Characteristics Of High Electron Mobility Transistors Research Articles

(Invited) Design and Reliability of GaN Power HEMT Technology

Gallium nitride (GaN) based high electron mobility transistors (HEMTs) are extensively being considered for power and RF applications by virtue of their unique properties. AlGaN/GaN HEMTs are depletion mode devices due to presence of high density 2DEG at the interface. However, enhancement mode devices prove beneficial for simpler and reliable power systems. F implant, p-GaN gate and AlGaN recess are some of the widely accepted processes to achieve the same. In spite of extensive research already being carried out in the domain, the search for a reliable technique for e-mode GaN with superior performance metric is still continued. A novel technique incorporating a p-type metal oxide in conjunction with partial recess of AlGaN has been established to demonstrate a high performing e-mode GaN/AlGaN HEMT. The p-type metal oxide helped in positive shift of the VTH which allowed the device to be e-mode even with partial recessing. This helped circumventing the challenges introduced by plasma damage in recess-based e-mode GaN HEMTs. Besides HEMTs, GaN based Schottky diodes have started gaining prominence for power applications. Schottky diodes with high ON current (1A/mm), 0.5 V cut in voltage and reduced reverse leakage were demonstrated by carrying out surface treatment post AlGaN recess. The high temperature anneals, and acid treatments helped reduce the surface damage induced due to plasma. Technology CAD (TCAD) based design approach goes a long way in designing improved and reliable devices. Hence a consistent and reliable TCAD strategy that can be employed for HEMT analysis and design of high performance HEMTs is the need of the hour. The presence of unique polarization effects combined with traps, hot electrons and self-heating make reproducing accurate device behavior for HEMTs a challenging task. A well-defined Technology CAD strategy is developed for the first time for GaN HEMT, incorporating all the physical aspects pertaining to HEMT. The simulation setup was well calibrated with the experimental data. TCAD based Co-Design guidelines were proposed, which can be effectively utilized to optimize the design parameters for maximum RF and breakdown performance. Further novel designs- RESURF, Superjunction and Stepgate were proposed having high breakdown voltages. Comprehensive study was done to study the physical effect of surface traps and buffer traps on the breakdown performance. GaN based HEMTs despite of its attractive performance/cost ratio, suffers from poor reliability which limits its penetration in the ever-growing power device market. Therefore, reliability of AlGaN/GaN HEMT is now a topic of intense research. The present understanding of underlying degradation physics needs to be improved further. Many aspects like ability of GaN HEMT to handle high power under extreme conditions and related SOA concerns are yet to be investigated. Characterization of HEMT can be done under dc and pulse stress. In pulse characterization, one can emulate real stress conditions present in power electronic circuits. On the fly dc I-V and C-V gives information about the device degradation whereas sub-bandgap UV is used to capture trapping effects. Besides, on the fly Raman captures evolution of piezoelectric/ thermal stress in HEMT. It has been observed that carrier trapping has significant impact on SOA of GaN HEMTs. High trap density causes electric field shift and peaking towards drain edge. The field enhancement at drain accelerates impact ionization and carrier injection into buffer leading to early filament formation. Non-uniform trapping along width triggers avalanche instability. The SOA boundary is found to deteriorate in OFF-state, due to compressive stress at drain-gate edge. ON-state SOA is also limited by tensile stress in the gate-to-drain region. The electric field profile in channel and buffer can be tuned by controlling the recess depth, which directly affects device degradation and SOA. Figure 1

Degradation in AlGaN/GaN HEMTs irradiated with swift heavy ions: Role of latent tracks

Abstract AlGaN/GaN high electron mobility transistor (HEMT) devices were irradiated with swift heavy ions at different fluences. From structural and electrical studies, it was found that SHI irradiation leads to a significant deterioration of structural and electrical properties of the devices. Positive threshold voltage Vth was found to increase by about 85% as a result of irradiation with 1540-MeV 209Bi ions at fluence of 1.7 × 1011 ions/cm2, while this threshold voltage value was increased by 55% after irradiation with 2300-MeV 129Xe at a fluence of 4 × 1011 ions/cm2. The maximum saturation drain current Ids was decreased by about two orders of magnitude in the device after irradiation with 209Bi ions. Quasi-continuous tracks were observed visually in the devices after irradiation with 209Bi ions. The observed defects and disorders induced in the devices by SHI irradiation were found responsible for the decrease in carrier mobility and sheet carrier density, and finally, these defects resulted in the degradation of electrical characteristics of HEMTs.

