Heterostructure Field-effect Transistor Structures Research Articles

Comparative study of BeMgZnO/ZnO heterostructures on c-sapphire and GaN by molecular beam epitaxy

Growth of ZnO directly on c-plane sapphire, with a large in-plane lattice mismatch of 18%, is relatively well understood for O-polar variety. However, the two-dimensional (2D) growth of Zn-polar ZnO on c-sapphire, needed for 2D electron gas formation, with low background electron density is in its infancy. While Zn-polar ZnO can be grown on GaN with the resulting small lattice mismatch (1.8%), the parallel conduction through GaN (bulk and/or surface) in fabricated BeMgZnO/ZnO heterostructure field effect transistors (HFETs) limits its utility for the time being. In this contribution, the authors report on the growth of high-quality Zn-polar BeMgZnO/ZnO HFET structures directly on sapphire substrates, in an effort to avoid the aforementioned parallel conduction, by employing an MgO and low-temperature ZnO buffer stack. The residual 2D equivalent concentration in the HFET structure on GaN is ∼2 × 1012 cm−2, which is detrimental, while highly resistive (>100 Ω.cm) Zn-polar ZnO layers with smooth surfaces on c-sapphire have been obtained via controlling the buffer stack growth conditions, which is vital to the realization of HFET device structures. Compared with the highest room temperature electron mobility of ∼250 cm2/V s in BeMgZnO/ZnO HFETs on GaN templates, a slightly lower electron mobility of ∼220 cm2/V s was achieved on c-sapphire due to a somewhat lower overall crystalline quality.

Novel GaN-based vertical heterostructure field effect transistor structures using crystallographic KOH etching and overgrowth

A novel V-groove vertical heterostructure field effect transistor structure is proposed using semi-polar (11-22) GaN. A crystallographic potassium hydroxide self-limiting wet etching technique was developed to enable a damage-free V-groove etching process. An AlGaN/GaN HFET structure was successfully regrown by molecular beam epitaxy on the V-groove surface. A smooth AlGaN/GaN interface was achieved which is an essential requirement for the formation of a high mobility channel.

Properties of InGaAs/GaAs metal-oxide-semiconductor heterostructure field-effect transistors modified by surface treatment

GaAs-based heterostructures exhibit excellent carrier transport properties, mainly the high carrier velocity. An AlGaAs-GaAs heterostructure field-effect transistor (HFET) with an InGaAs channel was prepared using metal-organic chemical vapor deposition (MOVPE). An AlOx layer was formed on the AlGaAs barrier layer by the air-assisted oxidation of a thin Al layer deposited in-situ in an MOVPE reactor immediately after AlGaAs/InGaAs growth. The HFETs and MOSHFETs exhibited a very low trap state density in the order of 1011cm−2eV−1. Capacitance measurement yielded no significant difference between the HFET and MOSHFET structures. The formation of an AlOx layer modified the surface by partially eliminating surface states that arise from Ga-and As-based native oxides. The presence of an AlOx layer reflected in a reduced gate leakage current, which was evidenced by the two-terminal transistor measurement. Presented preparation procedure and device properties show great potential of AlGaAs/InGaAs-based MOSHFETs.

Charge movement in a GaN-based hetero-structure field effect transistor structure with carbon doped buffer under applied substrate bias

Charge trapping and transport in the carbon doped GaN buffer of a GaN-based hetero-structure field effect transistor (HFET) has been investigated under both positive and negative substrate bias. Clear evidence of redistribution of charges in the carbon doped region by thermally generated holes is seen, with electron injection and capture observed during positive bias. Excellent agreement is found with simulations. It is shown that these effects are intrinsic to the carbon doped GaN and need to be controlled to provide reliable and efficient GaN-based power HFETs.

AlGaN/GaN MOS-HFETs based on InGaN/GaN MQW structures with Ta2O5dielectric

We report on metal-oxide-semiconductor (MOS) AlGaN/GaN heterostructure field effect transistors (HFETs) based on InGaN/GaN multiple quantum well (MQW) structure using Ta 2 O 5 dielectric deposited by electron beam evaporation (EBE) simultaneously for surface passivation and as a gate insulator. The device features a 5-pair MQW layer inserted into the AlGaN/GaN two-channel HFET structure. It results in a raised potential barrier, which leads to better carrier confinement and effective access to the InGaN layer. However, it revealed a pronounced leakage current which may be generated from the bottom Si-doped GaN and/or the sidewall leakage paths due to the exposure of channels after mesa etching. Both passivated MQW-HFET and MOS MQW-HFET present enhanced dc- and pulsed-mode performance compared to unpassivated one. In terms of transfer characteristics, MOS MQW-HFET exhibits the larger and broader main peak yet smaller satellite peak relative to passivated MQW-HFET. The reduced gate and mesa-to-mesa leakage current indicates the successful passivation effect from EBE-Ta 2 O 5 dielectric.

