HEMT Structures Research Articles

Correlation of carbon doping variations with the vertical breakdown of GaN-on-Si for power electronics

In this work gallium nitride (GaN) grown on silicon substrates was investigated in order to determine critical defects responsible for differences in the vertical breakdown of HEMT structures. Cathodoluminescence studies at the SEM revealed a direct correlation between the intensity of the blue luminescence (BL) band and the carbon doping concentration. Observing this, carbon depletion zones were found around threading dislocations in the active GaN layer, as well as a deep depletion in growth columns concluded from a reduction of the BL intensity. Using the given results a model of a defect, here called the deep carbon depletion (DCD), is proposed to explain the correlation between carbon variations and the vertical breakdown.

Correlation of gate leakage and local strain distribution in GaN/AlGaN HEMT structures

GaN/AlGaN HEMT structures are observed to undergo a reversible, drastic change in the leakage current when covered with an additional polymer passivation layer. The polymer layer induces a stress on the HEMT structures, which initiates material migration processes and the formation of structural defects, influencing the electrical performance. Local strain measurements were performed in the semiconductor, at the critical HEMT gate electrode, to evaluate the impact of the stress on the Schottky gates. The strain distributions in the structures were measured with nanobeam electron diffraction from electron-transparent samples at cross sections and longitudinal sections at the positions of high leakage currents. A variation of the strain distribution underneath the gate electrode was detected in a cross-sectional sample. On the contrary, only minor differences in the strain values were measured in the longitudinal sections at different photoemission sites. Finally, localized metal interdiffusion was detected at the sites with the highest photoemission intensities.

Terahertz Response of Tightly Concatenated Two Dimensional InGaAs Field-Effect Transistors Integrated on a Single Chip

THz response of AlGaAs/InGaAs/GaAs HEMT structure has been investigated. The structure consists of the serpentine chain of series connected HEMTs. The source of one is the drain for the subsequent transistor. Experiments have been showed THz response peculiarities of such structures and enhanced noise equivalent power.

Carbon-Doped GaN on SiC Materials for Low-Memory-Effect Devices

AlGaN/GaN on SiC HEMT structures suitable for high power, high frequency applications are demonstrated. The material manifests record breaking thermal management and electron mobility. Moreover, thanks to the fact that the buffer layer is doped with carbon, the memory effect of processed devices is very low making system design and manufacturing significantly easier and less expensive.

Impact of nanomaterial arrangement on the reliability and the electron mobility in AlGaN/GaN HEMTs

The obtained results demonstrate that the improvement of nanomaterial arrangement in AlGaN/GaN HEMT structures quantitatively characterized with the use of a multifractal parameter (the degree of disorder) results in the increase at several times in the electron mobility values at 2DEG channel in HEMT structures and the reliability of HEMT parameters.

Growth condition optimization and mobility enhancement through inserting AlAs monolayer in the InP-based InxGa1−xAs/In0.52Al0.48As HEMT structures**Project supported by the National Natural Science Foundation of China (Grant No. 61434006).

The structure of InP-based InxGa1−xAs/In0.52Al0.48As pseudomorphic high electron mobility transistor (PHEMT) was optimized in detail. Effects of growth temperature, growth interruption time, Si δ-doping condition, channel thickness and In content, and inserted AlAs monolayer (ML) on the two-dimensional electron gas (2DEG) performance were investigated carefully. It was found that the use of the inserted AlAs monolayer has an enhancement effect on the mobility due to the reduction of interface roughness and the suppression of Si movement. With optimization of the growth parameters, the structures composed of a 10 nm thick In0.75Ga0.25As channel layer and a 3 nm thick AlAs/In0.52Al0.48As superlattices spacer layer exhibited electron mobilities as high as 12500 cm2·V−1·s−1 (300 K) and 53500 cm2·V−1·s−1 (77 K) and the corresponding sheet carrier concentrations (Ns) of 2.8 × 1012 cm−2 and 2.9 × 1012 cm−2, respectively. To the best of the authors’ knowledge, this is the highest reported room temperature mobility for InP-based HEMTs with a spacer of 3 nm to date.