AlGaN/GaN heterostructures with an AlGaN layer grown directly on reactive-ion-etched GaN showing a high electron mobility (>1300 cm2 V−1 s−1)

In this study, the metal–organic-vapor-phase-epitaxial growth behavior and electrical properties of AlGaN/GaN structures prepared by the growth of an AlGaN layer on a reactive-ion-etched (RIE) GaN surface without regrown GaN layers were investigated. The annealing of RIE-GaN surfaces in NH3 + H2 atmosphere, employed immediately before AlGaN growth, was a key process in obtaining a clean GaN surface for AlGaN growth, that is, in obtaining an electron mobility as high as 1350 cm2 V−1 s−1 in a fabricated AlGaN/RIE-GaN structure. High-electron-mobility transistors (HEMTs) were successfully fabricated with AlGaN/RIE-GaN wafers. With decreasing density of dotlike defects observed on the surfaces of AlGaN/RIE-GaN wafers, both two-dimensional electron gas properties of AlGaN/RIE-GaN structures and DC characteristics of HEMTs were markedly improved. Since dotlike defect density was markedly dependent on RIE lot, rather than on growth lot, surface contaminations of GaN during RIE were believed to be responsible for the formation of dotlike defects and, therefore, for the inferior electrical properties.

Effects of proton irradiation at different incident angles on InAlAs/InGaAs InP-based HEMTs**Project supported by the National Natural Science Foundation of China (Grant Nos. 11775191, 61404115, 61434006, and 11475256), the Program for Innovative Research Team (in Science and Technology) in University of Henan Province, China (Grant No. 18IRTSTHN016), and the Development Fund for Outstanding Young Teachers in Zhengzhou University of China

InP-based high electron mobility transistors (HEMTs) will be affected by protons from different directions in space radiation applications. The proton irradiation effects on InAlAs/InGaAs hetero-junction structures of InP-based HEMTs are studied at incident angles ranging from 0 to 89.9° by SRIM software. With the increase of proton incident angle, the change trend of induced vacancy defects in the InAlAs/InGaAs hetero-junction region is consistent with the vacancy energy loss trend of incident protons. Namely, they both have shown an initial increase, followed by a decrease after incident angle has reached 30°. Besides, the average range and ultimate stopping positions of incident protons shift gradually from buffer layer to hetero-junction region, and then go up to gate metal. Finally, the electrical characteristics of InP-based HEMTs are investigated after proton irradiation at different incident angles by Sentaurus-TCAD. The induced vacancy defects are considered self-consistently through solving Poisson’s and current continuity equations. Consequently, the extrinsic transconductance, pinch-off voltage and channel current demonstrate the most serious degradation at the incident angle of 30°, which can be accounted for the most severe carrier sheet density reduction under this condition.

Effect of self-heating on electrical characteristics of AlGaN/ GaN HEMT on Si (111) substrate

In order to study the effect of self-heating of AlGaN/ GaN high electron mobility transistors (HEMTs) characteristics fabricated on Si(111) substrate, simulations of 2DEG temperature on different drain voltages have been carried out by Sentaurus TCAD simulator tool. Prior to the electrical direct-current (DC) characteristics studies, structural properties of the HEMT structures were examined by scanning transmission electron microscopy. The comparative analysis of simulation and experimental data provided sheet carrier concentration, mobility, surface traps, electron density at 2DEG by considering factors such as high field saturation, tunneling and recombination models. Mobility, surface trap concentration and contact resistance were obtained by TCAD simulation and found out to be ∼1270cm2/Vs, ∼2×1013 cm-2 and ∼0.2 Ω.mm, respectively, which are in agreement with the experimental results. Consequently, simulated current-voltage characteristics of HEMTs are in good agreement with experimental results. The present simulator tool can be used to design new device structures for III-nitride technology.

Characterization of high electron mobility transistor fabricated on hydride vapor phase epitaxy‐grown GaN templates containing various Fe concentrations

An AlGaN/GaN high electron mobility transistor (HEMT) structure was grown on a semi‐insulating (SI) GaN layer or substrate to suppress a leakage current and achieve a high breakdown voltage. Fe can be used as doping material to obtain SI‐GaN. However, the relationship between the Fe concentration in the GaN layers and its influence on HEMT characteristics has not yet been investigated. In this study, we therefore investigated the influence of Fe concentration in the GaN layers on HEMT characteristics. The GaN layers containing several Fe concentrations were grown by hydride vapor phase epitaxy (HVPE). The Fe concentration was varied from 3 × 1017 to 3 × 1020 atoms cm−3. It was determined that the source‐drain current (IDS) decreased as the Fe concentration increased. In the case of a low Fe concentration of 3 × 1017 atoms cm−3, IDS was almost the same as that of HEMT on the non‐Fe‐doped GaN template. Thus, HEMT characteristics deteriorated as the Fe concentration increased. The channel layer played a role in blocking the adverse effects of Fe doping on the two‐dimensional electron gas concentration in AlGaN/GaN. Moreover, it was shown that the GaN channel layer must become thicker as the Fe concentration of the GaN layers increases.