Degradation in InAlN/GaN-based heterostructure field effect transistors: Role of hot phonons

We report on high electric field stress measurements at room temperature on InAlN/AlN/GaN heterostructure field effect transistor structures. The degradation rate as a function of the average electron density in the GaN channel (as determined by gated Hall bar measurements for the particular gate biases used), has a minimum for electron densities around 1×1013 cm−2, and tends to follow the hot phonon lifetime dependence on electron density. The observations are consistent with the buildup of hot longitudinal optical phonons and their ultrafast decay at about the same electron density in the GaN channel. In part because they have negligible group velocity, the build up of these hot phonons causes local heating, unless they decay rapidly to longitudinal acoustic phonons, and this is likely to cause defect generation which is expected to be aggravated by existing defects. These findings call for modified approaches in modeling device degradation.

Electron mobility in InGaN channel heterostructure field effect transistor structures with different barriers

InGaN possesses higher electron mobility and velocity than GaN, and therefore is expected to lead to relatively better performances for heterostructure field effect transistors (HFETs). However, the reported mobilities for AlGaN∕InGaN HFETs are lower than GaN channel HFETs. To address this issue, we studied the effect of different barriers on the Hall mobility for InGaN channel HFETs grown by metal organic chemical vapor deposition. Unlike the conventional AlGaN barrier, the AlInN barrier can be grown at the same temperature as the InGaN channel layer, alleviating some of the technological roadblocks. Specifically, this avoids possible degradation of the thin InGaN channel during AlGaN growth at high temperatures; and paves the way for better interfaces. An undoped In0.18Al0.82N∕AlN∕In0.04Ga0.96N HFET structure exhibited a μH=820cm2∕Vs, with a ns=2.12×1013cm−2 at room temperature. Moreover, with an In-doped AlGaN barrier, namely, Al0.24In0.01Ga0.75N, grown at 900°C, the μH increased to 1230cm2∕Vs with a ns of 1.09×1013cm−2 for a similar InGaN channel. Furthermore, when the barrier was replaced by Al0.25Ga0.75N grown at 1030°C, μH dropped to 870cm2∕Vs with ns of 1.26×1013cm−2 at room temperature. Our results suggest that to fully realize the potential of the InGaN channel HFETs, AlInN or AlInGaN should be used as the barrier instead of the conventional AlGaN barrier.

GaN-based FETs using Cat-CVD SiN passivation for millimeter-wave applications

We have found that SiN passivation by catalytic chemical vapor deposition (Cat-CVD) can significantly increase an electron density of an AlGaN/GaN heterostructure field-effect transistor (HFET). This effect enables thin-barrier HFET structures to have a high-density two-dimensional electron gas and leads to suppression of short-channel effects. We fabricated 30-nm-gate Al 0.4Ga 0.6N(8 nm)/GaN HFETs using Cat-CVD SiN. The maximum drain current density and extrinsic transconductance were 1.49 A/mm and 402 mS/mm, respectively. Current-gain cutoff frequency and maximum oscillation frequency of the HFETs were 181 and 186 GHz, respectively. These high-frequency device characteristics are sufficiently high enough for millimeter-wave applications.

A monolithic Cockcroft‐Walton voltage multiplier based on AlGaN/GaN HFET structure

AbstractA five‐stage monolithic Cockcroft‐Walton voltage multiplier is reported which uses metal‐insulator‐metal capacitor and Schottky diode based on AlGaN/GaN heterostructure field‐effect transistor (HFET) structure. The voltage multiplier includes 5 capacitors and 5 Schottky diodes. AlGaN/GaN HFET with L/W = 1 μm/10 μm is used as the Schottky diode, where the gate made of Ni/Au/Ti is used for the anode and the ohmic contact is used for the cathode. The size of the chip is 0.567 mm×0.445 mm. The breakdown voltage of the capacitor is around 40 V and the diode breakdown voltage in reverse direction is more than 100 V. At input AC peak to peak voltage of 20 V under frequency of 10 MHz, the measured output voltage is about 40 V with the output impedance less than 1 MΩ. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

DC characteristics improvement of recessed gate GaN-based HFETs grown by MOCVD

The effect of the recessed gate structure on DC characteristics of n-channel depletion mode heterostructure field effect transistors (HFETs) grown by metalorganic chemical vapor deposition (MOCVD) was demonstrated. The recessed gate process was carried out by using photo-enhanced chemical (PEC) wet etching. There were improvements on the electrical characteristics after using the recessed gate AlGaN/GaN and AlGaN/GaN/InGaN/GaN HFET structures because of the better gate controllability to the drain current. It was also found that the gate voltage, V GS, of the transconductance peak value shifted toward positive bias for the recessed gate structure. The leakage current, the transconductance ( G m) and the saturation current of the recessed gate AlGaN/GaN/InGaN/GaN HFET with a 30-nm-thick Si-doped GaN cap layer are 1.95 mA/mm, 94.82 mA/mm and 230 mA/mm, respectively. This performance is quite better than that of the device with no recessed gate structure.