Quantitative SIMS depth profiling of Al in AlGaN/AlN/GaN HEMT structures with nanometer-thin layers

The possibilities of quantitative secondary ion mass spectrometry (SIMS) depth profiling of Al in AlxGa1 − xN/AlN/GaN transistor heterostructures are shown. Using a series of test structures for a TOF.SIMS-5 time-of-flight mass spectrometer, we obtained a refined linear calibration dependence of the secondary-ion yield on the composition ×, namely, Y(CsAl+)/Y(CsGa+) = K × x/(1 − x), with a high linear correlation coefficient, Rl = 0.9996, which permits quantitative SIMS analysis of relatively thick AlGaN barrier layers. The method of profile reconstruction with allowance for the main artifacts of ion sputtering has been first applied for the analysis of GaN/AlGaN/AlN/GaN high electron mobility transistor structure. This method permits to perform quantitative analysis of the thickness and composition of a nanometer-thin AlN sublayer and to estimate the measurement error. For the structure being studied, the AlN sublayer is 1.2 ± 0.2 nm thick. Copyright © 2016 John Wiley & Sons, Ltd.

An unambiguous identification of 2D electron gas features in the photoluminescence spectrum of AlGaN/GaN heterostructures

A fast and non-destructive method for probing the true signatures of 2D electron gas (2DEG) states in AlGaN/GaN heterostructures is presented. Two broad features superimposed with interference oscillations are observed in the low temperature photoluminescence (PL) spectrum. The two features are identified as the ground and excited 2DEG states which are confirmed by comparing the PL spectra of as-grown and top barrier layer etched samples. Broad PL features disappear at a certain temperature along with the associated interference oscillations. Furthermore, the two broad PL features depicts specific temperature and excitation intensity dependencies which make them easily distinguishable from the bandedge excitonic or defect related PL features. The presence of strong interference oscillations associated with the 2DEG PL features is explained by considering the localized generation of PL signal at the AlGaN/GaN heterointerface. Finally, a large value of the polarization induced electric field of ~1.01 MV cm−1 is reported from PL measurements for AlGaN/GaN HEMT structures. It became possible only when the true identification of 2DEG features was made possible by the proposed method.

Properties of two‐dimensional electron gas in AlGaN/GaN HEMT structures determined by cavity‐enhanced THz optical Hall effect

AbstractIn this work we employ terahertz (THz) ellipsometry to determine two‐dimensional electron gas (2DEG) density, mobility and effective mass in AlGaN/GaN high electron mobility transistor structures grown on 4H‐SiC substrates. The effect of the GaN interface exposure to low‐flow‐rate trimethylaluminum (TMA) on the 2DEG properties is studied. The 2DEG effective mass and sheet density are determined tobe in the range of 0.30‐0.32m0 and 4.3‐5.5×1012 cm–2, respectively. The 2DEG effective mass parameters are found to be higher than the bulk effective mass of GaN, which is discussed in view of 2DEG confinement. It is shown that exposure to TMA flow improves the 2DEG mobility from 2000 cm2/Vs to values above 2200 cm2/Vs. A record mobility of 2332±61 cm2/Vs is determined for the sample with GaN interface exposed to TMA for 30 s. This improvement in mobility is suggested to be due to AlGaN/GaN interface sharpening causing the reduction of interface roughness scattering of electrons in the 2DEG. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High Performance AlGaN/GaN Hemts on GaN Substrates

AlGaN/GaN HEMTs grown on GaN substrates have the potential to achieve significant improvement in device performance and reliability. We present a comprehensive study of high performance AlGaN/GaN HEMTs on GaN substrate with an emphasis on correlation of defects with device performance and reliability. Three AlGaN/GaN HEMT structures (AlGaN/GaN-HEMTs-on-GaN, AlGaN/GaN-HEMTs-on-Si and AlGaN/GaN-HEMTs-on-sapphire) were grown and used for comparison. Temperature-dependent electrical performance of AlGaN/GaN HEMTs were analyzed and correlated with defects. The results show that very smooth surface morphology of AlGaN/GaN-HEMTs-on-GaN is demonstrated with significantly reduced structural defects (characterized by HR-TEM) and surface defects (characterized by AFM). Leakage current reduction of more than four orders of magnitude is achieved with AlGaN/GaN-HEMTs-on-GaN structure. The effects of defects on surface passivation and dynamic on-resistance of AlGaN/GaN HEMTs will also be presented and analyzed.

