Advances in Linear Modeling of Microwave Transistors

Heterojunction field effect transistors (HFET) based on gallium nitride (AlGaN/GaN) and metal semiconductor field effect transistors (MESFETs) based on silicon carbide (SiC) are the preferred transistors for high-power amplifier circuit designs rather than MESFETs, high electron mobility transistors (HEMTs) and pseudomorphic HEMTs based on gallium arsenide (GaAs) or indium phosphide (InP) semiconductor technology. While AlGaN/GaN and SiC are good candidates for high-power applications, GaAs and InP semiconductor technologies are the preferred transistors in low-power, low-voltage, and low-noise applications [1].     

Electrothermal analysis of AlGaN/GaN high electron mobility transistors

Efficiency of AlGaN/GaN HEMTs used in high power, high frequency applications is thought to be limited by parasitic thermal effects. In this study, we investigate coupled electrical and thermal transport in AlGaN/GaN HEMTs using an ensemble Monte Carlo model. Calculation of the non-equilibrium phonon population reveals a hot spot in the channel that is localized at low drain-source bias, but expands towards the drain at higher bias, significantly degrading channel mobility.     

Strain effects in SiN-passivated GaN-based HEMT devices

The use of a passivating layer can reduce or even eliminate surface effects responsible for limiting both the RF current and breakdown voltage of AlGaN/GaN HEMTs. To study the effect of passivation on electrical characteristics of GaN-based devices, we have developed a macroscopic model of strain in SiN/AlGaN/GaN heterostructure, considering the system as a free-standing one. Basing on the strain results, we have calculated the strain map for a SiN-passivated structure and the electron sheet charge density in the channel. Results have been compared with experimental measurements and with an alternative passivation model.     

Aging and Stability of GaN High Electron Mobility Transistors and Light-Emitting Diodes With $hbox{TiB}_{2}$- and Ir-Based Contacts

There is interest in developing more stable contacts to a variety of GaN-based devices. In this paper, we give two examples of devices that show improved thermal stability when boride or Ir diffusion barriers are employed in ohmic-contact stacks. AlGaN/GaN high electron mobility transistors (HEMTs) were fabricated with Ti/Al/X/Ti/Au source-drain ohmic (where X is or Ir) contacts and were subjected to long-term annealing at 350 . By comparison with companion devices with conventional Ti/Al/Ni/Au ohmic contacts, the HEMTs with boride-based ohmic metal showed superior stability of both source-drain current and transconductance after 25 days of aging at 350 . The gate current for standard HEMTs increases during aging, and the standard ohmic contacts eventually fail by shorting to the gate contact. Similarly, InGaN/GaN multiple quantum well light-emitting diodes (MQW-LEDs) were fabricated with either Ni/Au//Ti/Au or Ni/Au/Ir/Au p-ohmic contacts. Both of these contacts showed superior long-term thermal stability compared to LEDs with conventional Ni/Au contacts.     

Quantum Corrected Full-Band Cellular Monte Carlo Simulation of AlGaN/GaN HEMTs

A full-band Cellular Monte Carlo (CMC) approach is applied to the simulation of electron transport in AlGaN/GaN HEMTs with quantum corrections included via the effective potential method. The best fit Gaussian parameters of the effective potential method for different Al contents and gate biases are calculated from the equilibrium electron density. The extracted parameters are used for quantum corrections included in the full-band CMC device simulator. The charge set-back from the interface is clearly observed. However, the overall current of the device is close to the classical solution due to the dominance of polarization charge.     

Is self-heating responsible for the current collapse in GaN HEMTs?

AlGaN/GaN high-electron mobility transistors (HEMTs) are a very promising technology for switching and radio frequency power applications due to the high saturation velocity and large breakdown field of the GaN material. However, the electrical reliability of this material system in both the on and the off-state operation regimes is still a fundamental problem to be solved before the widespread use of this technology can be made.     

Scaling effects in AlGaN/GaN HEMTs: Comparison between Monte Carlo simulations and experimental data

AlGaN/GaN based HEMTs are showing an unexpected behaviour of the transport characteristic compared to the GaAs and InGaAs HEMT technology. A downscaling of the source-gate and gate-drain lengths of GaN HEMTs produced a maximum output current increase, which is not compatible with a framework where the electrons velocity is saturated. Monte Carlo simulations are able to reproduce this experimental behaviour showing how this phenomena is related to intrinsic velocity-field relation in GaN. We attribute the lack of overshooting phenomena to the high scattering rate of III–V nitrides and to the strong localisation of the high electric field region. We also have found that such suppression could be avoided with a proper scaling of the devices.     