Capacitance–Voltage Spectroscopy of Trapping States in GaN/AlGaN Heterostructure Field-Effect Transistors

In AlGaN/GaN heterostructure field-effect transistor structures, the surface defects and dislocations may serve as trapping centers and affect the device performance via leakage current and lowfrequency noise. In this paper we report results of our investigation of the trapping characteristics of SiO2-passivated Al0� 2Ga0� 8N/GaN heterostructure field-effect transistors using the capacitance– voltage (CV) profiling technique. From the measured frequency dependent CV data, we identified the characteristics of the traps at the AlGaN/GaN interface adjoining the channel and on the surface along the ungated region between the gate and drain. Based on the measured data, the influence of the channel traps on the low-frequency noise spectra and the effect of the surface traps on possible leakage noise are analyzed and compared with the previous studies.

Demonstration of undoped quaternary AlInGaN∕GaN heterostructure field-effect transistor on sapphire substrate

Undoped AlInGaN∕GaN heterostructure field-effect transistors (HFETs) have been demonstrated on sapphire substrate. The maximum drain current of 758mA∕mm and extrinsic transconductance of 123mS∕mm were obtained from the device with 2μm gate length and 15μm gate width. Such performance was comparable to that of a conventional modulation-doped (MOD) AlGaN∕GaN HFET fabricated at same the condition. In spite of the high-density two-dimentional electron gas formed at the heterointerface, the AlInGaN∕GaN HFET structure showed relatively low Hall mobility (689cm2∕Vs at 300K), which was due to the high concentration background doping in the undoped AlInGaN barrier region. Encouragingly, both the gate leakage current and drain current collapse showed lower values than that of the MOD-AlGaN∕GaN HFET, which implied the potential application of quaternary AlInGaN to high-power and high-frequency devices.

Pre-metallization processing effects on Schottky contacts to AlGaN∕GaN heterostructures

Changes in the Schottky barrier height of Ni on AlGaN∕GaN heterostructure field effect transistor structures are characterized by internal photoemission spectroscopy (IPE) as a function of pre-metallization processing conditions and postmetallization ultrahigh vacuum annealing. Low energy electron-excited nanoluminescence spectroscopy and mapping reveal AlGaN near band edge emission variations that correlate with IPE Schottky barrier height. Ni∕AlGaN interface impurities measured by secondary ion mass spectrometry are also correlated with IPE Schottky barrier height. We show that changes in the Schottky barrier height and the appearance of dual barriers are dominated by changes in the local Al mole fraction. Interfacial oxygen and carbon have secondary but systematic effects as well.

III-nitride metal-insulator-semiconductor heterojunction field-effect transistors using sputtered AlON thin films

In this letter, a III-nitride metal-insulator-semiconductor heterostructure field-effect transistors (MISHFET) was demonstrated by incorporating a sputtered AlON layer in the AlGaN∕GaN heterostructure field-effect transistors (HFET). The AlON layer was deposited on the HFET structure by magnetron sputtering, followed by rapid thermal annealing at 850°C for 45s. A reverse gate leakage current that is four orders of magnitude lower was obtained in the MISHFET, compared to that in HFET. The MISHFET also shows 20% increase in the drain saturation current. For a MISHFET with 1-μm-long gate, the current gain cutoff frequency, ft and the power gain cutoff frequency, fmax are measured to be 13 and 37GHz, respectively.

An experimental set-up for in situ Hall measurements under high-energy electronirradiation for wide-bandgap materials

A dc Hall-effect apparatus, based on conventional van der Pauw specimen geometry, hasbeen developed for in situ measurements under van de Graaff electron irradiation(0.7–2.2 MeV). An associated cryostat permits ambient temperatures of 95–300 K.A key element is a flat permanent magnet of field strength 3.55 kG. Initial testmeasurements have been performed on wide-bandgap semiconductor materials, includingan HVPE-grown AlGaN/GaN heterostructure field-effect transistor structure.