A Case Study on Common Mode Electromagnetic Interference Characteristics of GaN HEMT and Si MOSFET Power Converters for EV/HEVs

Wide bandgap semiconductors, such as gallium nitride (GaN)-based power devices have become increasingly popular in the automotive industry due to their low on-state resistance and fast switching capabilities. These devices are sought to replace silicon (Si) devices in power electronics converters for vehicular applications. GaN devices dissipate less energy during each switching event, thus, GaN converter designers can significantly increase the switching frequency without increasing switching losses, relative to Si converters. However, one item of concern is that the high $dv/dt$ of GaN devices, due to the increased switching speed, has the potential to deteriorate the electromagnetic interference (EMI) emission of power converters, and thus may fail the corresponding regulations for vehicles. To understand these effects, this paper presents an experimental case study on the switching characteristics and common mode (CM) noise generation of a GaN-based half-bridge configuration operating in the synchronous boost mode. A comprehensive comparison has been made between the chosen GaN high electron mobility transistors (HEMTs) and Si MOSFET in terms of switching voltage waveforms, ground leakage currents, and CM noise spectrum. By parametrically increasing the gate resistance of GaN HEMT and Si MOSFET, the tradeoff between converter efficiency and CM noise generation is also quantified.

Theoretical study of the effect of temperature differential and ionizing radiation on the current–voltage characteristics of HEM transistors

Requirements to the modeling of the effects of temperature differential and ionizing radiation on the current–voltage characteristics of high electron mobility transistors (HEMTs) were formulated. The results of modeling of the effects of temperature differential on the current–voltage characteristics of HEMTs were described. The results of analysis of the effects of ionizing radiation on the current–voltage characteristics of HEMTs were given. The results of modeling of the effects of ionizing radiation on the current–voltage characteristics of HEMTs were presented.

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.

Strain- and temperature-induced effects in AlGaN/GaN high electron mobility transistors

This paper presents a physics-based model for computing the combined effect of applied strain and temperature on the device characteristics of aluminium gallium nitride (AlGaN/GaN) high electron mobility transistors (HEMTs). More specifically, the electrical response of the HEMT is predicted under applied biaxial strain from ±1% over a wide range of temperatures (300–500 K). In addition, the interface state densities at the Schottky-AlGaN interface are introduced in the model. This physics-based model calculates the charge due to applied, thermal and lattice mismatch strain and temperature effects at the two-dimensional electron gas (2DEG) interface of the HEMT. Coupled with a model for the 2DEG mobility that includes strain and temperature effects, current–voltage characteristics for the HEMT are derived above the threshold voltage. Regimes with large strain sensitivity and temperature compensation are identified and vice-versa. The analysis from the model clarifies the large range of strain response variations observed in the experimentally measured characteristics of HEMTs in literature. Furthermore, the developed model is a useful tool for predicting the response of HEMTs used in sensing and under the influence of packaging in extreme environments, especially when temperature fluctuation and strain coupling is of concern.

Strain-effect transistors: Theoretical study on the effects of external strain on III-nitride high-electron-mobility transistors on flexible substrates

This paper presents strain-effect transistors (SETs) based on flexible III-nitride high-electron-mobility transistors (HEMTs) through theoretical calculations. We show that the electronic band structures of InAlGaN/GaN thin-film heterostructures on flexible substrates can be modified by external bending with a high degree of freedom using polarization properties of the polar semiconductor materials. Transfer characteristics of the HEMT devices, including threshold voltage and transconductance, are controlled by varied external strain. Equilibrium 2-dimensional electron gas (2DEG) is enhanced with applied tensile strain by bending the flexible structure with the concave-side down (bend-down condition). 2DEG density is reduced and eventually depleted with increasing compressive strain in bend-up conditions. The operation mode of different HEMT structures changes from depletion- to enchantment-mode or vice versa depending on the type and magnitude of external strain. The results suggest that the operation modes and transfer characteristics of HEMTs can be engineered with an optimum external bending strain applied in the device structure, which is expected to be beneficial for both radio frequency and switching applications. In addition, we show that drain currents of transistors based on flexible InAlGaN/GaN can be modulated only by external strain without applying electric field in the gate. The channel conductivity modulation that is obtained by only external strain proposes an extended functional device, gate-free SETs, which can be used in electro-mechanical applications.