Analytical modeling and analysis of Al m Ga1−m N/GaN HEMTs employing both field-plate and high-k dielectric stack for high-voltage operation

In this paper, a new high-voltage Al _m Ga_1?m N/GaN HEMT (High Electron Mobility Transistors) with Field-Plate and high-k dielectric stack, Graded two-dimensional electron gas (2DEG) Channel Field-Plate Stack dielectric (GCFPS) HEMTs structure has been reported. The proposed structure has shown enhancements of the performances of the GaN-based HEMTs taking into account the effects of spontaneous and piezoelectric polarization fields. In order to analyze this structure, a 2D analytical model has been developed where the expressions for 2D channel potential and electric field distribution have been derived. It was shown that the GCFPS design exhibits significantly reduction of the electric field peaks along the 2DEG channel. Therefore, the breakdown voltage (BV) is greatly improved in comparison with the standard AlGaN/GaN FP-HEMTs. The developed model is validated by the good agreement with the 2D simulated data.     

Temperature-Dependent Characterization of AlGaN/GaN HEMTs: Thermal and Source/Drain Resistances

This paper shows the application of simple dc techniques to the temperature-dependent characterization of AlGaN/ GaN HEMTs in terms of the following: 1) thermal resistance and 2) ohmic series resistance (at low drain bias). Despite their simplicity, these measurement techniques are shown to give valuable information about the device behavior over a wide range of ambient/channel temperatures. The experimental results are validated by comparison with independent measurements and numerical simulations.     

A novel efficiency‐enhancement topology: GaN high‐electron mobility transistors with waveform‐modulation structure composed of Schottky diodes

To improve the power‐added efficiency (PAE) of the gallium nitride (GaN) high‐electron mobility transistor (HEMT) in radio frequency applications, this paper studies the relationship between the nonlinearity of the gate capacitance and the PAE of the GaN HEMTs. The theoretical analysis and simulation results demonstrate that the nonlinearity of the gate capacitance modulates the signal phase at the GaN HEMT input and increases the average drain current, leading to increased power consumption and reduced PAE. Then, an efficiency‐enhancement topology for GaN HEMTs that employs the waveform‐modulation effect of Schottky diodes to reduce power consumption and improve efficiency is presented. The efficiency‐enhancement topology for a 4 × 100‐μm GaN HEMT with waveform‐modulation diodes is then fabricated. Results of load‐pull test demonstrate that the novel topology can increase the PAE of the 4 × 100‐μm GaN HEMT by more than 5% at 8 GHz. The novel efficiency‐enhancement topology for GaN HEMTs proposed in this paper will be suitable for applications that demand high‐efficiency GaN HEMTs or circuits.     

Electrothermal Monte Carlo simulation of submicron wurtzite GaN/AlGaN HEMTs

We present results from the simulation of the electrothermal behaviour of submicron wurtzite GaN/AlGaN High Electron Mobility Transistors (HEMTs). The simulator uses an iterative procedure which couples a Monte Carlo simulation with a fast Fourier series solution of the Heat Diffusion Equation (HDE). The results demonstrate the dependence of the extent of the thermal droop observed in the I_ds-V_ds characteristics and the device peak temperature on the device bias. The paper also investigates the effect of the inclusion of thermal self-consistency on the device microscopic properties and studies the dependence of the device electrothermal characteristics on the type of substrate material used.     

GaN高电子迁移率晶体管的研究进展

GaN高电子迁移率晶体管(HEMT)是GaN微波功率器件的主要形式,是第3代半导体技术领域发展和竞争的焦点.针对GaN HEMT的目标特性和电流崩塌现象,从材料结构设计和器件设计两方面概括了十几年来GaN HEMT器件性能优化的研究方法,并给出了国内外GaN HEMT器件微波功率特性目前的研究进展水平.     

Double aperture double-gate vertical high-electron-mobility transistor

In this paper, we propose a double aperture double-gate AlGaN/GaN vertical high-electron-mobility transistor (HEMT) to improve the device characteristics, such as the current and the ON resistance (\(\hbox {\textit{R}}_{\mathrm{ON}}\)). The proposed vertical HEMT results are compared to the conventional single aperture single-gate vertical HEMT of equal dimensions, and increased drain current and lower \(\hbox {\textit{R}}_{\mathrm{ON}}\) are shown. A comprehensive simulation study has also been carried out for the proposed device, to analyse the impact of thickness and doping concentration of aperture, drift region, and current blocking layer. In addition, the effect of different materials in current blocking layer on device characteristics is also studied. The obtained results and their effect on device characteristics have been thoroughly analysed and explained accordingly.     

A simulation study of the impact of traps in the GaN substrate on the electrical characteristics of an AlGaN/GaN HEMT with a thin channel layer

Journal of Computational Electronics - Gallium nitride (GaN) substrates are promising candidates for GaN high-electron-mobility transistors (HEMTs) because of their epitaxial layer growth with low...     

