Susana Perez

Microscopic analysis of generation-recombination noise in semiconductors under dc and time-varying electric fields

We present a microscopic analysis of current fluctuations in a semiconductor sample in the presence of trapping–detrapping processes and conventional scattering mechanisms. An ensemble Monte Carlo simulation is used for calculations. To ensure the linearity of the system, we use a model where the characteristic times of the different microscopic mechanisms are considered as energy independent. We analyze the behavior of thermal and generation-recombination noise spectra under static (dc field) and time-varying (ac field) conditions. Under dc bias we confirm the validity of the microscopic model by comparing the results of the simulation with analytical predictions. When an ac field is applied, amplitude modulation of the semiconductor response takes place due to generation-recombination processes. This modulation leads to the upconversion of the low-frequency generation-recombination spectrum, which is evidenced (even in the absence of dc current) and analyzed under different physical conditions.

Microscopic investigation of large-signal noise in semiconductor materials and devices

The investigation of noise in electronic devices operating under large-signal conditions is attracting increasing attention in recent years. Theoretical analyses on this subject are typically performed in the framework of the impedance field method, implemented under the drift-diffusion approximation. As an alternative, a more general microscopic approach including a more detailed physical description of the systems is mandatory. This work reviews recent results of Monte Carlo simulations of electronic noise in bulk materials and submicron semiconductor structures subject to high-frequency large-amplitude periodic electric fields or applied voltages/currents. The peculiarity of the noise analysis under large-signal operation is that velocity or current/voltage fluctuations appear simultaneously with the regular response of the nonlinear medium or device, so that the regular response and noise spectra are overlapped in the whole frequency range of interest. Here, various correlation functions of fluctuations, their instantaneous and integrated spectral densities, etc. are calculated under large-signal operation for compound semiconductors, such as GaAs, and InN, as well as for GaAs Schottky-barrrier diodes and n+nn+ structures. A comparison with the results obtained under stationary conditions is performed. Under these large-signal cyclostationary working conditions, when the system response becomes nonlinear, several modifications and anomalies appear in the noise spectra with respect to static stationary conditions. In particular, an increase of the low-frequency noise and a resonant-like enhancement of the spectra near the fundamental frequency (and eventually high-order harmonics) of the applied signal is observed under some specific conditions. These anomalies are interpreted as a manifestation of dynamical effects under sufficiently high frequency and amplitude of the applied signal. Similarities and differences of the noise resonant-like enhancement around the fundamental frequency with noise upconversion processes are discussed.

An Assessment of Available Models for the Design of Schottky-Based Multipliers Up to THz Frequencies

This paper evaluates the ranges of application and physical limitations of lumped equivalent circuits and drift-diffusion models for the design of THz circuits. The predictions of these models have been compared with a Monte Carlo model, which was considered as a reference, and with measurements from doublers and triplers designed and fabricated by the Jet Propulsion Laboratory. Additionally, the usefulness of Schottky diodes as frequency multipliers above 3 THz is analyzed with the Monte Carlo model.

Noise in Schottky-barrier diodes: from static to large-signal operation

We report Monte Carlo particle (MCP) simulations of the current response and noise spectrum in heavily doped nanometric GaAs Schottky-barrier diodes (SBDs) operating under static, cyclostationary and resonant-circuit conditions in the forward bias region. Main attention is paid to the SBDs application in the THz frequency region. General features of the regular response and noise as well as their modifications under various operation modes are obtained from MCP simulations and analyzed in the framework of a simple analytical model based on the static I-V and C-V relations obtained from simulations.

A Generalized Drift-Diffusion Model for Rectifying Schottky Contact Simulation

We present a discussion on the modeling of Schottky barrier rectifying contacts (diodes) within the framework of partial-differential-equation-based physical simulations. We propose a physically consistent generalization of the drift-diffusion model to describe the boundary layer close to the Schottky barrier where thermionic emission leads to a non-Maxwellian carrier distribution, including a novel boundary condition at the contact. The modified drift-diffusion model is validated against Monte Carlo simulations of a GaAs device. The proposed model is in agreement with the Monte Carlo simulations not only in the current value but also in the spatial distributions of microscopic quantities like the electron velocity and concentration.