Polar semiconductor heterojunction structure energy band diagram considerations

The unique nature of built-in electric field induced positive/negative charge pairs of polar semiconductor heterojunction structure has led to a more realistic device model for hexagonal III-nitride HEMT. In this modeling approach, the distribution of charge carriers is dictated by the electrostatic potential profile instead of Femi statistics. The proposed device model is found suitable to explain peculiar properties of GaN HEMT structures, including: (1) Discrepancy in measured conventional linear transmission line model (LTLM) sheet resistance and contactless sheet resistance of GaN HEMT with thin barrier layer. (2) Below bandgap radiation from forward biased Nickel Schottky barrier diode on GaN HEMT structure. (3) GaN HEMT barrier layer doping has negligible effect on transistor channel sheet charge density.

Electron Mobilities and Effective Masses in InGaAs/InAlAs HEMT Structures with High In Content

In\(_{x}\)Ga\(_{1-{x}}\)As/In\(_{y}\)Al\(_{1-{y}}\)As HEMT structures \({\updelta }\)-doped by Si were grown by molecular beam epitaxy on InP substrate. We investigated the influence of the In content on the electron mobilities and effective masses in dimensionally quantized subbands. The electron effective masses were determined by the temperature dependence of the amplitude of the Shubnikov–de Haas effect at 1.6 and 4.2 K. We found that the more the In content in quantum well (QW), the less the electron effective masses. The mobilities are higher in HEMT structures with wider and deeper QW. The energy band diagrams were calculated by using Vegard’s law for basic parameters. The calculated band diagrams are in a good agreement with the experimental data of photoluminescence spectra.

Analytical model and new structure of the enhancement-mode polarization-junction HEMT with vertical conduction channel

A novel enhancement-mode (E-mode) polarization-junction HEMT with vertical conduction channel (PVC-HEMT) is proposed, and its analytical model for threshold voltage (Vth) is presented. It has two features: one is GaN/AlGaN/GaN double hetero-structure, the other is that source and drain locate at the same side of trench-type MOS gate (T-gate), and the source contacts with the T-gate, which forms vertical conduction channel (VC). The 2-D hole gas (2-DHG) and 2-D electron gas (2-DEG) are formed at the GaN-top/AlGaN and AlGaN/GaN-buffer interface, respectively, forming the polarization-junction. First, the E-mode operation is realized because 2-DHG under the source prevents the electrons injecting from source to 2-DEG, breaking through the conventional E-mode method by depleting 2-DEG under the gate. Second, a uniform electric field (E-field) distribution is achieved due to the assisted depletion effect by polarization-junction. Third, the source reduces the E-field peak at the T-gate side and modulates the E-field distribution. The breakdown voltage (BV) of PVC-HEMT is 705 V and specific ON-resistance (RON,sp) is 1.18 mΩ cm2. Compared with conventional HEMT (C-HEMT), PVC-HEMT has a smaller size due to the special location of the source and T-gate. An analytic threshold voltage model is presented and the analytical results agree well with the simulated results.

Effect of GaN buffer polarization on electron distribution of AlGaN/GaN heterostructure

The formation of 2DEG in AlGaN/GaN heterostructure is discussed in detail. A misunderstanding about the 2DEG sheet density expression is clarified. It is predicted by theoretical analysis and validated by self-consistent Schrodinger–Poisson numerical simulation that under the force of GaN polarization, large amounts of electrons will accumulate at the GaN/substrate interface in AlGaN/GaN/substrate HEMT structure.

Direct observation of nanometer-scale gate leakage paths in AlGaN/GaN and InAlN/AlN/GaN HEMT structures

This paper investigates the effects of MOVPE growth parameters on the gate leakage characteristics of InAlN HEMT structures and compares leakage current paths in AlGaN and InAlN HEMT structures on a nanometer scale. The gate leakage characteristics of AlGaN and InAlN HEMT were compared, and the large leakage current in InAlN HEMT was found to result from the high-density 2DEG-induced strong electric field accumulation in the barrier layer. Both the increasing growth rate and decreasing growth temperature in the InAIN layer of InAlN HEMT caused the deterioration of gate leakage characteristics and surface morphologies. Conductive AFM measurements were performed for AlGaN and InAlN HEMT structures, and a one-to-one clear correlation between surface pits and leakage paths was found for AlGaN HEMT. On the contrary, for the InAlN HEMT grown at a low temperature, high-density localized leakage paths were observed. We believe the observed leakage paths originate from nanometer-scale material fluctuations, which is most likely an indium composition fluctuation. Our results indicate that it is necessary to grow the InAlN barrier layer at 740 °C or higher and that the growth rate must be 1.25 nm min−1 or less to realize low-gate leakage in InAlN HEMT. A further reduction of the gate leakage current requires the reduction of the dislocation density and use of gate insulators.

