A Self-Consistent Numerical Method for Simulation of Quantum Transport in High Electron Mobility Transistor; Part 1: The Boltzmann-Poisson-Schrodinger Solver

A self-consistent Boltzmann-Poisson-Schrödinger solver for High Electron Mobility Transistor is presented. The quantization of electrons in the quantum well normal to the heterojunction is taken into account by solving the two higher moments of Boltzmann equation along with the Schrödinger and Poisson equations, self-consistently. The Boltzmann transport equation in the form of a current continuity equation and an energy balance equation are solved to obtain the transient and steady-state transport behavior. The numerical instability problems associated with the simulator are presented, and the criteria for smooth convergence of the solutions are discussed. The current-voltage characteristics, transconductance, gate capacitance, and unity-gain frequency of a single quantum well HEMT is discussed. It has been found that a HEMT device with a gate length of 0.7 µm, and with a gate bias voltage of 0.625 V, has a transconductance of 579.2 mS/mm, which together with the gate capacitance of 19.28 pF/cm, can operate at a maximum unity-gain frequency of 47.8 GHz

When self-consistency makes a difference

Compound semiconductor power RF and microwave device modeling requires, in many cases, the use of selfconsistent electrothermal equivalent circuits. The slow thermal dynamics and the thermal nonlinearity should be accurately included in the model; otherwise, some response features subtly related to the detailed frequency behavior of the slow thermal dynamics would be inaccurately reproduced or completely distorted. In this contribution we show two examples, concerning current collapse in HBTs and modeling of IMPs in GaN HEMTs. Accurate thermal modeling is proved to be be made compatible with circuit-oriented CAD tools through a proper choice of system-level approximations; in the discussion we exploit a Wiener approach, but of course the strategy should be tailored to the specific problem under consideratio

A multisubband self-consistent two-dimensional numerical model for Hemt including intersubband and intrasubband scattering mechanisms in the quantum well

The previous one-subband model is extended to include transport of electrons in the quantum well with two subbands. The two higher moments of Boltzmann Transport Equations are solved for the two lowest subbands and the bulk system. The Schrodinger\u27s and Poisson\u27s Equations are solved self-consistently. The wavefunctions obtained are used to calculate the ionized impurity and the polar optical-phonon scattering mechanisms. The scattering rates obtained are in good agreement with those reported by Yokoyama and Hess. Coupling terms between the two subbands in the quantum well and the bulk system are derived from the scattering rates; We obtain lower transconductance and unity gain frequency which were overestimated in the previous model. At a gate bias of 0.625 V, we obtained a transconductance of 316 mS/mm, a gate capacitance of 17.68 pF/cm, and a unity-gain frequency of 28.44 GHz. (Abstract shortened with permission of author.)

High Electron Mobility Transistors: Performance Analysis, Research Trend and Applications

In recent years, high electron mobility transistors (HEMTs) have received extensive attention for their superior electron transport ensuring high speed and high power applications. HEMT devices are competing with and replacing traditional field‐effect transistors (FETs) with excellent performance at high frequency, improved power density and satisfactory efficiency. This chapter provides readers with an overview of the performance of some popular and mostly used HEMT devices. The chapter proceeds with different structures of HEMT followed by working principle with graphical illustrations. Device performance is discussed based on existing literature including both analytical and numerical models. Furthermore, some notable latest research works on HEMT devices have been brought into attention followed by prediction of future trends. Comprehensive knowledge of up‐to‐date results, future directions, and their analysis methodology would be helpful in designing novel HEMT devices

Ensemble Monte Carlo Based Simulation Analysis of GaN HEMTs for High-Power Microwave Device Applications

The high electron mobility transistors (HEMTs) fabricated using wide-bandgap semiconductors show promise as high-gain, low-noise devices with superior frequency response. The structure and operation principle of HEMT are first briefly discussed. The distinguishing and unique properties of GaN are reviewed and compared with those of GaAs. Calculations of the electronic mobility and drift velocity have been carried out for bulk GaN based on a Monte Carlo approach, which serves as a validity check for the simulation model. By taking account of polarization effects, degeneracy and interface roughness scattering, important microwave performance measures such as the dynamic range, harmonic distortion and inter-modulation characteristics are fully studied. Monte Carlo based calculations of the large-signal nonlinear response characteristics of GaN-AlGaN HEMTs with particular emphasis on intermodulation distortion (IMD) have been performed. The nonlinear electrical transport is treated on first principles, including all scattering mechanisms. Both memory and distributed effects are built into the model. The results demonstrate an optimal operating point for low intermodulation distortion (IMD) at reasonably large output power due to the exist of a minima in the IMD curve. Dependence of the nonlinear characteristics on the barrier mole fraction “x” is also demonstrated and analyzed. High-temperature predictions of the IMD have also been made by carrying out the simulations at 600 K. Due to a relative suppression of interface roughness scattering, an increase in dynamic range with temperature is predicted. Finally, towards the end of the research, real-space transfer (RST) phenomena are included in the Monte Carlo simulator to accurately describe the electron transport behavior in HEMTs. The RST is shown to affect the velocity overshoot and inter-modulation distortion behavior and to lead to enhanced substrate leakage current as well as lowered overall performance speed. The potential for drain current compression has also been examined through simulations. Comparisons with and without RST have been performed based on Monte Carlo simulations. Results show that the velocity, IMD and dynamic range are all affected by the applied bias, temperature, internal electric field and gate length characteristics

