High Electron Mobility Transistor Heterostructures Research Articles

Polarization dependent charge control model for microwave performance assessment of AlGaN/GaN/AlGaN double heterostructure HEMTs

An accurate polarization dependent charge control-based analytical model is proposed for microwave performance assessment of $$\hbox {Al}_{0.15}\hbox {Ga}_{0.85}\hbox {N/GaN/Al}_{0.15}\hbox {Ga}_{0.85}\hbox {N}$$ double heterostructure high electron mobility transistors (DH-HEMTs) in terms of current, transconductance, gate capacitances and cutoff frequency. An analytical expression correlating the sheet carrier concentration in the two 2DEGs formed at the upper and lower heterointerfaces of a DH-HEMT is obtained. AlGaN/GaN/AlGaN DH-HEMTs are found to exhibit superior RF performance as compared to its single heterostructure counterpart in terms of higher drain current, improved transconductance, higher gate capacitance and higher unity-gain cutoff frequency. This improvement in the DH-HEMT is mainly attributed to the formation of two 2DEGs (at top and the bottom heterointerface) as compared to the single 2DEG in a SH-HEMT. The variation of drain current with drain voltage and with gate voltage of AlGaN/GaN SH-HEMTs and AlGaN/GaN/AlGaN DH-HEMTs is obtained analytically and found to agree reasonably well with that obtained using ATLAS 2D device simulation, thereby validating the proposed model.

Investigation of Photoluminescence, Stimulated Emission, Photoreflectance, and 2DEG Properties of Double Heterojunction AlGaN/GaN/AlGaN HEMT Heterostructures Grown by Ammonia MBE

A series of double heterojunction AlGaN/GaN/AlGaN high electron mobility transistor (HEMT) heterostructures are grown by ammonia MBE on sapphire substrates using AlN buffer layers with varying thicknesses and growth conditions. The grown heterostructures are investigated by atomic force microscopy as well as photoluminescence (PL), stimulated emission (SE), and photoreflectance (PR) measurements. It is shown that PL intensity cannot be used as a reliable criterion for quality control of HEMT heterostructures because of an influence of surface roughness. A new method of quality control of the active GaN layer in HEMTs is proposed based on determination of the SE threshold excitation level. An advantage of the method is that no reference sample is needed for comparison of different HEMT structures. Internal electric field strength values are determined from PR spectra and their correlation with 2DEG density is demonstrated.

Contactless Method to Measure 2DEG Charge Density and Band Structure in HEMT Structures

We present a contactless method that is capable of characterizing a high electron mobility transistor heterostructure at the wafer stage, right after its growth, before any production process has been attempted, to provide the equilibrium band structure and the density of charge of the 2-dimensional electron gas in the quantum well. The method can thus evaluate critical transistor parameters and help to screen out low performance wafers before the actual fabrication. To this end, we use a simple optical spectroscopy at room temperature that measures the surface photovoltage band-edge responses in the heterostructure and uses a model that takes into account the effect of the built-in electric fields on optical absorption in the layers and heterojunctions to evaluate bandgaps, band offsets, and built-in fields. The quantum well charge is then calculated from the built-in fields. The main advantage of the method is in its capability to provide information on all the different layers in the typical heterostructure simultaneously in a single measurement. The method is not limited to the high electron mobility transistor structure but may be used on any other heterostructure. It opens the door for a new type of characterization methods suitable for the post-silicon multi-layer multi-semiconductor heterostructure device era.

(Invited) What's Next after GaN? How about More GaN!