Mechanisms of current collapse and gate leakage currents in AlGaN/GaN heterostructure field effect transistors

In order to clarify the mechanisms of drain current collapse and gate leakage currents in the AlGaN/GaN heterostructure field effect transistor (HFET), detailed electrical properties of the ungated portion and Schottky-gated portion of the device were investigated separately, using a gateless HFET structure and an AlGaN Schottky diode structure. The gateless device was subjected to plasma treatments and surface passivation processes including our novel Al2O3-based surface passivation. dc I–V curves of gateless HFETs were highly nonlinear due to virtual gating by surface states. After drain stress, air-exposed, H2 plasma-treated and SiO2-deposited gateless HFETs showed an initial large-amplitude exponential current transient followed by a subsequent smaller, slow, and highly nonexponential response. The former was explained by emission from deep donors at Ec−0.37 eV, and the latter by emission from surface states. Capture transients with stress-dependent capture barriers were also observed. An x-ray photoelectron spectroscopy (XPS) study indicated that 0.37 eV-deep donors are N-vacancy related. On the other hand, no current transients took place in N2 plasma treated and Al2O3-passivated samples. Temperature dependences of I–V curves of Schottky diodes were extremely small and reverse currents were anomalously large. They were explained by the “thin surface barrier” (TSB) model where thermionic field emission and field emission through the TSB region formed by deep donors produce leakage current paths. By combining the results on gateless HFETs and Schottky diodes, a new unified model of near-surface electronic states for the free surface and Schottky interface of AlGaN is proposed. It consists of a U-shaped surface state continuum and N-vacancy related near-surface discrete deep donors. The model can explain the observed large gate leakage and drain current collapse in AlGaN/GaN HFETs in a unified way. It is also shown that our novel Al2O3 passivation, when also used as a gate insulator, can completely suppress current collapse and gate leakage.

Effects of surface processing on 2DEG current transport at AlGaN/GaN interface studied by gateless HFET structure

Effects of hydrogen and nitrogen electron cyclotron resonance (ECR) plasma surface treatments on the current transport of two-dimensional electron gas (2DEG) at AlGaN/GaN interface were investigated by using a gateless heterostructure field-effect transistors (HFETs) test device. Current–voltage ( I– V) characteristics of the gateless device exhibited linear and saturation current behavior similar to that of a gated device, indicating presence of strong Fermi level pinning. As compared with the air-exposed surface, the current reduced significantly with appearance of hysteresis after H 2-plasma treatment, whereas it slightly increased after N 2-plasma treatment. After experiencing a positive pulse voltage, current in the H 2-plasma-treated device dropped and then showed a fast exponential recovery followed by highly non-exponential slow recovery. Temperature dependence of initial recovery gave an activation energy of 0.37 eV. Observed current transients could be reproduced by numerical simulation assuming presence of dominant discrete near-surface deep donors and a U-shaped surface state continuum. X-ray photoelectron spectroscopy (XPS) analysis indicated near-surface reduction of N atoms after the H 2-plasma treatment, indicating that the discrete states are related to N-vacancies. Since generation of N-vacancies is likely to occur in growth and processing of GaN-related materials, the present observation explains surface-related instabilities such as current collapse in AlGaN/GaN HFETs. For oxide removal, the N 2-plasma treatment is much better than H 2-plasma treatment, effectively removing oxides without producing defects.

Insulating gate III-N heterostructure field-effect transistors for high-power microwave and switching applications

Describes the properties of novel III-N-based insulating gate heterostructure field-effect transistors (HFETs). For the gate isolation, these devices use either SiO/sub 2/ layer (in metal-oxide-semiconductor HFET (MOSHFET) structures) or Si/sub 3/N/sub 4/ layer (in metal-insulator-semiconductor HFET structures). These insulating gate HFETs have the gate-leakage currents 4-6 orders of magnitude lower than HFETs, even at elevated temperatures up to 300/spl deg/C. A double-heterostructure MOSHFET with SiO/sub 2/ gate isolation exhibits current collapse-free performance with extremely low gate-leakage current. Insulating gate devices, including large periphery multigate structures, demonstrate high-power stable operation and might find applications in high-performance power amplifiers and microwave and high-power switches with operating temperatures up to 300/spl deg/C or even higher.

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 < x < 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 > 20 GHz × μm were demonstrated on HFET structure grown on SiC substrates.

Delta-doped AlGaN/GaN Heterostructure Field-Effect Transistors with Incorporation of AlN Epilayers

ABSTRATCThe simulation and experiment results of delta-doped AlGaN/GaN heterostructure field-effect transistors (HFETs) with the incorporation of highly-resistive AlN epilayer are reported. The high quality AlN epilayer is used as the dislocation filter for the HFET structure growth, and the high resistivity of AlN also removes the parasitic conduction related with the GaN bulk buffer. Delta doping can reduce gate leakage, further more, our simulation and growth results demonstrate that delta-doping in the barrier is more effective than uniform doping scheme to increase the sheet electron density. The influence of spacer layer thickness on the electron mobility and sheet electron density is also presented. The DC characterization of the fabricated devices shows our structure has a very high performance with a maximum current ∼ 1 A/mm.