A new expression of Ns versus Ef to an accurate control charge model for AlGaAs/GaAs

Semi-conductor components become the privileged support of information and communication, particularly appreciation to the development of the internet. Today, MOS transistors on silicon dominate largely the semi-conductors market, however the diminution of transistors grid length is not enough to enhance the performances and respect Moore law. Particularly, for broadband telecommunications systems, where faster components are required. For this reason, alternative structures proposed like hetero structures IV-IV or III-V [1] have been.The most effective components in this area (High Electron Mobility Transistor: HEMT) on IIIV substrate. This work investigates an approach for contributing to the development of a numerical model based on physical and numerical modelling of the potential at heterostructure in AlGaAs/GaAs interface. We have developed calculation using projective methods allowed the Hamiltonian integration using Green functions in Schrodinger equation, for a rigorous resolution “self coherent” with Poisson equation. A simple analytical approach for charge-control in quantum well region of an AlGaAs/GaAs HEMT structure was presented. A charge-control equation, accounting for a variable average distance of the 2-DEG from the interface was introduced. Our approach which have aim to obtain ns-Vg characteristics is mainly based on: A new linear expression of Fermi-level variation with two-dimensional electron gas density in high electron mobility and also is mainly based on the notion of effective doping and a new expression of AEc

Deep Traps in AlGaN/GaN High Electron Mobility Transistors on SiC

Capacitance-voltage (C-V) and current-voltage (I-V) characteristics and deep trap spectra were measured for high-power HEMT structures grown by metallorganic chemical vapor deposition on semi-insulating 4H-SiC substrates and comprised of an AlGaN barrier and GaN(Fe) semi-insulating buffer with the upper (∼100 nm) portion unintentionally doped and lightly n-type. The C-V measurements were performed in the 85–400 K range in the dark and showed a decrease in the threshold voltage for temperatures higher than 200 K. The change was most prominent for temperatures from 290 K to ∼330 K and occured due to deep traps capturing electrons in the AlGaN barrier and near the AlGaN/GaN interface. C-V measurements at low temperature showed that the traps in the AlGaN barrier are located near Ec−2 eV and have a density of ∼1012 cm−2, while the traps at the interface have an optical ionization threshold near 1.7 eV and the density of ∼4 × 1012 cm−2. Capacitance deep level transient spectroscopy (DLTS) and current DLTS (CDLTS) revealed electron traps with levels near Ec−0.56 eV and hole traps with levels near Ev+0.9 eV and Ev+0.5 eV. The Ec−0.56 eV traps are known to be located near the AlGaN/GaN interface in the GaN buffer. The Ev+0.9 eV hole traps are located in the buffer.

Si3N4 layers for the in-situ passivation of GaN-based HEMT structures

A method for the in situ passivation of GaN-based structures with silicon nitride in the growth chamber of a metal organic vapor phase epitaxy (MOVPE) reactor is described. The structural and electrical properties of the obtained layers are investigated. The in situ and ex situ passivation of transistor structures with silicon nitride in an electron-beam-evaporation device are compared. It is shown that ex situ passivation changes neither the initial carrier concentration nor the mobility. In situ passivation makes it possible to protect the structure surface against uncontrollable degradation upon the finishing of growth and extraction to atmosphere. In the in situ passivated structure, the carrier concentration increases and the mobility decreases. This effect should be taken into account when manufacturing passivated GaN-based transistor structures.