AlGaN/GaN High Electron Mobility Transistor Structures: Self-Heating Effect and Performance Degradation

This paper reports on the results of the experimental and numerical investigation into the self-heating effect in AlGaN/GaN heterostructures grown on sapphire and SiC substrates. It shows that temperature increase has an opposite dependence on the buffer thickness for sapphire and SiC substrates. Noise spectroscopy is also used to monitor the self-heating effect. Moreover, it is shown that the room-temperature spectra can be used to determine the activation energy of the traps. An irreversible improvement in mobility and quantum scattering time is registered after the irradiation of AlGaN/GaN heterostructures at a total dose of 1 times10⁶ rad of ⁶⁰Co gamma rays.     

基于GaN器件的固态射频电源应用研究

随着电力电子技术的发展,射频电源由电子管电源发展成现在的晶体管射频电源。氮化镓GaN(gallium nitride)作为第三代宽禁带半导体材料的典型代表,具有宽禁带、高临界击穿场强、高电子饱和漂移速度以及高导通的AlGaN/GaN异质结二维电子气2DEG(two-dimensional electrons gas)等优点。GaN功率器件与硅(Si)功率器件相比,具有导通阻抗低,输入、输出电容小等特性,这些特性使得GaN功率器件高开关速度、低损耗。在E类功率射频电源的基础上,采用GaN功率器件设计制作了一款开关频率为4 MHz、功率可调的全固态射频电源实验样机。通过电路的设计和优化,样机的输出功率为21.4 W时,效率达到了96.7%;同时,采用专为射频电源生产的Si功率器件替换掉样机上的GaN器件,实验数据验证了GaN器件开关速度快、损耗低,可大幅度提高射频电源的效率。     

A deterministic study of hot phonon effects in a 2D electron gas channel formed at an AlGaN/GaN heterointerface

We investigate the hot phonon effects on the transport properties of a two-dimensional electron gas formed in an AlGaN/GaN heterostructure. For this purpose, we use a deterministic numerical scheme to solve the coupled system of Boltzmann transport equations. The envelope wave functions of the confined carriers are self-consistently calculated from the Schrödinger-Poisson system. The simulation results show that the electron drift velocity is reduced by hot phonons for moderate and high electric fields, but become enhanced for low fields. This interesting behavior is elucidated by virtue of the energy balance equation using an analytic electron temperature model. We find good agreement to experimental data when hot phonon and degeneracy effects are taken into account.     

High performance 1 mm AlGaN/GaN HEMT based on SiC substrate

This is our first report on the high performance 1 mm AlGaN/GaN high electron mobility transistor (HEMT) which was developed using home-made AlGaN/GaN epitaxy structures based on SiC substrate. Metal-organic chemical vapor deposition (MOCVD) was used to generate the epitaxy layers. Corresponding experiments show that the device has a gate length of 0.8 μm exhibiting drain current density of 1.16 A/mm, transconductance of 241 ms/mm, a gate-drain breakdown voltage larger than 80 V, maximum current gain frequency of 20 GHz and maximum power gain frequency of 28 GHz. In addition, the power gain under the continues wave condition is 14.2 dB with a power density of 4.1 W/mm, while under the pulsed wave condition, power gain reaches 14.4 dB with power density at 5.2 W/mm. Furthermore, the two-port network impedance characteristics display great potential in microwave application. __________ Translated from Chinese Journal of Semiconductors, 2006, 27(11): 1981–1983 [译自: 半导体学报]     

A physics-based electrical model of a magnetically sensitive AlGaN/GaN HEMT

Journal of Computational Electronics - The electrical model of a magnetically sensitive two-drain contact AlGaN/GaN HEMT (MagHEMT) is described for the first time and implemented in...     

An improved model to assess temperature-dependent DC characteristics of submicron GaN HEMTs

A modified analytical model for the current–voltage (I–V) characteristics of AlGaN/GaN high-electron-mobility transistors (HEMTs) is presented, considering the temperature-dependent: (a) Schottky barrier height, (b) energy bandgap discontinuity, (c) carrier mobility, and (d) saturation velocity. It is demonstrated that the Schottky barrier height and energy bandgap discontinuity decrease with increase of the temperature. The effective mobility of the two-dimensional electron gas (2-DEG) also decreases with increasing temperature, causing a reduction in the output current of the device. The model was tested over a wide range of temperatures (300–500 K) and bias, and it was observed that the developed model can successfully predict the I–V characteristic of the device with reasonable accuracy, especially at high temperatures (\(\sim 500\) K). It is shown that the developed model offers, on average, a 39 % improvement for the temperature variation, from 300–500 K, relative to the best model reported in literature.