Heterostructure Field Effect Transistors Devices Research Articles

Ultra-thin AlGaN/GaN HFET with a high breakdown voltage on sapphire substrates

In this Letter, an ultra-thin AlGaN/GaN heterostructure field effect transistor (HFET) with a total thickness of ∼200 nm was fabricated on sapphire substrates by combing physical vapor deposition and metal organic chemical vapor deposition growth methods. Thanks to the absence of a conventional semi-insulating thick GaN buffer by taking advantage of an ex situ sputtered AlN nucleation layer, we achieved a profound soft breakdown voltage of 1700 V accompanied by a 12.5 Ω·mm on-resistance and a low off-state leakage of 0.1 μA/mm in such ultra-thin HFET devices. Our work demonstrates an alternative strategy to fabricate GaN based power devices with high breakdown voltage and low cost.

On a transistor-type hydrogen gas sensor prepared by an electrophoretic deposition (EPD) approach

A Pd/GaN/AlGaN heterostructure field-effect transistor (HFET)-type hydrogen gas sensor, based on an electrophoretic deposition (EPD) approach, is fabricated and studied. Due to the formation of good Schottky gate contact by an EPD approach, the studied HFET shows improved DC performance including the suppressed gate current and better thermal stabilities on current–voltage (I–V) characteristics. This is mainly attributed to the reduction of interface trap density and improved Pd morphology. The EPD-based Pd morphologies are examined by X-ray diffraction, energy dispersive spectroscopy, Auger electron spectroscopy, scanning electron microscopy, and atomic force microscopy. For the used gate-dimension of 1 μm × 100 μm, an EPD-based HFET shows low gate current of 2.9 nA, maximum drain saturation current of 490 mA/mm, and maximum extrinsic transconductance of 78.9 mS/mm at room temperature. Also, solid thermal stabilities on maximum drain saturation current (−0.46 mA/mm K) and maximum extrinsic transconductance (−0.08 mS/mm K) are found as the temperature is increased from 300 to 600 K. For hydrogen gas sensing application, at 370 K, the maximum hydrogen sensitivity of 600.1 μA/mm ppm H2/air under a 5 ppm H2/air ambiance and fast response time (30 s) and recovery time (47 s) under a 10,000 ppm H2/air ambiance are obtained. The EPD approach also demonstrates advantages of low cost, simple apparatus, easy process, little restriction on the shaped substrate, composited deposition, and adjustable alloy grain size. Therefore, the proposed EPD approach gives the promise for fabricating high-performance HFET devices and hydrogen gas sensors.

Enhanced effect of side-Ohmic contact processing on the 2DEG electron density and electron mobility of In0.17Al0.83N/AlN/GaN heterostructure field-effect transistors

From the capacitance–voltage curves and current–voltage characteristics of the In0.17Al0.83N/AlN/GaN heterostructure field-effect transistors (HFETs) with side-Ohmic contacts and normal-Ohmic contacts, two-dimensional electron gas (2DEG) electron mobility was calculated. It is found that the polarization Coulomb field scattering (PCF) is closely related to the normal-Ohmic contact processing, and the PCF was weakened by side-Ohmic contact processing in In0.17Al0.83N/AlN/GaN HFETs, similar to that in AlGaN/AlN/GaN HFET devices. Further, due to the stronger spontaneous polarization in the thinner In0.17Al0.83N barrier layer, the influence of the gate bias on the PCF in In0.17Al0.83N/AlN/GaN HFETs is greater than that in AlGaN/AlN/GaN HFETs. As a result, the PCF in In0.17Al0.83N/AlN/GaN HFETs with side-Ohmic contacts is stronger than that in AlGaN/AlN/GaN HFETs with side-Ohmic contacts. Moreover, the 2DEG electron density in the In0.17Al0.83N/AlN/GaN HFETs with side-Ohmic contacts is increased by more than twice compared with the 2DEG electron density in the In0.17Al0.83N/AlN/GaN HFETs with normal-Ohmic contacts.

Comprehensive study of hydrogen sensing phenomena of an electroless plating (EP)-based Pd/AlGaN/GaN heterostructure field-effect transistor (HFET)

A Pd/AlGaN/GaN heterostructure field-effect transistor (HFET) type hydrogen sensor, with sensitization, activation, and electroless plating (EP) deposition approaches, is fabricated and investigated. Based on sensitization and activation processes, the dense and uniform Pd layer exhibits small grain size (38.14±7.24nm) and less surface roughness in gate region. Experimentally, for a 1μm gate-length HFET device, advantages of lower temperature-dependent variation of the maximum drain saturation current (−0.52mA/mmK), maximum extrinsic transconductance (−0.13mS/mmK), and threshold voltage (−1.05mV/K) are obtained over the temperature range from 300 to 600K. For three-terminal-controlled hydrogen gas sensing phenomena, a high current variation of 34.9mA/mm is found in a 1% H2/air gas at 375K. The exothermic hydrogen adsorption reaction with the enthalpy and entropy of −45.3kJmol−1 and −118.2Jmol−1K−1, respectively, are obtained. Furthermore, the response analysis reveals that the initial response rate is significantly increased with the hydrogen concentration and temperature. The related activation energy for hydrogen adsorption is about 11.65kJmol−1.