Particle-Based Modeling of Reliability for Millimeter-Wave GaN Devices for Power Amplifier Applications

abstract: In this work, an advanced simulation study of reliability in millimeter-wave (mm-wave) GaN Devices for power amplifier (PA) applications is performed by means of a particle-based full band Cellular Monte Carlo device simulator (CMC). The goal of the study is to obtain a systematic characterization of the performance of GaN devices operating in DC, small signal AC and large-signal radio-frequency (RF) conditions emphasizing on the microscopic properties that correlate to degradation of device performance such as generation of hot carriers, presence of material defects and self-heating effects. First, a review of concepts concerning GaN technology, devices, reliability mechanisms and PA design is presented in chapter 2. Then, in chapter 3 a study of non-idealities of AlGaN/GaN heterojunction diodes is performed, demonstrating that mole fraction variations and the presence of unintentional Schottky contacts are the main limiting factor for high current drive of the devices under study. Chapter 4 consists in a study of hot electron generation in GaN HEMTs, in terms of the accurate simulation of the electron energy distribution function (EDF) obtained under DC and RF operation, taking into account frequency and temperature variations. The calculated EDFs suggest that Class AB PAs operating at low frequency (10 GHz) are more robust to hot carrier effects than when operating under DC or high frequency RF (up to 40 GHz). Also, operation under Class A yields higher EDFs than Class AB indicating lower reliability. This study is followed in chapter 5 by the proposal of a novel π-Shaped gate contact for GaN HEMTs which effectively reduces the hot electron generation while preserving device performance. Finally, in chapter 6 the electro-thermal characterization of GaN-on-Si HEMTs is performed by means of an expanded CMC framework, where charge and heat transport are self-consistently coupled. After the electro-thermal model is validated to experimental data, the assessment of self-heating under lateral scaling is considered.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Simulation and optimization of HEMTs

We have developed a simulation system for nanoscale high-electron mobility transistors, in which the self-consistent solution of Poisson and Schr\"odinger equations is obtained with the finite element method. We solve the exact set of nonlinear differential equations to obtain electron wave function, electric potential distribution, electron density, Fermi surface energy and current density distribution in the whole body of the device. For more precision, local dependence of carrier mobility on the electric field distribution is considered. We furthermore compare the simulation to a recent experimental measurement and observe perfect agreement. We also propose a graded channel design to improve the transconductance and thereby the threshold frequency of the device.Comment: 8 pages, 19 figure

A two-dimensional numerical model of the high electron mobility transistor

A two-dimensional numerical model of the high electron mobility transistor (HEMT) with consideration of quantization in the channel is presented. In this model, the spatial spread of the electron concentration in the quantum well normal to the heterojunction is taken into consideration by solving Schrodinger\u27s and Poisson\u27s equations self-consistently. The Boltzmann transport equation in the form of a current continuity equation and an energy transport equation is solved to obtain the transient transport behavior. Transport of carriers takes place in two layers in the GaAs region: the lowest subband of the quantum well and a non-quantized bulk layer; The simulation program investigates the effects on the overall performance of the device due to variation of the gate length and the impurity doping concentration in AlGaAs. A reduction in the gate length results in an increase of the drain current which is partly due to a shift in the threshold voltage. (Abstract shortened with permission of author.)

High-frequency response and thermal effects in GaN diodes and transistors: modeling and experimental characterization