Over the course of the past few years, transistors based on gallium nitride (GaN) and related III-N materials have proven to be clear contenders for solid-state power amplifier applications in the commercial and military market spaces. In microwave technology nodes of lateral GaN high-electron-mobility transistor (HEMT) technology (with gate lengths greater than 0.25 microns), the material-driven electrical performance of GaN has been demonstrated with exceptional reliability in production-released processes. While a power density of several W/mm can be easily achieved at S- and X-band with GaN HEMTs based on an AlGaN/GaN heterojunction, it generally becomes more difficult to obtain the same output power level at higher frequency nodes. This is primarily due to limitations on the operating voltage that can be used; as device dimensions scale shorter, internal electric fields increase and field management techniques, such as source-connected field plates, generally are not used because they introduce undesirable parasitic capacitance. To address these shortcomings, significant technical research and development has been carried out over the past few years focusing on extending GaN HEMT frequency performance into the millimeter wavelength (MMW) range of 30 – 300 GHz. To prevent the onset of short-channel effects in short gate length devices, vertically-scaled barrier heterostructures have been investigated. In a number of examples, materials growth development of novel III-N materials has enabled heterojunctions with desirable two-dimensional electron gas transport properties. In this talk, we will review a number of recent developments in the community related to MMW GaN HEMT technology. In addition to reported literature, we will discuss NRL’s internal materials and MMW device development using molecular beam epitaxy. Novel HEMT heterostructures based on AlN, ScxAl1-xN, and InxAl1-xN (metal- and N-polar) will be discussed.

Influence of a low-temperature GaN cap layer on the electron concentration in AlGaN/GaN heterostructure

The influence of low-temperature passivating GaN cap layers on the electrophysical parameters of a 2D electron gas (2DEG) in heterostructure high-electron mobility transistors has been studied. It has been found that thin GaN layers deposited in situ at 550°C do not exhibit polar properties and do not change the carrier concentration in the 2DEG. However, GaN layers deposited at 830°C decrease the carrier concentration in the 2DEG, which is in agreement with theoretical calculations. Using the reflected high-energy electron diffraction technique, it has been established that this effect may be associated with different structures and morphologies of GaN layers deposited at different temperatures.

Trap state analysis in AlGaN/GaN/AlGaN double heterostructure high electron mobility transistors at high temperatures

In this work, frequency-dependent capacitances and conductance measurements are adopted to investigate high temperature characteristics of trap states in AlGaN/GaN/AlGaN double heterostructure high electron mobility transistors (DH-HEMTs). It is found that fast and slow trap states are present in DH-HEMTs, while only fast traps exist in AlGaN/GaN single heterostructure (SH) HEMTs. In the former, the fast trap state density ranges from 4.6 × 1012 cm−2 eV−1 to 1.9 × 1013 cm−2 eV−1 located at an energy below the conduction band between 0.273 eV and 0.277 eV, and the slow deep trap state density decreases from 2.4 × 1013 cm−2 eV−1 to 8.7 × 1012 cm−2 eV−1 located at an energy ranging from 0.384 eV to 0.423 eV in DH-HEMTs with a 14 nm GaN channel layer. These active trap energy levels in DH-HEMTs become deeper as the thickness of the channel layer decreases. In addition, the active trap energy levels in SH- and DH-HEMTs gradually become deeper as the measurement temperature increases. Also, the change in amplitude of the active trap energy levels in DH-HEMTs is larger than that in SH-HEMTs, which indicates that DH is efficient in suppressing the reverse gate leakage current at high temperatures.

Optical properties of AlGaN nanowires synthesized via ion beam techniques

AlGaN plays a vital role in hetero-structure high electron mobility transistors by employing a two-dimensional electron gas as an electron blocking layer in multi-quantum well light emitting diodes. Nevertheless, the incorporation of Al into GaN for the formation of the AlGaN alloy is limited by the diffusion barrier formed by instant nitridation of Al adatoms by reactive atomic N. The incorporation of Al above the miscibility limit, however, can be achieved by the ion beam technique. The well known ion beam mixing (IBM) technique was carried out with the help of Ar+ irradiation for different fluences. A novel approach was also adopted for the synthesis of AlGaN by the process of post-irradiation diffusion (PID) as a comparative study with the IBM technique. The optical investigations of AlGaN nanowires, synthesized via two different methods of ion beam processing, are reported. The effect of irradiation fluence and post-irradiation annealing temperature on the random alloy formation was studied by the vibrational and photoluminescence (PL) spectroscopic studies. Vibrational studies show one-mode phonon behavior corresponding to the longitudinal optical (LO) mode of A1 symmetry [A1(LO)] for the wurtzite phase of AlGaN nanowires in the random alloy model. A maximum Al atomic percentage of ∼6.3%–6.7% was calculated with the help of band bowing formalism from the Raman spectral analysis for samples synthesized in IBM and PID processes. PL studies show the extent of defects present in these samples.