A Wireless-Transfer-Based Hydrogen Gas Sensing System With a Pd/AlGaN/GaN Heterostructure Field-Effect Transistor (HFET)

In this paper, a wireless hydrogen sensing system using a Pd/AlGaN/GaN heterostructure field-effect transistor (HFET) is developed and demonstrated. This HFET device shows good hydrogen sensing performance, including higher drain current variation (0.17 mA @ 1 ppm H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /air gas), fast response speed, and superior repeatability behaviors. For the sensing application, a readout circuit is designed to display the value of hydrogen concentration. Then, these data are fed into single chip microprocessor for later processing and wireless transmission. In addition, a grey polynomial differential model algorithm is employed to significantly reduce the redundant data for alleviate the wireless transmission load. The wireless monitoring of hydrogen concentration in a remote site longer than 50 m in distance is successfully demonstrated. From the experimental results, the proposed wireless hydrogen sensing system shows the advantages of high hydrogen sensing performance, easy operation, low cost, high portability, and effective wireless transmission.

Polarization Coulomb field scattering in In0.18Al0.82N/AlN/GaN heterostructure field-effect transistors

Using the measured capacitance-voltage curves of Ni Schottky contacts with different areas and the current-voltage characteristics for the rectangular and circular In0.18Al0.82N/AlN/GaN heterostructure field-effect transistors (HFETs) at low drain-source voltage, we found that the Ohmic contact processing and the gate bias cause the irregular distribution of the polarization charges at the In0.18Al0.82N/AlN interface which generates the polarization Coulomb field, and the polarization Coulomb field scattering has an important influence on the two-dimensional electron gas electron mobility in both our rectangular and circular In0.18Al0.82N/AlN/GaN HFET devices as same as in AlGaN/AlN/GaN HFET devices.

Current collapse transient behavior and its mechanism in submicron-gate AlGaN∕GaN heterostructure transistors

The current collapse transient behavior of a practical submicron AlGaN∕GaN heterostructure field effect transistor (HFET) is investigated, and its mechanism is proposed. First, the steady-state and transient characteristics of the Schottky diode obtained by connecting the source and drain electrodes of the transistor have been investigated. The steady-state characteristics can be explained by the thin surface barrier model, indicating the presence of tunneling injection of electrons. Turn-on and turn-off transient characteristics of the reverse current of Schottky diode showed very slow nonexponential transients covering six orders of magnitude of time scale from milliseconds to thousands of seconds. They are very similar to those of a large planar Schottky diode studied recently by the authors. The HFET device showed a clear current collapse behavior after a gate stress beyond pinch off. Pulsed gate stress visualized drain current transients which again included very slow nonexponential transients covering six orders of magnitude of time scale. The whole experimental results are explained consistently by a model in which the current collapse is due to surface state charging near the source side and drain side of the gate edge where its rate limiting process is not the usual Shockley–Read–Hall capture-emission process but the dispersive electron transport through the surface states by time-continual hopping, which is triggered by the tunneling injection process at the gate edge.

2 MeV ion irradiation effects on AlGaN/GaN HFET devices

AlGaN/GaN heterostructure field effect transistors (HFETs) were irradiated with 2MeV protons, carbon, oxygen, iron and krypton ions with fluences ranging from 1×109cm−2 to 1×1013cm−2. DC, pulsed I–V characteristics, loadpull and S-parameters of the AlGaN HFET devices were measured before and after irradiation. In parallel, a thick GaN reference layer was also irradiated with the same ions and was characterized by X-ray diffraction, photoluminescence, Hall measurements before and after irradiation. Small changes in the device performance were observed after irradiation with carbon and oxygen at a fluence of 5×1010cm−2. Remarkable changes in device characteristics were seen at a fluence of 1×1012cm−2 for carbon, oxygen, iron and krypton irradiation. Similarly, remarkable changes were also observed in the GaN layer for irradiations with fluence of 1×1012cm−2. The results found on devices and on the GaN layer were compared and correlated.

