differences each stack has on several small-signal RF figures-of-merit and on the RF noise parameters, and demonstrate that devices with fully silicided gates will enable ultralow-power/low-noise RF applications, while the performance of transistors using multilayer gate stacks are limited by large contact resistance. Although offering better bandwidth and noise characteristics than the poly/silicide stack, the deposited metal stack will lose its advantage in devices requiring higher gate work functions than in planar bulk CMOS transistors.

On the response of nanoelectrode capacitive biosensors to DNA and PNA strands

This paper investigates the response of a nanoelectrode based capacitive biosensor to the presence of single and double strands of DNA/PNA. The expected admittance spectrum for an idealized system with cylindrical symmetry, where the biomolecule is represented by a simple dielectric and charged rod, is calculated over a broad frequency range extending from below to above the electrolytes dielectric relaxation cut-off frequency. DNA and PNA hybridization is also considered. The results provide indications on optimum detection conditions for admittance based DNA biosensors.

Derivation and Numerical Verification of a Compact Analytical Model for the AC Admittance Response of Nanoelectrodes, Suitable for the Analysis and Optimization of Impedance Biosensors

This paper presents a compact analytical model for the AC response of nanoelectrode-based impedimetric biosensors to dielectric nanoparticles suspended in the electrolyte. The model highlights the functional dependence of the impedance change on the nanoparticle and the system geometrical and physical parameters. The model is carefully verified by means of 2-D simulations carried out with an ad hoc numerical solver of the Poisson-Nernst-Planck (Poisson-Drift-Diffusion) equations. The results can be useful to determine optimum detection conditions for impedimetric nanobiosensors, and to interpret experimental results.

Efficient DC and AC simulation of nanoelectrode-nanoparticle interactions in capacitive biosensors

This paper presents a case study of the interaction between nanoelectrodes and dielectric nanoparticles possibly representative of biomolecules in a simple cylindrical capacitive biosensor. The small signal admittance change due to the insertion of the biomolecule in the biosensor electrolyte is studied as a function of the position, aspect ratio and charge of the biomolecule and of the signal frequency. Results suggest clear advantages in operating the biosensor beyond the electrolyte cutoff frequency.