Simulated x-ray diffraction from pseudomorphic GaAs/In0.3Ga0.7As superlattice high electron mobility transistor heterostructures on GaAs (001) substrates

In this paper, the authors report a study of the simulated dynamical x-ray diffraction from GaAs/In0.3Ga0.7As superlattice high electron mobility transistor heterostructures on GaAs (001) substrates both with (metamorphic) and without (pseudomorphic) dislocations. The analysis of dynamical x-ray diffraction for 004, 115, 026, and 117 reflection profiles was conducted for the case of Cu kα1 radiation. The authors show that the threading dislocation density may be estimated from nondestructive x-ray rocking curve measurements, using the rocking curve peak intensity ratios or widths for superlattice diffraction peaks. Despite the complexity of these multilayered device structures and the resulting x-ray diffraction profiles, analysis of the 004 x-ray diffraction profile allows characterization of the pseudomorphic–metamorphic transition in them and is of considerable practical importance for device realization.

InAlN/AlN/GaN heterostructures for high electron mobility transistors

The results of development of InAlN/AlN/GaN heterostructures, grown on sapphire substrates by metal-organic chemical vapour deposition, and high electron mobility transistors (HEMTs) based on them are presented. The dependencies of the InAlN/AlN/GaN heterostructure properties on epitaxial growth conditions were investigated. The optimal indium content and InAlN barrier layer thicknesses of the heterostructures for HEMT s were determined. The possibility to improve the characteristics of HEMTs by in-situ passivation by Si3N4 thin protective layer deposited in the same epitaxial process was demonstrated. The InAlN/AlN/GaN heterostructure grown on sapphire substrate with diameter of 100 mm were obtained with sufficiently uniform distribution of sheet resistance. The HEMTs with saturation current of 1600 mA/mm and transconductance of 230 mS/mm are demonstrated.

The influence of an In0.52Al0.48As transition layer design on the transport characteristics of a metamorphic high-electron-mobility transistor

We have used the atomic force microscopy and Hall effect measurements to study the influence of In0.52Al0.48As transition layer design on the electron mobility in the InAlAs/InGaAs/GaAs channel of a highelectron- mobility transistor (HEMT) with the metamorphic buffer. The optimum buffer layer favors suppression of the misfit dislocation threading into upper layers of the HEMT heterostructure and prevents development of the surface microrelief.

A compact drain current model for heterostructure HEMTs including 2DEG density solution with two subbands

An explicit and precise model for two dimensional electron gas (2DEG) charge density and Fermi level (Ef) in heterostructure high electron mobility transistors (HEMTs) is developed. This model is from a consistent solution of Schro¨dinger’s and Poisson’s equations in the quantum well with two important energy levels. With these closed-form solutions, a unified surface potential calculation valid for all the operation regions is derived. With the help of surface potential, a single-piece drain current model is developed which is also capable of describing the current collapse effect by using a semi-empirical expression of source/drain access region resistances. Comparisons with numerical and measured data show that the proposed model gives an accurate description of Ef and drain current in all regions of operation.

Al0.30Ga0.70N/GaN/Al0.07Ga0.93N Double Heterostructure High Electron Mobility Transistors with a Record Saturation Drain Current of 1050mA/mm**Supported by the National Science and Technology Major Project of China under Grant No 2013ZX02308-002, and the National Natural Science Foundation of China under Grant Nos 11435010 and 61474086.

We report Al0.30Ga0.70N/GaN/Al0.07Ga0.93N double heterostructure high electron mobility transistors with a record saturation drain current of 1050 mA/mm. By optimizing the graded buffer layer and the GaN channel thickness, both the crystal quality and the device performance are improved significantly, including electron mobility promoted from 1535 to 1602 cm2/V·s, sheet carrier density improved from 0.87×1013 to 1.15×1013 cm−2, edge dislocation density reduced from 2.4×109 to 1.3×109 cm−2, saturation drain current promoted from 757 to record 1050 mA/mm, mesa leakage reduced by two orders in magnitude, and breakdown voltage promoted from 72 to 108 V.