A Novel Backside Gate Structure to Improve Device Performance

AlGaN/GaN HEMTs have gained more attention recently because of the superior mobility (~1400 cm2/V-s) and high breakdown voltage. However, the reported breakdown voltage (~2000V at Lds=40µm) was still below GaN limit. The reason might be due to surface traps, defects in GaN buffer layer, etc. Methods to improve breakdown voltage include acceptor doping in buffer layer, insulator passivation or field plate over dielectric passivation. In this paper, we propose a new structure, which opened a via through the substrate as a backside gate and a field plate, and demonstrate the improvement of device performance such as transport characteristics and breakdown voltage. The HEMT structures consisted of a 25 nm AlGaN barrier layer, a 0.8 µm C doped GaN buffer layer, a 1.4µm AlGaN transition layer and a thin AlN nucleation layer on 300µm Si substrate. HEMT fabrication began with Ti/Al/Ni/Au Ohmic metallization and rapid thermal annealing in N2 at 825ºC. Isolation was accomplished by use of multiple energy N+ implantations. After implantation, the devices were passivated with 70-nm-thick SiNx by PECVD at 300ºC. Gate definition was achieved by patterning the gate and selectively removing the SiNx layer. The contact windows to the Ohmic contact pads were also opened simultaneously as defining the gate. After SiNx etching, wider gate patterns and Ohmic contact pads were opened. Ni/Au-based gate metallization was deposited on the gate and Ohmic contact pads simultaneously. The devices were then passivated with another 400-nm SiNx layer at 300ºC. The contact windows were opened by dry etching. The LDS was 4 µm. The LG was 0.45 and WG was 100 µm, respectively. There was an additional metal deposition for source field plate (FP). It was connected to the source and extended by 1 µm out over the gate to the gate-to-drain region. Above process flows are our standard process flow for reference. For our new structure, to drill via holes on the backside of the devices, the wafers were first polished to 150µm. Photoresist AZ9260 were used to pattern the via holes on the backside of the samples and aligned to the front side by Aligner MJB3. The high aspect ratio of the via holes were fabricated by BOSCH process. After forming via holes, Ti/Au was sputtered to form the backside gate (BG). Figure 1 showed effect of backside voltage (VBG) to drain I-V characteristics. As shown in Figure 1, the drain saturation current decreased and the channel resistance became higher after applying VBG at -10 V. Also, the drain current at pinch off region get decreased. This is due to the introduction of another depletion region from the backside of the device. In other words, the BG provided an additional control of the device, which made the device more robust. Figure 2 showed the transfer characteristics of device with and without applying VBG. As shown in Figure 2, both of the on current and off current became lower. Besides, the sub-threshold slope became steeper after applying -10 V at the BG, which meant better modulation of the device. Due to the better pinch off of the device, the breakdown voltage (VBD) was significantly improved. Table 1 showed the summary of the effect of VBG. As shown in TABLE 1, although IDS was slightly decreased, but the more reduction of gate current improved on/off ratio for one order, sub-threshold swing from 204 to 137 mV/dec, and VBD for 40%. These improvements were all achieved because of the backside gate. Besides applying VBG to improve VBD, the BG could be used as a FP, too. Studies have shown that FP can redistribute the electric field, and thus increase the VBD. In this study, we demonstrate the similar result. When connecting the BG to front-side gate, the VBD could be increased around 8 % as shown in Figure 3. When connecting the BG to front-side source, VBD didn’t have noticeable improvement. The reason that source FP didn’t have obvious effect in our study might be the existence of front-side FP. On the other hand, drain FP actually decrease the VBD. For drain FP, the backside FP was at positive bias. This positive bias along with the front-side positive bias would generate hot electrons and enhance the tunneling. In summary, VBD could be improved by simply connecting the backside gate to front-side gate. In conclusion, we demonstrate a new structure for AlGaN/GaN HEMT. This structure could improve the on/off ratio, sub-threshold swing, and breakdown voltage. It is worth to see if it could improve short channel effect, self-heating effect, or current collapse. Figure 1

Optical band-gap shift in (InAs)GaAs/AlGaAs HEMTs structures studied by photoluminescence spectroscopy

The optical band-gap has been investigated in detail in (InAs)GaAs/AlGaAs HEMTs structures using a photoluminescence spectroscopy. A power dependent PL study allows highlighting the different causes of the shift band-gap. We can explain the shift by more effects: quantum confined Stark effect (QCSE), band-gap renormalisation (BGR) and Burstein–Moss (BM) effects. We are interested in these effects based on the carriers’ behaviour inside GaAs and InAs/GaAs channels. These effects were affected by carrier concentration depending on Si-δ-doping density and AlGaAs spacer layer thickness as well as the strain relaxation effect. The photoluminescence characterisations have been studied in the 1–10Wcm−2 power range.