Two-dimensional Electron Gas Channel Research Articles

Creation of a Short-Range Ordered Two-Dimensional Electron Gas Channel in Al2O3/In2O3 Interfaces.

The tuning of electrical properties in oxides via surface and interfacial two-dimensional electron gas (2DEG) channels is of great interest, as they reveal the extraordinary transition from insulating or semiconducting characteristics to metallic conduction or superconductivity enabled by the ballistic transport of spatially confined electrons. However, realizing the practical aspects of this exotic phenomenon toward short-range ordered and air-stable 2DEG channels remains a great challenge. At the heterointerface formed after deposition of an Al2O3 layer on a nanocrystalline In2O3 layer, a dramatic improvement in carrier conduction equivalent to metallic conduction is obtained. A conductivity increase by a factor of 1013 times that in raw In2O3, a sheet resistance of 850 Ω/cm2, and a room temperature Hall mobility of 20.5 cm2 V-1 s-1 are obtained, which are impossible to achieve by tuning each layer individually. The physicochemical origin of metallic conduction is mainly ascribed to the 2D interfacially confined O-vacancies and semimetallic nanocrystalline InOx (x < 2) phases by the clustered self-doping effect caused by O-extraction from In2O3 to the Al2O3 phase during ALD. Unlike other submetallic oxides, this 2D channel is air-stable by complete Al2O3 passivation and thereby promises applicability for implementation in devices.

Highly Sensitive Detection of Deoxyribonucleic Acid Hybridization Using Au-Gated AlInN/GaN High Electron Mobility Transistor-Based Sensors**Supported by the National Key Research and Development Program of China under Grant Nos 2016YFB0400104 and 2016YFB0400301, the National Natural Sciences Foundation of China under Grant No 61334002, and the National Science and Technology Major Project.

Gallium nitride- (GaN) based high electron mobility transistors (HEMTs) provide a good platform for biological detection. In this work, both Au-gated AlInN/GaN HEMT and AlGaN/GaN HEMT biosensors are fabricated for the detection of deoxyribonucleic acid (DNA) hybridization. The Au-gated AlInN/GaN HEMT biosensor exhibits higher sensitivity in comparison with the AlGaN/GaN HEMT biosensor. For the former, the drain-source current ( V) shows a clear decrease of 69 μA upon the introduction of 1 μmolL (μM) complimentary DNA to the probe DNA at the sensor area, while for the latter it is only 38 μA. This current reduction is a notable indication of the hybridization. The high sensitivity can be attributed to the thinner barrier of the AlInN/GaN heterostructure, which makes the two-dimensional electron gas channel more susceptible to a slight change of the surface charge.

Improved reverse recovery characteristics of low turn-on voltage AlGaN/GaN Schottky barrier diodes with anode edge AlON spacers

Low reverse recovery charge and low turn-on voltage AlGaN/GaN Schottky barrier diodes were fabricated on a 6-inch Si (111) substrates using a recessed anode and N2O plasma-oxidized AlON compound spacer. The N2O plasma-treated areas at the anode edge were oxidized because the AlON passivation layers and spacers under the field plate metals formed a favorable interface with the GaN layer that reduced the nitrogen vacancies and surface leakage currents. Relative to traditional recessed barrier diodes, the barrier diodes fabricated with a recessed anode and an AlON compound spacer realized a higher breakdown voltage (825 V), lower specific on-resistance (3.36 mΩ-cm2), and lower turn-on voltage (0.43 V). In addition, the reverse recovery and low-frequency noise measurements indicated that the N2O plasma-oxidized AlON compound spacer reduced the surface traps from the plasma bombardment during the anode recess process and improved the carrier confinement in the two-dimensional electron gas channel.

Ion-assisted gate recess process induced damage in GaN channel of AlGaN/GaN Schottky barrier diodes studied by deep level transient spectroscopy

Deep traps in AlGaN/GaN Schottky barrier diodes have been investigated using deep level transient spectroscopy. It has been found that ion-assisted gate recess process leads to the formation of electron traps. The defects related to these traps are mainly located in the two-dimensional electron gas channel below the Schottky contact, meaning that the partial etching of the AlGaN layer produces damage on the top of the underlying GaN layer. The activation energies of the electron traps, extracted from the data, range between 0.28 and 0.41 eV. We believe that these centers are complexes linked with nitrogen vacancies which may behave as extended defects.