PHEMT as a circuit element for high impedance nanopower amplifiers for ultra-low temperatures application

In this work, high electron mobility transistor (HEMT) was studied as a circuit element for amplifiers operating at temperatures of the order of 10–100mK. To characterize the HEMT, the relative parameters are proposed to be used. HEMT characteristics were measured at a temperature of 50mK for the first time. It follows from the reported studies that the power consumption of high-impedance HEMT-based amplifiers can be reduced down to hundreds of nanowatt or even lower.

Low Current Collapse and Low Leakage GaN MIS-HEMT Using AlN/SiN as Gate Dielectric and Passivation Layer

Gallium nitride-based high-electron-mobility transistors (HEMTs) have demonstrated outstanding performance for high-power and high-frequency applications for defense and communication systems. However, there are many undesirable effects such as current collapse and increase in dynamic ON-resistance due to the surface donor states and the high polarization nature of the material [1], [2]. Nitride-based materials are more desirable for GaN passivation because the oxide-based materials have many oxygen contaminations on GaN. SiN has been proved as an effective passivation dielectric to reduce the surface states and can efficiently suppress current collapse in the GaN HEMTs [3]. However, the bandgap of SiN (∼5 eV) is not high enough to suppress leakage current. AlN has a large bandgap (∼6.2 eV) can effectively reduce leakage current as passivation layer [4]. In this work, we demonstrate GaN MIS-HEMT using AlN/SiN bilayer gate dielectric and passivation layer which combine the advantages of SiN and AlN. For comparing the performance of GaN MIS-HEMT with AlN/SiN bilayer gate dielectric, we also prepared reference devices with single layers dielectric. The wafer was divided into three samples after mesa and ohmic contact process. The dielectric and passivation layer were prepared differently for each sample: sample A with 5-nm SiN, sample B with 1-nm AlN/5-nm SiN, sample C with 5-nm AlN. Fig. 1 shows the I D-V D characteristics, where the drain current curves were measured at V G = 0 V. The SiN first passivated devices have higher I D and lower R ON as compared to AlN first passivated device. The R ON were 0.3 mΩ‧cm2, 0.3 mΩ‧cm2, and 0.33 mΩ‧cm2 for sample A, sample B, and sample C, respectively. For gate leakage shown in Fig. 2, GaN MIS-HEMTs using AlN as gate dielectric have lower gate leakage current. It means AlN can effectively suppress leakage current. Fig. 3 shows the percent degradation of R ON,DYN versus time. The OFF-state quiescent stress bias were set at V DSQ = 100 V and V GSQ = -10 V. The ON-state R ON,DYN is extracted at a gate voltage of 0 V and a drain voltage of 2 V. R ON,DYN degradation reflects the passivation quality including surface, interface, and barrier traps. GaN MIS-HEMT with AlN/SiN obviously suppresses R ON,DYN degradation compared to the single layer samples. Also, the SiN first passivated devices have slight R ON,DYN degradation. It indicates SiN has low interface trapping density on GaN surface and AlN can reduce trapping electrons charged through gate leakage. In conclusion, SiN was been proved that it has many good effects for GaN passivation such as decreasing of channel resistance, low surface state, low interface trapping density and low current collapse effect. AlN with high bandgap nature can suppress the leakage current. However, it would increase channel resistance, and cause severe current collapse effect. In this study, an effective AlN/SiN bilayer dielectric and passivation layer have been demonstrated for reducing current collapse effect and leakage current in GaN MIS-HEMT.Fig. 1 I D–V D characteristics of GaN HEMTs with different surface passivation layers.Fig. 2 Gate leakage current comparison in MIS-HEMTs structure.Fig. 3 Percent degradation of R ON,DYN versus time.[1] S. Binari, K. Ikossi, J. A. Rousos, et al., “Trapping effects and microwave power performance in AlGaN/GaN HEMTs,” IEEE Trans. Electron Devices, vol. 48, no. 3, pp. 465–471, Mar. 2001.[2] D. Jin and J. A. Alamo, “Methodology for the Study of Dynamic ON-Resistance in High-Voltage GaN Field-Effect Transistors,” IEEE Trans. Electron Devices, vol. 60, no. 10, pp. 3190–3196, Oct. 2013.[3] B. Green and K. Chu, “The effect of surface passivation on the microwave characteristics of undoped AlGaN/GaN HEMTs,” IEEE Device Lett., vol. 21, no. 6, pp. 268–270, Jun. 2000.[4] Z. Tang, S. Huang, Q. Jiang, S. Liu, C. Liu, and K. J. Chen, “High-Voltage (600-V) Low-Leakage Low-Current-Collapse AlGaN/GaN HEMTs With AlN/SiNx Passivation,” IEEE Device Lett., vol. 34, no. 3, pp. 366–368, Mar. 2013.