Irradiation effects on AlGaN HFET devices and GaN layers

AlGaN/GaN heterostructure field effect transistors (HFETs) were irradiated with protons as well as carbon, oxygen, iron and krypton ions of high (68 and 120 MeV) and low (2 MeV) energy with fluences in the range from 1 × 107 to 1 × 1013 cm−2. High energy irradiation with protons, carbon and oxygen produced no degradation in devices while krypton irradiation at the fluence of 1 × 1010 cm−2 resulted in a small reduction of 2% in the transconductance. Similarly, for GaN samples irradiated with protons, carbon and oxygen at high energy no changes were seen by XRD, PL and Hall effect, while changes in lattice constant and a reduction in PL intensity were observed after irradiation with high energy krypton. Low energy irradiation with carbon and oxygen at a fluence of 5 × 1010 cm−2 results in small change in the device performance while remarkable changes in device characteristics are seen at a fluence of 1 × 1012 cm−2 for carbon, oxygen, iron and krypton irradiation. Similarly changes are also observed by XRD, PL and Hall effect for the thick GaN layer irradiated at the fluence of 1 × 1012 cm−2. The device results and GaN layer properties are strongly correlated.

Large reduction of leakage currents in AlGaN Schottky diodes by a surface control process and its mechanism

Leakage currents in AlGaN Schottky diodes were investigated systematically by using a rigorous computer simulation based on the thin surface barrier model taking account of unintentionally doped surface donors. The leakage currents in AlGaN Schottky diodes have stronger bias dependence and smaller temperature dependences as compared with those of GaN diodes. It was shown that these features were associated with shallow oxygen donors located near the AlGaN surface. Then, an attempt was made to remove oxygen and suppress leakage currents by a surface control process using an ultrathin Al layer and subsequent annealing. An in situ x-ray photoelectron spectroscopy analysis indicated the formation of Al2O3 layer during the surface control process, suggesting efficient gettering of oxygen from the surface. C-V analysis directly indicated the reduction of shallow donors by the surface control process. A remarkable reduction of reverse leakage currents of four to five orders of magnitude took place in large area AlGaN Schottky diodes after the application of the surface control process. This process also reduced leakage currents of the gate of the heterostructure field effect transistor device by more than one order of magnitude and increased temperature dependences of current.

Characterization and modeling of AlGaN/GaN MOS capacitor with leakage for large signal transistor modeling

Dual mode AlGaN/GaN metal oxide semiconductor (MOS) heterostructure field-effect transistor (HFET) devices were fabricated and characterized. In HFET mode of operation the devices showed an f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> /spl middot/L/sub g/ product of 12GHz/spl middot/μm at Vgs=-2 V. The AlGaN devices showed formation of an accumulation layer under the gate in forward bias and a f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> /spl middot/L/sub g/ product of 6GHz/spl middot/μm was measured at Vgs=5 V. A novel piecewise small signal model for the gate capacitance of MOS HFET devices is presented and procedures to extract the capacitance in presence of gate leakage are outlined. The model accurately fits measured data from 45MHz to 10GHz over the entire bias range of operation of the device.

Impact of native oxides beneath the gate contact of AlGaN/GaN HFET devices

We report on the decisive role of oxides at the surface of AlGaN/GaN Heterostructure Field Effect Transistors (HFETs). The effect of oxides at the surface is twofold, on one hand the 2DEG in the channel is directly influenced by surface charges and surface potential correlated to surface oxides (D. Mistele et al., phys. stat. sol. (a) 194 (2), 452 (2002). [1]), on the other hand, a surface oxide below a subsequently deposited gate contact increases the barrier height and therefore reduces leakage currents by several orders of magnitude. This study includes various surface treatments on AlGaN/GaN heterostructures such as etching by HCl, oxidation by O2-plasma, and subsequent passivation by Si3N4. Next, we report on the correlation between gate leakage current and the drain current and dedicate this behavior to the Schottky barrier and to surface related charging effects. A model with the surface related charging effects on the 2DEG and the barrier height is discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

MOCVD AlGaN/GaN HFET's Material Optimization and Devices Characterization

ABSTRACTAn optimization of growth parameters of AlxGa1*xN/AlN/GaN heterostructure field effect transistors (HFET) grown by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) technique on SiC and sapphire substrates with relatively high Al mole fraction in the barrier layer (0.3 &lt; x &lt; 0.5) has been presented. The properties of the two-dimensional electron gas (2DEG) forming at the AlxGa1-xN/GaN heterojunction can be tuned by careful adjustments of AlxGa1-xN barrier layer thickness and Al mole fraction, x. The 2DEG sheet conductivities (μ ns) as high as 2.6 × 1016 V-1s-1 at μ ∼ 2200 cm2/Vs and ns ∼ 1.2 × 1013 cm-2 has been achieved on AlxGa1-xN/AlN/GaN HFET structures on SiC substrate at x = 0.47. HFET devices processed on these structures exhibited improved low field conductivities and DC and high frequency performance. Saturation currents above 1.2 A/mm at 0 V gate bias, transconductance as high as 340 mS/mm at Lg = 0.25 μm and FT × Lg &gt; 20 GHz × μm were demonstrated on HFET structure grown on SiC substrates.