Polarization-mediated Debye-screening of surface potential fluctuations in dual-channel AlN/GaN high electron mobility transistors

A dual-channel AlN/GaN/AlN/GaN high electron mobility transistor (HEMT) architecture is proposed, simulated, and demonstrated that suppresses gate lag due to surface-originated trapped charge. Dual two-dimensional electron gas (2DEG) channels are utilized such that the top 2DEG serves as an equipotential that screens potential fluctuations resulting from surface trapped charge. The bottom channel serves as the transistor's modulated channel. Two device modeling approaches have been performed as a means to guide the device design and to elucidate the relationship between the design and performance metrics. The modeling efforts include a self-consistent Poisson-Schrodinger solution for electrostatic simulation as well as hydrodynamic three-dimensional device modeling for three-dimensional electrostatics, steady-state, and transient simulations. Experimental results validated the HEMT design whereby homo-epitaxial growth on free-standing GaN substrates and fabrication of the same-wafer dual-channel and recessed-gate AlN/GaN HEMTs have been demonstrated. Notable pulsed-gate performance has been achieved by the fabricated HEMTs through a gate lag ratio of 0.86 with minimal drain current collapse while maintaining high levels of dc and rf performance.

Electrical characterization of a gate-recessed AlGaN/GaN high-electron-mobility transistor with a p-GaN passivation layer

A novel passivation technique was developed that reduces the electron-surface-hopping-induced leakage current of AlGaN/GaN high-electron-mobility transistors (HEMTs) and enhances their electrical properties under high drain bias operation. The key aspect of this passivation technique entailed growing a p-type GaN layer on a traditional depletion-mode AlGaN/GaN HEMT; this p-GaN passivation layer was also used as the spacer layer of a field-plate metal. The originally exposed gate-to-source and gate-to-drain areas were passivated by the p-GaN cap layer. Thus, a surface depletion region was formed between the p-GaN passivation layer and an n-type AlGaN/GaN two-dimensional electron gas channel. This extra surface depletion region depleted the channel carriers far from the surface to reduce the probability of the carriers being trapped by surface defects. Therefore, the carrier-hopping-induced leakage current in the gate-to-drain area, which was strongly temperature-dependent, was suppressed. Low-frequency noise (LFN) measurement revealed that the traditional chemical-vapor-deposited SiO2 layer still exhibited a surface trap center. Moreover, the slope of a plot of noise spectra versus gate overdrive voltage was approximately −3. At a cryogenic temperature, the LFN of the p-GaN—passivated device improved by 3 orders of magnitude, whereas the high surface trap density of the standard passivated device resulted in a high LFN, even at a cryogenic temperature.

Suppression of surface-originated gate lag by a dual-channel AlN/GaN high electron mobility transistor architecture

A dual-channel AlN/GaN high electron mobility transistor (HEMT) architecture is demonstrated that leverages ultra-thin epitaxial layers to suppress surface-related gate lag. Two high-density two-dimensional electron gas (2DEG) channels are utilized in an AlN/GaN/AlN/GaN heterostructure wherein the top 2DEG serves as a quasi-equipotential that screens potential fluctuations resulting from distributed surface and interface states. The bottom channel serves as the transistor's modulated channel. Dual-channel AlN/GaN heterostructures were grown by molecular beam epitaxy on free-standing hydride vapor phase epitaxy GaN substrates. HEMTs fabricated with 300 nm long recessed gates demonstrated a gate lag ratio (GLR) of 0.88 with no degradation in drain current after bias stressed in subthreshold. These structures additionally achieved small signal metrics ft/fmax of 27/46 GHz. These performance results are contrasted with the non-recessed gate dual-channel HEMT with a GLR of 0.74 and 82 mA/mm current collapse with ft/fmax of 48/60 GHz.

Effects of proton irradiation and thermal annealing on off-state step-stressed AlGaN/GaN high electron mobility transistors