High-performance AlGaN/GaN High-electron-mobility transistors employing H2O annealing

We have proposed and fabricated high performance AlGaN/GaN high-electron-mobility transistors (HEMTs), of which the breakdown voltage is considerably high and the leakage current is efficiently suppressed employing H2O annealing. We also investigated the effects of H2O annealing on the formation of GaOx film on the surface of AlGaN/GaN HEMT as well as reverse blocking characteristics including the breakdown voltage and the leakage current. We compared the reverse blocking characteristics of the AlGaN/GaN HEMTs fabricated by the proposed H2O annealing treatment with those of the HEMT by widely used O2 annealing method. Also, we investigated the reverse blocking characteristics of HEMT fabricated without any annealing treatment. We used the smaller molecular size, higher permeation probability, and the lower activation energy of H2O than O2. The breakdown voltage of AlGaN/GaN HEMT with the LGD of 20μm employing H2O and O2 annealing was 1674 and 1512V, respectively. The drain leakage current of these devices using H2O and O2 annealing was 13.1 and 60.1nA/mm under VDS of 100V and VGS of −10V, respectively. A conventional device without any annealing process exhibited the breakdown voltage of 598V and the leakage current of 666.0μA/mm. In order to investigate the amounts of deep traps in the interface of AlGaN/GaN HEMT, we measured the pulsed I–V. A low drain current of the device under pulsed bias indicated that the GaOx, which was formed by H2O annealing, induced the deeper level and longer emission time of traps compared to the shallow ones. These may be related to the surface leakage current in the AlGaN/GaN HEMTs.

Validation of a triangular quantum well model for GaN-based HEMTs used in pH and dipole moment sensing

Gallium nitride (GaN) is a relatively new semiconductor material that has the potential of replacing gallium arsenide (GaAs) in some of the more recent technological applications, for example chemical sensor applications. In this paper, we introduce a triangular quantum well model for an undoped AlGaN/GaN high electron mobility transistor (HEMT) structure used as a chemical and biological sensor for pH and dipole moment measurements of polar liquids. We have performed theoretical calculations related to the HEMT characteristics and we have compared them with experimental measurements carried out in many previous papers. These calculations include the current–voltage (I–V) characteristics of the device, the surface potential, the change in the drain current with the dipole moment and the drain current as a function of pH. The results exhibit good agreement with experimental measurements for different polar liquids and electrolyte solutions. It is also found that the drain current of the device exhibits a large linear variation with the dipole moment, and that the surface potential and the drain current depend strongly on the pH. Therefore, it can distinguish molecules with slightly different dipole moments and solutions with small variations in pH. The ability of the device to sense biomolecules (such as proteins) with very large dipole moments is investigated.

Impact of the lateral width of the gate recess on the DC and RF characteristics of InAlAs/InGaAs HEMTs

We fabricated 88 nm gate-length InP-based InAlAs/InGaAs high electron mobility transistors (HEMTs) with a current gain cutoff frequency of 100 GHz and a maximum oscillation frequency of 185 GHz. The characteristics of HEMTs with side-etched region lengths (Lside) of 300, 412 and 1070 nm were analyzed. With the increase in Lside, the kink effect became notable in the DC characteristics, which resulted from the surface state and the effect of impact ionization. The kink effect was qualitatively explained through energy band diagrams, and then eased off by reducing the Lside. Meanwhile, the Lside dependence of the radio frequency characteristics, which were influenced by the parasitic capacitance, as well as the parasitic resistance of the source and drain, was studied. This work will be of great importance in fabricating high-performance InP HEMTs.

