T. González

Voltage tuneable terahertz emission from a ballistic nanometer InGaAs∕InAlAs transistor

Terahertz emission from InGaAs∕InAlAs lattice-matched high electron mobility transistors was observed. The emission appears in a threshold-like manner when the applied drain-to-source voltage UDS is larger than a threshold value UTH. The spectrum of the emitted signal consists of two maxima. The spectral position of the lower-frequency maximum (around 1 THz) is sensitive to UDS and UGS, while that of the higher frequency one (around 5 THz) is not. The lower-frequency maximum is interpreted as resulting from the Dyakonov–Shur instability of the gated two-dimensional electron fluid, while the higher frequency is supposed to result from current-driven plasma instability in the ungated part of the channel. The experimental results are confirmed by and discussed within Monte Carlo calculations of the high-frequency current noise spectra.

Ballistic nanodevices for terahertz data processing: Monte Carlo simulations

By using a semi-classical two-dimensional Monte Carlo simulation, simple devices (T-branch junctions (TBJs) and rectifying diodes) based on AlInAs/InGaAs ballistic channels are analysed. Initially, the model is validated by means of Hall-effect measurements of mobility and electron concentration in long (diffusive) channels. Then, quasi-ballistic transport at room temperature is confirmed in a 100 nm channel. Our simulations qualitatively reproduce the experimental results of electric potential measured in a TBJ appearing as a result of electron ballistic transport, and in close relation with the presence of space charge inside the structure. As examples of devices exploiting the ballistic transport of electrons, preliminary simulations of a multiplexor/demultiplexor and a rectifying diode are presented, demonstrating their capability for terahertz operation.

Microscopic modeling of nonlinear transport in ballistic nanodevices

By using a semi-classical two-dimensional (2-D) Monte Carlo simulation, simple ballistic devices based on AlInAs/InGaAs channels are analyzed. Our simulations qualitatively reproduce the experimental results in T- and Y-branch junctions as well as in a ballistic rectifier appearing as a result of electron ballistic transport. We show that a quantum description of electron transport is not essential for the physical explanation of these results since phase coherence plays no significant role. On the contrary, its origin can be purely classical: the presence of classical electron transport and space charge inside the structures.

Improved Monte Carlo algorithm for the simulation of /spl delta/-doped AlInAs/GaInAs HEMTs

A classical Monte Carlo (MC) device simulation has been modified to locally introduce the effects of electron degeneracy and nonequilibrium screening. Its validity in the case of AlInAs/GaInAs HEMTs has been checked through the comparison, first, with a quantum Schrodinger-Poisson (SP) simulation in the case of a complicated layer structure, which is actually used in the fabrication of real devices, and second, with experimental results of static characteristics of recessed /spl delta/-doped HEMTs.

Operation and high-frequency performance of nanoscale unipolar rectifying diodes

By means of the microscopic transport description supplied by a semiclassical two-dimensional Monte Carlo simulator, we provide an in depth explanation of the operation (based on electrostatic effects) of the nanoscale unipolar rectifying diode, so called self-switching diode, recently proposed in A. M. Song, M. Missous, P. Omling, A. R. Peaker, L. Samuelson, and W. Seifert, Appl. Phys. Lett. 83, 1881 (2003). The simple downscaling of this device and the intrinsically high electron velocity of InGaAs channels allows one to envisaging the fabrication of structures working in the THz range. We analyze the high-frequency performance of the diodes and provide design considerations for the optimization of the downscaling process.

Microscopic simulation of electronic noise in semiconductor materials and devices

We present a microscopic interpretation of electronic noise in semiconductor materials and two-terminal devices. The theory is based on Monte Carlo simulations of the carrier motion self-consistently coupled with a Poisson solver. Current and voltage noise operations are applied and their respective representations discussed. As application we consider the cases of homogeneous materials, resistors, n/sup +/nn/sup +/ structures, and Schottky-barrier diodes. Phenomena associated with coupling between fluctuations in carrier velocity and self-consistent electric field are quantitatively investigated for the first time. At increasing applied fields hot-carrier effects are found to be of relevant importance in all the cases considered here. As a general result, noise spectroscopy is found to be a source of valuable information to investigate and characterize transport properties of semiconductor materials and devices. >

Monte Carlo determination of the intrinsic small-signal equivalent circuit of MESFET's

A Monte Carlo technique for the determination of the intrinsic elements of a broad-band small-signal equivalent circuit (SSEC) of MESFETs (and FETs in general) is described. The values of the different elements are calculated from the Y parameters of the intrinsic MESFET, which are obtained from the Fourier analysis of the device transient response to voltage-step perturbations at the drain and gate electrodes. An accurate estimator of the instantaneous currents at the terminals is used, which guarantees the precision of the method. Three different MESFET geometries have been analyzed. For low drain currents under saturation the intrinsic elements are found to be independent of frequency in the whole range of device operation. This fact validates the technique and the proposed equivalent circuit under these conditions. However, for high drain currents the gate-drain capacitance and the drain conductance depend on frequency due to the appearance of charge-accumulation effects. >

Comparison Between the Dynamic Performance of Double- and Single-Gate AlInAs/InGaAs HEMTs

The static and dynamic behavior of InAlAs/InGaAs double-gate high-electron mobility transistors (DG-HEMTs) is studied by means of an ensemble 2-D Monte Carlo simulator. The model allows us to satisfactorily reproduce the experimental performance of this novel device and to go deeply into its physical behavior. A complete comparison between DG and similar standard HEMTs has been performed, and devices with different gate lengths have been analyzed in order to check the attenuation of short-channel effects expected in the DG-structures. We have confirmed that, for very small gate lengths, short-channel effects are less significant in the DG-HEMTs, leading to a better intrinsic dynamic performance. Moreover, the higher values of the transconductance over drain conductance ratio gm /gd, and, especially, the lower gate resistance Rg also provide a significant improvement of the extrinsic fmax.

Physical models of ohmic contact for Monte Carlo device simulation

Abstract This paper investigates the problem of modelling ohmic contacts for Monte Carlo simulation of semiconductor devices. Several models are proposed with different velocity distributions for the injected carriers. The influence of each model on the device physics near the contact is discussed. As a prototype for this analysis we investigate the role of the ohmic contact on the electrical characteristics of a GaAs Schottky-barrier diode under forward-bias condition. To get accurate results from the simulations of this device, correct modelling of the ohmic contact is crucial. We have found that the best simulation of the carrier dynamics near the contact is achieved by using a velocity-weighted Maxwellian distribution for injecting the carriers, which provides flat profiles of the different magnitudes near the boundary and a zero voltage drop at the contact. In addition, an appropriate time and space algorithm for carrier injection must be applied.

Influence of the surface charge on the operation of ballistic T-branch junctions: a self-consistent model for Monte Carlo simulations

We analyse the influence of the surface charge on the operation of ballistic T-branch junctions by means of a semi-classical 2D Monte Carlo simulator. We propose a new self-consistent model in which the local value of the surface charge is dynamically adjusted depending on the surrounding carrier density. The well-known parabolic behaviour of the central branch potential VC when biasing right and left branches in a push–pull fashion is found to be much influenced by the value of the surface charge in both the horizontal and vertical branches. With the help of experimental measurements performed in real devices, the influence of the width of the central branch on the values of VC and its relation to surface charge effects are also studied.