Comparative Monte Carlo analysis of InP- and GaN-based Gunn diodes

In this work, we report on Monte Carlo simulations to study the capability to generate Gunn oscillations of diodes based on InP and GaN with around 1 μm active region length. We compare the power spectral density of current sequences in diodes with and without notch for different lengths and two doping profiles. It is found that InP structures provide 400 GHz current oscillations for the fundamental harmonic in structures without notch and around 140 GHz in notched diodes. On the other hand, GaN diodes can operate up to 300 GHz for the fundamental harmonic, and when the notch is effective, a larger number of harmonics, reaching the Terahertz range, with higher spectral purity than in InP diodes are generated. Therefore, GaN-based diodes offer a high power alternative for sub-millimeter wave Gunn oscillations.

Experimental assessment of anomalous low-frequency noise increase at the onset of Gunn oscillations in InGaAs planar diodes

In this work, the presence of anomalous low-frequency fluctuations during the initiation of higher frequency oscillations in InGaAs-based Gunn planar diodes has been evidenced and investigated. Accurate measurements showing the evolution of the power spectral density of the device with respect to the applied voltage have been carried out. Such spectra have been obtained in the wide frequency range between 10 MHz and 43.5 GHz, simultaneously covering both the low-frequency noise and the fundamental oscillation peak at some tens of GHz. This provides valuable information to better understand how these fluctuations appear and how these are distributed in frequency. For much higher frequency operation, such understanding can be utilized as a simple tool to predict the presence of Gunn oscillations without requiring a direct detection.

Electrical and Noise Modeling of GaAs Schottky Diode Mixers in the THz Band

This paper presents a simulation tool for the analysis and design of Schottky mixers which is able to evaluate self-consistently both the conversion losses and the noise temperature. The tool is based on a Monte Carlo model of the diode coupled with a multi-tone harmonic balance technique. A remarkable feature of this tool is that it avoids the need of analytical or empirical models. The validation of the tool has been carried out by comparing simulation results with measurements of mixers published in the literature up to 2.5 THz. The usefulness of Schottky diodes as frequency mixers above 2.5 THz is analyzed. Additionally, the range of application and the limitations of simpler simulation tools based on lumped equivalent circuits or drift-diffusion models have been evaluated using the Monte Carlo model as a reference.

Self-consistent electro-thermal simulations of AlGaN/GaN diodes by means of Monte Carlo method

In this contribution we present the results from the simulation of an AlGaN/GaN heterostructure diode by means of a Monte Carlo tool where thermal effects have been included. Two techniques are investigated: (i) a thermal resistance method (TRM), and (ii) an advanced electro-thermal model (ETM) including the solution of the steady-state heat diffusion equation. Initially, a systematic study at constant temperature is performed in order to calibrate the electronic model. Once this task is performed, the electro-thermal methods are coupled with the Monte Carlo electronic simulations. For the TRM, several values of thermal resistances are employed, and for the ETM method, the dependence on the thermal-conductivity, thickness and die length is analyzed. It is found that the TRM with well-calibrated values of thermal resistances provides a similar behavior to ETM simulations under the hypothesis of constant thermal conductivity. Our results are validated with experimental measurements finding the best agreement when the ETM is used with a temperature-dependent thermal conductivity.

UPCONVERSION OF INTERGROUP HOT-CARRIER NOISE IN SEMICONDUCTORS OPERATING UNDER PERIODIC LARGE-SIGNAL CONDITIONS

By means of Monte Carlo simulations of carrier transport in bulk semiconductors operating under periodic large-signal regime, we show the existence of upconversion of low-frequency hot-carrier noise associated with velocity fluctuations into a high-frequency region centered around the fundamental frequency of the large-signal. It is found that the upconverted noise corresponds to long-time fluctuations of relative populations of two groups of carriers characterized by different dynamical properties in momentum space. The appearance of the upconversion process is related to kinks of the static velocity-field characteristic when the dynamics of carrier motion in momentum space undergoes drastic changes.