Impact of $\hbox{N}_{2}$ Plasma Power Discharge on AlGaN/GaN HEMT Performance

The effects of power and time conditions of in situ <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{N}_{2}$</tex></formula> plasma treatment, prior to silicon nitride (SiN) passivation, were investigated on an AlGaN/GaN high-electron mobility transistor (HEMT). These studies reveal that <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{N}_{2}$</tex></formula> plasma power is a critical parameter to control the SiN/AlGaN interface quality, which directly affects the 2-D electron gas density. Significant enhancement in the HEMT characteristics was observed by using a low power <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{N}_{2}$</tex> </formula> plasma pretreatment. In contrast, a marked gradual reduction in the maximum drain–source current density <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(I_{DS \max})$</tex></formula> and maximum transconductance <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$(g_{m \max})$</tex></formula> , as well as in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $f_{T}$</tex></formula> and <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$f_{\max}$</tex></formula> , was observed as the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{N}_{2}$</tex></formula> plasma power increases (up to 40% decrease for 210 W). Different mechanisms were proposed to be dominant as a function of the discharge power range. A good correlation was observed between the device electrical characteristics and the surface assessment by atomic force microscopy and Kelvin force microscopy techniques.

Improvement of Breakdown Characteristics of a GaN HEMT with AlGaN Buffer Layer

High-electron-mobility transistors (HEMTs) with a highly resistive two-layer buffer layer (AlGaN/GaN) were grown on 6H-SiC substrates by metalorganic chemical vapor deposition. The characteristics were compared with those of conventional HEMTs utilizing GaN as the high-resistivity buffer. The results of x-ray diffraction and atomic force microscopy indicate that the crystal quality of the HEMT heterostructure is not deteriorated by the AlGaN buffer layer. The direct-current (DC) characteristics of the HEMTs with the two different structures are similar, while the off-state breakdown voltage is enhanced and the mobility of the two-dimensional electron gas is improved by the AlGaN buffer layer. The reasons for the effects of the AlGaN buffer layer are discussed systematically.

Influence of deep-pits on the device characteristics of metal-organic chemical vapor deposition grown AlGaN/GaN high-electron mobility transistors on silicon substrate

We have investigated the regions around deep-pits and their limitations on the performance of metal-organic chemical vapor deposition grown AlGaN/GaN high-electron-mobility transistors (HEMTs) on Si. The device characteristics such as maximum drain current density (IDSmax), threshold voltage (Vth), and three terminal-OFF breakdown (BVOFF) of a HEMT were affected based on the distance of deep-pits from a device active region. The deterioration of HEMT characteristics are significant if the active region is present within 50 μm radius of a deep-pit and becomes insignificant for distance beyond 50 μm. These device characteristics were compared with optical measurements and it was found the region around 50 μm radius of a pit have large dislocations and defects as confirmed by cathodoluminescence. Further, the E2 Raman shift measurements reveal additional stress in the 50 μm region around a pit.

Open-Gate Liquid-Phase Sensor Fabricated on Undoped-AlGaN/GaN HEMT Structure

Sensing responses of an open-gate liquid-phase sensor fabricated on undoped-AlGaN/GaN high-electron-mobility-transistor (HEMT) structure are investigated in aqueous solution. In air-exposed ambient, the open-gate undoped AlGaN/GaN HEMT shows the only presence of linear region of currents while Si-doped AlGaN/GaN shows linear and saturation regions of currents, very similar to those of gated devices. This seems to show that very low Fermi level pinning by surface states exists in undoped AlGaN/GaN sample compared to Si-doped sample. In aqueous solution, the typical current-voltage (I-V) characteristics of HEMTs with reasonably good gate controllability are observed. The potential of the AlGaN surface at the open-gate area is effectively controlled via aqueous solution by Ag/AgCl gate electrode. The open-gate undoped AlGaN/GaN HEMT structure is capable of distinguishing pH level in aqueous electrolytes and exhibits linear sensitivity, where high sensitivity of 1.9 mA/pH or 3.88 mA/mm/pH at drain-source voltage, VDS = 5 V is obtained. Due to large leakage current where it increases with the negative gate voltage, the Nernstian’s like sensitivity cannot be determined as what commonly reported in literatures. This large leakage current may be caused by the technical factors rather than the characteristics of the devices themselves. Suprisingly, although there is imperfection in the device preparation, the fabricated devices work very well in distinguishing the pH levels. Suppression of current leakage is likely to improve the device performance. The fabricated open-gate undoped-AlGaN/GaN structure is expected to be suitable for pH sensing application.