The effects of proton irradiation doses on dc characteristics of reference, electrically stressed under off-state conditions and stressed/annealed AlGaN/GaN high electron mobility transistors (HEMTs) were investigated. The HEMTs were irradiated with protons at a fixed energy of 5 MeV and doses ranging from 1013 to 1015 cm−2. As expected, in all three types of HEMTs, more degradation of the device dc characteristics was observed for higher doses due to the larger displacement damage in two-dimensional electron gas channel of the HEMTs. The electrically stressed HEMTs after proton irradiation showed more degradation compared with reference and stressed/annealed HEMTs. After proton irradiation at a dose of 1015 cm−2, the drain saturation current and maximum transconductance of stressed HEMTs were decreased by 28.5% and 15%, respectively, compared to 24% and 11.5%–12%, respectively, for reference and stressed/annealed devices. The dc characteristics of stressed/annealed HEMTs after proton irradiation showed similar degradation trends compared with those of reference HEMTs, confirming that annealing is effective in removing defects created by the off-state stressing. In some cases, the authors also annealed stressed/irradiated HEMTs after the proton irradiation step. The drain current and transconductance of stressed/irradiated HEMTs were slightly increased after subsequent thermal annealing at 450 °C for 10 min, while reverse gate leakage current after annealing was decreased more than an order of magnitude. The interface trap density of stressed HEMTs after proton irradiation at a dose of 1015 cm−2 increased from 3.05 × 1012 to 1.37 × 1013/cm2 V and were reduced to 6.01 × 1012/cm2 V following thermal annealing. Our results are consistent with the notion that off-state stressing creates defects that have a common origin with those created by proton irradiation, and thus, irradiation of off-state stressed devices leads to more defects than in unstressed devices that are subsequently irradiated. Annealing the stressed devices prior to irradiation makes them behave the same as unstressed HEMTs when both are irradiated with protons.

Temperature-dependent characteristics of AlGaN/GaN FinFETs with sidewall MOS channel

AlGaN/GaN fin-shaped field-effect transistors (FinFETs) with variable fin width have been fabricated and characterized. Low-temperature measurements reveal distinct operation modes for wide FinFET, narrow FinFET and planar FET. The wide fin device exhibits broad transconductance (gm) that decreases sublinearly with increasing temperature due to the existence of the sidewall metal–oxide–semiconductor (MOS) channel. By comparison, the conventional planar AlGaN/GaN metal–insulator–semiconductor heterostructure FET (MISHFET) features relatively narrow gm curve and near-exponentially decay of gm with temperature. The effect of the sidewall channel becomes more prominent for the narrow fin device and leads to two distinct gm peaks. The first peak at negative gate voltage corresponds to the two-dimensional electron gas (2-DEG) channel, while the second peak at positive gate voltage is related to the sidewall MOS channel. Measurements also show that the electrons in 2-DEG channel experience polar-optical-phonon scattering unlike the electrons in the sidewall MOS channel which are mainly subject to Coulomb scattering.

The modulation of multi-domain and harmonic wave in a GaN planar Gunn diode by recess layer

In this paper, a novel structure of a gallium nitride (GaN) planar Gunn diode with isosceles trapezoidal recess layers in the aluminum gallium nitride (AlGaN) barrier layer is proposed to enhance the harmonic components of Gunn oscillation waveforms. Numerical simulations demonstrate that the oscillation frequency rises from 99.1 GHz up to 300.4 GHz with the recess number increasing from one to four, at which the maximum radio frequency (RF) output power and conversion efficiency are obtained. The output performance of the diode enhances at high harmonic frequencies and is mainly due to the multiple recess layer structure that can trigger the formation of multiple Gunn domains in the two-dimensional electron gas channel of the GaN planar Gunn diode. This indicates that the long channel GaN Gunn diode has the ability to output the available RF power associated with the device operating in a submillimeter-wave and terahertz (THz) regime.

Investigation of trap states in Al2O3 InAlN/GaN metal–oxide–semiconductor high-electron-mobility transistors**Project supported by the Program for National Natural Science Foundation of China (Grant Nos. 61404100 and 61306017).

In this paper the trapping effects in Al2O3/In0.17Al0.83N/GaN MOS-HEMT (here, HEMT stands for high electron mobility transistor) are investigated by frequency-dependent capacitance and conductance analysis. The trap states are found at both the Al2O3/InAlN and InAlN/GaN interface. Trap states in InAlN/GaN heterostructure are determined to have mixed de-trapping mechanisms, emission, and tunneling. Part of the electrons captured in the trap states are likely to tunnel into the two-dimensional electron gas (2DEG) channel under serious band bending and stronger electric field peak caused by high Al content in the InAlN barrier, which explains the opposite voltage dependence of time constant and relation between the time constant and energy of the trap states.