Small signal modeling of high electron mobility transistors on silicon and silicon carbide substrate with consideration of substrate loss mechanism

In this paper, we present a comparative study on small-signal modeling of AlN/GaN/AlGaN double hetero-structure high electron mobility transistors (HEMTs) grown on silicon (Si) and silicon carbide (SiC) substrate. The traditional small signal equivalent circuit model is modified to take into account the transmission loss mechanism of coplanar waveguide (CPW) line which cannot be neglected at high frequencies. CPWs and HEMTs-on-AlN/GaN/AlGaN epitaxial layers are fabricated on both the Si and SiC substrates. S-parameter measurements at room temperature are performed over the frequency range from 0.5GHz to 40GHz. Transmission loss of CPW lines are modeled with a distributed transmission line (TL) network and an equivalent circuit model is included in the small-signal transistor model topology. Measurements and simulations are compared and found to be in good agreement.

Identification of photoluminescence bands in AlGaAs/InGaAs/GaAs PHEMT heterostructures with donor-acceptor-doped barriers

The photoluminescence of AlGaAs/InGaAs/GaAs pseudomorphic high-electron mobility transistor heterostructures with donor-acceptor-doped AlGaAs barriers is studied. It is found that the introduction of additional p +-doped AlGaAs layers into the design brings about the appearance of new bands in the photoluminescence spectra. These bands are identified as resulting from transitions (i) in donor-acceptor pairs in doped AlGaAs layers and (ii) between the conduction subband and acceptor levels in the undoped InGaAs quantum well.

Growth and characterization of AlGaN/GaN/AlGaN double-heterojunction high-electron-mobility transistors on 100-mm Si(111) using ammonia-molecular beam epitaxy

To improve the confinement of two-dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistor (HEMT) heterostructures, AlGaN/GaN/AlGaN double heterojunction HEMT (DH-HEMT) heterostructures were grown using ammonia-MBE on 100-mm Si substrate. Prior to the growth, single heterojunction HEMT (SH-HEMT) and DH-HEMT heterostructures were simulated using Poisson-Schrödinger equations. From simulations, an AlGaN buffer with “Al” mole fraction of 10% in the DH-HEMT was identified to result in both higher 2DEG concentration (∼1013 cm−2) and improved 2DEG confinement in the channel. Hence, this composition was considered for the growth of the buffer in the DH-HEMT heterostructure. Hall measurements showed a room temperature 2DEG mobility of 1510 cm2/V.s and a sheet carrier concentration (ns) of 0.97 × 1013 cm−2 for the DH-HEMT structure, while they are 1310 cm2/V.s and 1.09 × 1013 cm−2, respectively, for the SH-HEMT. Capacitance-voltage measurements confirmed the improvement in the confinement of 2DEG in the DH-HEMT heterostructure, which helped in the enhancement of its room temperature mobility. DH-HEMT showed 3 times higher buffer break-down voltage compared to SH-HEMT, while both devices showed almost similar drain current density. Small signal RF measurements on the DH-HEMT showed a unity current-gain cut-off frequency (fT) and maximum oscillation frequency (fmax) of 22 and 25 GHz, respectively. Thus, overall, DH-HEMT heterostructure was found to be advantageous due to its higher buffer break-down voltages compared to SH-HEMT heterostructure.

Unstrained InAlN/GaN heterostructures grown on sapphire substrates by MOCVD

InAlN/GaN heterostructures were grown on sapphire substrates by low-pressure metal organic chemical vapor deposition. The influences of NH3 flux and growth temperature on the In composition and morphologies of the InAlN were investigated by X-ray diffraction and atomic force microscopy. It's found that the In composition increases quickly with NH3 flux decrease. But it's not sensitive to NH3 flux under higher flux. This suggests that lower NH3 flux induces a higher growth rate and an enhanced In incorporation. The In composition also increases with the growth temperatures decreasing, and the defects of the InAlN have close relation with In composition. Unstrained InAlN with In composition of 17% is obtained at NH3 flux of 500 sccm and growth temperature of 790 °C. The InAlN/GaN heterostructure high electron mobility transistor sample showed a high two-dimensional electron gas (2DEG) mobility of 1210 cm2/(V·s) with the sheet density of 2.3 × 1013 cm−2 at room temperature.