GaN–AlGaN heterostructure field-effect transistors over bulk GaN substrates

We report on AlGaN/GaN heterostructures and heterostructure field-effect transistors (HFETs) fabricated on high-pressure-grown bulk GaN substrates. The 2d electron gas channel exhibits excellent electronic properties with room-temperature electron Hall mobility as high as μ=1650 cm2/V s combined with a very large electron sheet density ns≈1.4×1013 cm−2. The HFET devices demonstrated better linearity of transconductance and low gate leakage, especially at elevated temperatures. We also present the comparative study of high-current AlGaN/GaN HFETs (nsμ&amp;gt;2×1016 V−1 s−1) grown on bulk GaN, sapphire, and SiC substrates under the same conditions. We demonstrate that in the high-power regime, the self-heating effects, and not a dislocation density, is the dominant factor determining the device behavior.

Si/Si 1−xGe x heterostructure field effect transistors fabricated using a low thermal budget CMOS process

Strained Si/Si 0.75Ge 0.25 heterostructure field effect transistors (HFETs) have been fabricated using a modified, low-thermal budget CMOS process with deposited gate oxides. Transmission electron microscopy demonstrates the integrity of the strained-Si quantum well after processing. The transconductances of the HFET devices are higher than the similarly processed Si MOSFET devices. Electrical characterisation data is presented which suggest that thinner gate oxides, higher Ge contents in the virtual substrate and optimisation of the p-type substrate doping profile will improve device performance.

InAs channel heterostructure-field effect transistors with InAs/AlSb short-period superlattice barriers

We report on the implementation of InAs-channel heterostructure-field-effect transistors (HFETs) fabricated with InAs/AlSb short-period superlattice barriers. The InAs/AlSb superlattice barrier structure is advantageous for InAs/AlSb HFETs because of its improved chemical stability against oxidation when compared to pure AlSb, and its compatibility with silicon as an n-type dopant during growth by molecular beam epitaxy. The structures examined here consist of a 200-Å-wide InAs quantum well inserted between 25/25 Å InAs/AlSb superlattice barriers that provide a 0.5 eV conduction band discontinuity between the quantum well and the superlattice barrier. Fabricated HFET devices display complete channel modulation, confirming the field-effect operation at room temperature. In addition, we demonstrate the modulation doping of an InAs quantum well clad by silicon-doped InAs/AlSb superlattice barriers.

Characterization of reactive ion etched AlGaAs/GaAs heterostructures by photoluminescence and low temperature Hall measurements

A low damage dry etch process has been developed for the gate recess of AlGaAs/GaAs heterostructure field effect transistor (HFET) with a selectivity of the etch rates greater than 750:1. The reactive ion etching induced damage has been compared to that reported from other etch processes in literature by the means of photoluminescence (PL) emission from multiple quantum well structures. Better resolution in amount and depth of created defects is obtained by PL measurements of undoped heterostructure layers. The temperature dependent Hall mobility μH and sheet carrier concentration ns of a doped heterostructure layer which directly correspond to the HFET device properties are evaluated in the dark and under illumination. Values of 360.000 cm2/V s for μH and 8.6×1011 cm−2 for ns at 15 K after the etch process show that a residual damage can be neglected.

Study of strain in pseudomorphic InGaAs heterostructures related to the enhanced performance of p-channel heterostructure field effect transistor devices

Results of a study of strain and its relation to InGaAs channel p‐heterostructure insulated gate field effect transistors (HIGFET) device performance are presented. Measurements of the strain in a pseudomorphic InGaAs channel p‐HIGFET device heterostructure using x‐ray rocking curves in conjunction with a dynamical x‐ray diffraction calculation are discussed. The strain in the InGaAs layer was found to increase by over 45% from the center to a position 2.5 cm from the center of a 3‐in wafer. This variation is discussed in terms of across‐wafer nonuniformity in the composition and thickness of the InGaAs. InGaAs channel p‐HIGFET devices fabricated on a number of positions across the same 3‐in. wafer yielded 1.0 μ gate length devices with room temperature transconductance (gm) values of up to 67 mS/mm. The relationship between the measured strain and p‐HIGFET device characteristics are also discussed.