AC-Capacitance Techniques for Interface Trap Analysis in GaN-Based Buried-Channel MIS-HEMTs

Effective interface trap characterization approaches are indispensable in the development of gate stack and dielectric surface passivation technologies in III-nitride (III-N) insulated-gate power switching transistors for enhanced stability and dynamic performance. In III-N metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) that feature a buried channel, the polarized barrier layer separates the critical dielectric/III-N interface from the two-dimensional electron gas (2DEG) channel and consequently complicates interface trap analysis. The barrier layer not only causes underestimation/uncertainty in interface trap extraction using conventional ac-conductance method but also allows the Fermi level dipping deep into the bandgap at the pinch-off of the 2DEG channel. To address these issues, we analyze the frequency/temperature dispersions of the second slope in capacitance-voltage characteristics and develop systematic ac-capacitance techniques to realize interface trap mapping in MIS-HEMTs. The correlation between ac-capacitance and pulse-mode hysteresis measurements show that appropriate gate bias need to be selected in the interface trap characterization of MIS-HEMTs, in order to match the time constant of interface traps at the Fermi level with ac frequency and pulsewidth.

Gate-tunable high mobility remote-doped InSb/In1−xAlxSb quantum well heterostructures

Gate-tunable high-mobility InSb/In1−xAlxSb quantum wells (QWs) grown on GaAs substrates are reported. The QW two-dimensional electron gas (2DEG) channel mobility in excess of 200 000 cm2/V s is measured at T = 1.8 K. In asymmetrically remote-doped samples with an HfO2 gate dielectric formed by atomic layer deposition, parallel conduction is eliminated and complete 2DEG channel depletion is reached with minimal hysteresis in gate bias response of the 2DEG electron density. The integer quantum Hall effect with Landau level filling factor down to 1 is observed. A high-transparency non-alloyed Ohmic contact to the 2DEG with contact resistance below 1 Ω·mm is achieved at 1.8 K.

Highly sensitive room temperature infrared hybrid organic-nanocrystal detector

In this paper we demonstrate a very sensitive near-infrared light detector device based on InAs nano crystals (NCs) acting as an optical gate on top of a high-mobility shallow two-dimensional electron gas channel. The ability to integrate electronics with the NCs using both top-down and bottom-up approaches allows to utilize the NCs unique quantum properties for future optoelectronic devices. We employ wet chemistry to self-assemble the organic monolayers and the NCs on top of the field effect transistor. By using shallow and very narrow channel, the device's quantum efficiency can go as high as 106V/W at room temperature, with a signal-to-noise ratio that enables sensitivity for very low photon power. We find that our experimental results are compatible with simulation. Lastly, the route to advance to the single photon detection limit is discussed.

Demonstration of Normally-Off Recess-Gated AlGaN/GaN MOSFET Using GaN Cap Layer as Recess Mask

Based on our proposed self-terminating gate recess etching technique, normally-off recess-gated AlGaN/GaN MOSFET has been demonstrated with a novel method using GaN cap layer (CL) as recess mask, which, as a result, simplifies the device fabrication process and lowers the fabrication cost. The GaN CL is capable of acting as an effective recess mask for the gate recess process, which includes a thermal oxidation for 45 min at 650 °C followed by 4-min etching in potassium hydroxide (KOH) at 70 °C. After gate recess process, no obvious change is observed in terms of the surface morphology of the GaN CL, the contact resistance of the Ohmic contact formed directly on the GaN CL as well as the sheet resistance of the two-dimensional electron gas (2-DEG) channel layer under the GaN CL. The fabricated device exhibits a threshold voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm {th}})$ </tex-math></inline-formula> as high as 5 V, a maximum drain current ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$I_{\mathrm {\rm dmax}})$ </tex-math></inline-formula> of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 200$ </tex-math></inline-formula> mA/mm, a high on/off current ratio of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 10^{10}$ </tex-math></inline-formula> together with a low forward gate leakage current of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 10^{\mathrm {-5}}$ </tex-math></inline-formula> mA/mm. Meanwhile, the OFF-state breakdown voltage ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$V_{\mathrm {br}})$ </tex-math></inline-formula> of the device with gate-drain distance of 6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu $ </tex-math></inline-formula> m is 450 V.