Effect of electron-beam irradiation on leakage current of AlGaN/GaN HEMTs on sapphire

This study examined the effect of electronbeam (E-beam) irradiation on the electrical properties of n-GaN, AlGaN and AlGN/GaN structures on sapphire substrates. E-beam irradiation resulted in a significant decrease in the gate leakage current of the n-GaN, AlGaN and HEMT structure from 4.0×10<SUP>-4</SUP> A, 6.5×10<SUP>-5</SUP> A, 2.7×10<SUP>-8</SUP> A to 7.7×10<SUP>-5</SUP> A, 7.7×10<SUP>-6</SUP> A, 4.7×10<SUP>-9</SUP> A, respectively, at a drain voltage of -10V. Furthermore, we also investigated the effect of E-beam irradiation on the AlGaN surface in AlGaN/GaN heterostructure high electron mobility transistors(HEMTs). The results showed that the maximum drain current density of the AlGaN/GaN HEMTs with E-beam irradiation was greatly improved, when compared to that of the AlGaN/GaN HEMTs without E-beam irradiation. These results strongly suggest that E-beam irradiation is a promising method to reduce leakage current of AlGaN/GaN HEMTs on sapphire through the neutralization the trap.

Ultrathin barrier GaN-on-Silicon devices for millimeter wave applications

In this paper, the possibility to use GaN-on-Silicon devices for millimeter wave applications by means of ultrathin barrier layers is reviewed. In particular, an emerging double heterostructure high electron mobility transistor (DHFET) based on AlN/GaN/AlGaN grown on silicon substrate is described, which enables a unique simultaneous achievement of high breakdown voltage and high frequency performance. This configuration system allowed for state-of-the-art GaN-on-Silicon DC, RF output power and noise performances at 40GHz, paving the way for high performance mmW cost-effective amplifiers. Preliminary reliability assessment has been performed for the first time on this new class of RF devices, showing promising device stability.

Microstructure of Ti/Al/Ni/Au ohmic contacts for N-polar GaN/AlGaN high electron mobility transistor devices

The microstructure of Ti/Al/Ni/Au ohmic contacts on N-polar GaN/AlGaN high electron mobility transistor heterostructures annealed from 800 °C to 900 °C has been studied using transmission electron microscopy and associated analytical techniques. Two ohmic metal stacks with different Ti/Al/Ni/Au layer thicknesses (20/200/40/50 nm and 20/100/10/50 nm) have been examined. Samples with low ohmic contact resistance after annealing were found to have two common characteristics: (1) the top GaN channel layer had completely reacted with Ti metal to form a polycrystalline TiN layer and (2) a ∼5 nm-thick Au-rich layer was present near the TiN/AlGaN interface. Possible conduction mechanisms related to the presence of Au in low ohmic contact resistance samples are discussed.

Modeling of 2DEG sheet carrier density and DC characteristics in spacer based AlGaN/AlN/GaN HEMT devices

In this paper, 2-D physics based model for two-dimensional electron gas (2DEG) sheet carrier density ns and DC characteristic of the proposed spacer layer based AlxGa1−xN/AlN/GaN High Electron Mobility Transistors (HEMTs) is modeled by considering the triangular quantum well. To obtain charge density ns, the variation of Fermi level with supply voltage and the formation of Energy sub-bands E0, E1 is considered. The obtained results are simple and easy to analyze the sheet charge density, DC characteristics model for spacer layer based AlxGa1−xN/AlN/GaN HEMT power devices. Due to large two-dimensional electron gas (2DEG) sheet carrier density and high velocity, the maximal drain current density achieved is very high. An expression for the two-dimensional electron gas (2-DEG) charge density ns, valid in all regions of device operation is studied and applied to derive drain current along with channel length modulation and velocity saturation effects. The spacer layer based AlGaN/AlN/GaN heterostructure HEMTs shows excellent promise as one of the candidates to substitute present AlGaN/GaN HEMTs for future high speed and high power applications. The proposed model is in a good agreement with experimental data for analyzing both sheet carrier density and drain current over a range of applied voltages and different gate geometries.