Se han analizado diodos autoconmutantes (SSDs) y transitores de alta movilidad de electrones (HEMTs) de GaN, tanto en el régimen DC como en AC, tanto desde el punto de vista experimental como de simulaciones. Las no linealidades presentes en las curvas corriente-voltaje permiten su operación como detectores de microondas a polarización nula. A pesar de las buenas propiedades del GaN, existen problemas tecnológicos relacionados con defectos, trampas y calentamiento que deben ser investigados para perfeccionar la electrónica de potencia en el futuro. Medidas pulsadas y de transitorios de corriente realizadas sobre el SSD han revelado la influencia de trampas volúmicas y superficiales, observándose anomalías en las características DC e impedancia AC. Los efectos superficiales son relevantes en canales estrechos puesto que la relación superficie-volumen del dispositivo aumenta, mientras que en los dispositivos más anchos prevalece la influencia de las trampas de tipo volúmico. Las medidas muestran un incremento anómalo de la detección a bajas temperaturas, mientras que a altas frecuencias el voltaje detectado muestra una caída que atribuimos a la presencia de trampas de tipo superficial y volúmico. Se ha observado una fuerte dispersión a baja frecuencia tanto de la transconductanciacomo de la conductancia de salida en HEMTs de AlGaN/AlN/GaN en el rango de microondas, que atribuimos a la presencia de trampas y defectos tanto en el volumen de canal de GaN como en los contactos de fuente y drenador. Estos efectos han sido modelados mediante un circuito equivalente (SSEC) modificado, obteniéndose un acuerdo excelente con los parámetros S medidos. La geometría del dispositivo afecta a los valores de los elementos del circuito equivalente y con ello a las frecuencias de corte, siendo la longitud de puerta el parámetro más influyente. Para LG = 75 nm, ft y fmax son 72 y 89 GHz, respectivamente, en los HEMTs estudiados. En los SSDs caracterizados, se ha observado una potencia equivalente del ruido (NEP) de 100 - 500 pW/Hz1=2 y una responsividad de decenas de V/W con una fuente de 50 ohmios. Se ha demostrado una frecuencia de corte de unos 200 GHz junto a una respuesta cuadrática hasta 20 dBm de potencia de entrada. A bajas frecuencias, las medidas RF muestran una responsividad que reproduce bien los cálculos realizados mediante un modelo cuasiestático (QS) basado en la pendiente y la curvatura de las curvas corriente-voltaje. Polarizar los dispositivos aumenta el voltaje detectado a costa del consumo de potencia y la aparición de ruido 1/f. El modelo QS predice que la reducción de la anchura del canal mejora la responsividad, hecho que ha sido confirmado experimentalmente. El aumento del número de diodos en paralelo reduce la impedancia; cuando coincide con el triple de la impedancia de la linea de transmisión o la antena, la NEP alcanza su valor mínimo. Los diodos con puerta (G-SSDs) muestran, en espacio libre a 300 GHz, una responsividad en torno a 600 V/W y una NEP en torno a 50 pW/Hz1=2 cerca del voltaje umbral. De nuevo, se obtiene un buen acuerdo entre los resultados del modelo QS, las medidas a 900 MHz y las medidas en espacio libre a 300 GHz, todo ello por encima de la zona subumbral. La NEP mejora al aumentar el número de canales en paralelo. Se han comparado los resultados de la detección inyectando la señal por el drenador (DCS) y la puerta (GCS) de los HEMTs hasta 40 GHz. Para DCS, se han obtenido una responsividad en torno a 400 V/W y una NEP de 30 pW/Hz1/2, en un HEMT con LG = 150 nm a temperatura ambiente bajo condiciones de polarización nula y puerta polarizada cerca del umbral. Por otro lado, la responsividad se incrementa en GCS hasta 1.4 kV/W, con la desventaja de polarizar con una corriente de drenador de ID = 1.2 mA. Ambas configuraciones muestran una frecuencia de corte, con -3 dB de caída, en torno a 40 GHz. Resulta interesante que en GCS y a unafrecuencia suficientemente alta para cortocircuitar la rama puerta-drenador con la de la no linealidad, se consigue detectar una responsividad no nula. El estudio del autocalentamiento se vuelve relevante cuando los dispositivos trabajan en condiciones de alta potencia. Las simulaciones se han realizado con una herramienta Monte Carlo (MC) desarrollada por el grupo y acoplada con dos modelos térmicos: (i) modelo de resistencia térmica (TRM) y (ii) un modelo electrotérmico avanzado y que se basa en la resolución autoconsistente de la ecuación del calor independiente del tiempo. A temperatura ambiente la herramienta MC se calibró comparando con resultados experimentales de TLMs (transfer length measurement ), lográndose reproducir la densidad supercial de portadores y la movilidad. Incluyendo la resistencia de contactos, la barrera Schottky y la barrera térmica, nuestros resultados se han validado con medidas experimentales de un HEMT de dimensiones LDS = 1.5 micras y LG = 150 nm, encontrándose un acuerdo razonable. El TRM da unos resultados similares al ETM con valores de la resistencia térmica (RTH) bien calibradas. La principal ventaja del ETM es la posibilidad de obtener mapas de temperatura dentro del canal e identificar la localización de los puntos calientes. También se discute el impacto de la polarización en el SSEC y las discrepancias entre los modelos ETM y TRM. Se utilizan medidas pulsadas hasta 500 K para estimar la temperatura del canal y el valor de la RTH. Para T 250 K.Nanoelectrónica de gap ancho y estrecho para la mejora de la eficiencia en aplicaciones de RF y THz (TEC2013-41640-R). Ministerio de Economía y Competitividad (MINECO). Estudio de efectos térmicos en dispositivos de RF. Modelado y caracterización experimental (SA052U13). Consejería de Educación de la Junta de Castilla y León. Emisores y detectores de terahercios basados en nanodiodos semiconductores para comunicaciones e imagen médica y de seguridad (SA022U16). Consejería de Educación de la Junta de Castilla y León. Tecnologías de diodos de GaN para generación y detección en la banda de subterahercios (TEC2017-83910-R). Ministerio de Economía y Competitividad (MINECO). Simulación y caracterización de efectos electrotérmicos en dispositivos de subterahercios para comunicaciones de alta velocidad (SA254P18). Consejería de Educación de la Junta de Castilla y León