A gate current 1/f noise model for GaN/AlGaN HEMTs

This work presents a theoretical and experimental study on the gate current 1/f noise in AlGaN/GaN HEMTs. Based on the carrier number fluctuation in the two-dimensional electron gas channel of AlGaN/GaN HEMTs, a gate current 1/f noise model containing a trap-assisted tunneling current and a space charge limited current is built. The simulation results are in good agreement with the experiment. Experiments show that, if Vg < Vx (critical gate voltage of dielectric relaxation), gate current 1/f noise comes from the superimposition of trap-assisted tunneling RTS (random telegraph noise), while Vg > Vx, gate current 1/f noise comes from not only the trap-assisted tunneling RTS, but also the space charge limited current RTS. This indicates that the gate current 1/f noise of the GaN-based HEMTs device is sensitive to the interaction of defects and the piezoelectric relaxation. It provides a useful characterization tool for deeper information about the defects and their evolution in AlGaN/GaN HEMTs.

AlGaN/AlN-GaN-SL HEMTs with Multiple 2DEG Channels

We report on a multichannel approach for AlGaN/GaN high-electron-mobility transistors (HEMTs) to increase the carrier mobility in the low and high sheet carrier concentration regimes. Between the AlGaN barrier and GaN buffer, alternating thin layers of AlN and GaN were inserted to create multiple two-dimensional electron gas (2DEG) channels. All samples were grown by metalorganic vapor-phase epitaxy (MOVPE) on sapphire substrates and compared with conventional single-channel HEMT with the same sheet carrier concentration. For low total 2DEG carrier concentrations ( 8 × 1012 cm−2), a mobility improvement is attributed to distribution of the sheet charge into multiple 2DEG channels. This has been found to reduce the carrier concentration per individual channel, which leads to larger distance between 2DEG and heterointerfaces and therefore less impact of interface roughness and alloy scattering. With increasing number of channels, phonon scattering is also reduced due to lowering of the three-dimensional (3D) electron density, which results from the added volume of the multiple 2DEGs. With this approach, the HEMT mobility was increased from 1620 cm2 V−1 s−1 to 1960 cm2 V−1 s−1 for a triple-channel device at sheet carrier concentration of 7.5 × 1012 cm−2.

Ultra-short channel GaN high electron mobility transistor-like Gunn diode with composite contact

We present a numerical analysis on an ultra-short channel AlGaN/GaN HEMT-like planar Gunn diode based on the velocity-field dependence of two-dimensional electron gas (2-DEG) channel accounting for the ballistic electron acceleration and the inter-valley transfer. In particular, we propose a Schottky-ohmic composite contact instead of traditional ohmic contact for the Gunn diode in order to significantly suppress the impact ionization at the anode side and shorten the “dead zone” at the cathode side, which is beneficial to the formation and propagation of dipole domain in the ultra-short 2-DEG channel and the promotion of conversion efficiency. The influence of the surface donor-like traps on the electron domain in the 2-DEG channel is also included in the simulation.

Characteristics of GaN and AlGaN/GaN FinFETs

AlGaN/GaN FinFETs, with high quality atomic layer deposited (ALD) Al2O3 gate dielectric, have been fabricated. The devices have a two-dimensional electron gas (2DEG) channel formed at AlGaN/GaN heterointerface and two sidewall GaN MOS channels. Two distinct transconductance peaks can be observed, one for the 2DEG channel and the other for the sidewall GaN MOS channels. On the other hand, we present heterojunction-free GaN FinFETs with junctionless configuration. The current flows through the volume of the heavily doped GaN fin rather than at the surface channel, which leads to superior off-state performance and less drain-induced virtual substrate biasing (DIVSB) effect.

A unified drain current 1/f noise model for GaN-based high electron mobility transistors

In this work, we present a theoretical and experimental study on the drain current 1/f noise in the AlGaN/GaN high electron mobility transistor (HEMT). Based on both mobility fluctuation and carrier number fluctuation in a two-dimensional electron gas (2DEG) channel of AlGaN/GaN HEMT, a unified drain current 1/f noise model containing a piezoelectric polarization effect and hot carrier effect is built. The drain current 1/f noise induced by the piezoelectric polarization effect is distinguished from that induced by the hot carrier effect through experiments and simulations. The simulation results are in good agreement with the experimental results. Experiments show that after hot carrier injection, the drain current 1/f noise increases four orders of magnitude and the electrical parameter degradation Δgm/gm reaches 54.9%. The drain current 1/f noise degradation induced by the piezoelectric effect reaches one order of magnitude; the electrical parameter degradation Δgm/gm is 11.8%. This indicates that drain current 1/f noise of the GaN-based HEMT device is sensitive to the hot carrier effect and piezoelectric effect. This study provides a useful reliability characterization tool for the AlGaN/GaN HEMTs.