The III–V nitride semiconductors, gallium nitride, aluminum nitride, and indium nitride, have, for some time now, been recognized as promising materials for novel electronic and optoelectronic device applications. As informed device design requires a firm grasp of the material properties of the underlying electronic materials, the electron transport that occurs within these III–V nitride semiconductors has been the focus of considerable study over the years. In an effort to provide some perspective on this rapidly evolving field, in this paper we review analyses of the electron transport within the III–V nitride semiconductors, gallium nitride, aluminum nitride, and indium nitride. In particular, we discuss the evolution of the field, compare and contrast results determined by different researchers, and survey the current literature. In order to narrow the scope of this review, we will primarily focus on the electron transport within bulk wurtzite gallium nitride, aluminum nitride, and indium nitride, for this analysis. Most of our discussion will focus on results obtained from our ensemble semi-classical three-valley Monte Carlo simulations of the electron transport within these materials, our results conforming with state-of-the-art III–V nitride semiconductor orthodoxy. A brief tutorial on the Monte Carlo approach will also be featured. Steady-state and transient electron transport results are presented. We conclude our discussion by presenting some recent developments on the electron transport within these materials.

Steady-State and Transient Electron Transport Within the III–V Nitride Semiconductors, GaN, AlN, and InN: A Review

Wide bandgap, high saturation velocity, and high thermal stability are some of the properties of GaN, which make it an excellent material for high-power, high-frequency, and high-temperature applications. As several application areas require the devices to operate at elevated temperatures, a proper modeling of the temperature dependences of the band structure and transport parameters is very important. We present two-dimensional hydrodynamic simulations of AlGaN/GaN high electron mobility transistors (HEMTs) at high temperatures. The simulator is calibrated against measurement data of a real device and delivers good predictive results for the DC and RF characteristics of another. The temperature dependence of the maximum current and cut-off frequency of submicron devices is further studied.

High-temperature modeling of AlGaN/GaN HEMTs

A temperature dependent analytical model has been presented for AlGaN/GaN power high electron mobility transistor (HEMT) to predict the DC performance at elevated temperatures. In this model the effects of temperature on band gap energy, sheet carrier density, threshold voltage, carrier mobility, and saturation velocity are taken into consideration. Channel length modulation in the saturation region of operation and spontaneous and piezoelectric polarization induced charges at the AlGaN/GaN heterointerface are also included in this model. Temperature- and bias-dependent on-wafer current–voltage measurements from 300 K to 500 K were carried out to verify the developed model. DC measurements and model predictions are presented for an AlGaN/GaN power HEMT fabricated on SiC substrate. The developed model shows good agreement with the measured data for a wide range of temperatures.

Temperature dependent analytical model for current-voltage characteristics of AlGaN/GaN power HEMT

Self-heating in AlGaN/GaN high electron mobility transistors (HEMTs) negatively impacts device performance and reliability. Under nominal operating conditions, a hot-spot in the device channel develops under the drain side corner of the gate due to a concentration of volumetric heat generation leading to nonequilibrium carrier interactions and non-Fourier heat conduction. These subcontinuum effects obscure identification of the most salient processes impacting heating. In response, we examine self-heating in GaN-on-Si HEMTs via measurements of channel temperature using above-bandgap UV thermoreflectance imaging in combination with fully coupled electrothermal modeling. The methods together highlight the interplay of heat concentration and subcontinuum thermal transport showing that channel temperature cannot be determined solely by continuum scale heat transfer principles. Under conditions of equal power dissipation (PDISS = VDS × IDS = 250 mW), for example, a higher VDS bias (∼23 V) resulted in an ∼44% larger rise in peak junction temperature compared to that for a lower VDS (∼7.5 V) condition. The difference arises primarily due to reduction in the heat generating volume when operating under partially pinched-off (i.e., high VDS) conditions. Self-heating amplifies with this reduction as heating now takes place primarily over length scales less than the mean free path of the phonons tasked with energy dissipation. Being less efficient, the subcontinuum transport restricts thermal transport away from the device hot-spot causing a net increase in channel temperature. Taken together, even purely thermally driven device mean-time-to-failure is not, therefore, based on power dissipation alone as both bias dependence and subcontinuum thermal transport influence device lifetime.

Nanoscale electro-thermal interactions in AlGaN/GaN high electron mobility transistors

In the present paper, the linear and nonlinear optical absorption coefficients and refractive index changes between the ground and the first excited states in double GaN/AlxGa(1−x)N quantum wells are studied theoretically. The electronic energy levels and their corresponding wave functions are obtained by solving Schrodinger-Poisson equations self-consistently within the effective mass approximation. The obtained results show that the optical absorption coefficients and refractive index changes can be red- and blue-shifted through varying the left quantum well width and the aluminum concentration xb2 of the central barrier, respectively. These structural parameters are found to present optimum values for carrying out the transition of 0.8 eV (1.55 μm). Furthermore, we show that the desired transition can also be achieved by replacing the GaN in the left quantum well with AlyGa(1−y)N and by varying the aluminum concentration yAl. The obtained results give a new degree of freedom in optoelectronic device ap...

Linear and nonlinear optical absorption coefficients and refractive index changes in GaN/AlxGa(1−x)N double quantum wells operating at 1.55 μm

The dc drain current characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) at high temperatures have been studied by numerical simulation from the self-consistent solution of Schrodinger's and Poisson's equations. The effects of temperature on the polarization and conduction band offset in the heterojunction have been considered. Our simulation results of the drain current at high temperature agree very well with reported experimental data. There is a significant reduction of saturation drain current when the temperature increases, and it is concluded that this is caused by both the decrease of saturation carrier velocity and two-dimensional electron density in the HEMT.

https://iopscience.iop.org/article/10.1088/0268-1242/20/2/016/pdf

Yuancheng Chang

Papers

Numerical simulation of current-voltage characteristics of AlGaN/GaN HEMTs at high temperatures