Two-dimensional Electron Gas Channel Research Articles

Effects of illumination on the excess carrier dynamics and variations of the surface states in an AlGaN/GaN heterostructure

Light illumination was observed to be able to change the charge status of surface states in an AlGaN/GaN heterostructure and vary the conductivity of the two-dimensional electron gas channel. The effects of several parameters, including sample temperature, intensity of the ultraviolet (UV) light, wavelength, and intensity of the laser light, were investigated and mathematically analyzed. A physical model that described the dynamics of excess carriers to different light illumination was proposed. Excess carriers created by UV light are able to reach the surface states after overcoming the valence and conduction band discontinuities by absorbing thermal energy and energy of the subsequently incident photon, respectively. Understanding the effect of the ambient illumination can help to achieve a higher sensitivity for an AlGaN/GaN sensor and discover new applications in the future.

Sub-Bandgap Laser Light-Induced Excess Carrier Transport Between Surface States and Two-Dimensional Electron Gas Channel in AlGaN/GaN Structure

Variations of the channel resistance of a AlGaN/GaN high electron mobilities transistor caused by subsequently incident photons with sub-bandgap energies after ultraviolet light-induced changes became stable were studied. Temperature-dependent measurements yielded a 0.342 eV thermal activation energy and wavelength-varying measurements yielded a 0.8 eV cut-off photon energy. The ratio between the two values is very close to the theoretical value of the ratio between the valence and conduction band discontinuities. A qualitative description about the transports of excess carriers through the band discontinuities, is also proposed and consistent with the experimental results.

Plasmon-enhanced heat dissipation in GaN-based two-dimensional channels

Decay of nonequilibrium longitudinal optical (LO) phonons is investigated at room temperature in two-dimensional electron gas channels confined in nearly lattice-matched InAlN/AlN/GaN structures. A nonmonotonous dependence of the LO-phonon lifetime on the supplied electric power is reported for the first time and explained in terms of plasmon–LO-phonon resonance tuned by applied bias at a fixed sheet density (8×1012 cm−2). The shortest lifetime of 30±15 fs is found at the power of 20±10 nW/electron.

Various Recipes of SiNx Passivated AlGaN/GaN High Electron Mobility Transistors in Correlation with Current Slump

The current slump of different recipes of SiNx passivated AlGaN/GaN high electron mobility transistors (HEMTs) is investigated. The dc and pulsed current-voltage curves of AlGaN/GaN HEMTs using different recipes are analyzed. It is found that passivation leakage has a strong relationship with NH3 flow in the plasma-enhanced chemical vapor phase deposition process, which has impacted on the current collapse of SiNx passivated devices. We analyze the pulsed IDS – VDS characteristics of different recipes of SiNx passivation devices for different combinations of gate and drain quiescent biases (VGS0, VDS0) of (0, 0), (−6, 0), (−6, 15) and (0, 15)V. The possible mechanisms are the traps in SiNx passivation capturing the electrons and the surface states at the SiNx/AlGaN interface, which can affect the channel of two-dimensional electron gas and cause the current collapse.

Direct contact mechanism of Ohmic metallization to AlGaN/GaN heterostructures via Ohmic area recess etching

The effects of recess etching of the Ohmic contact area on the performance of Ti/Al/Mo/Au metallization and its interfacial reactions with AlGaN/AlN/GaN epilayers were investigated. The best Ohmic contact performances of 0.31, 0.41, and 0.26 Ω mm for three epilayers from two different sources were obtained only when the two-dimensional electron gas (2DEG) channels were completely removed under the Ohmic contact metallization. This is due to the direct sideway contact made by the electrode to the 2DEG around the edges of the active-layer mesas or pads; this is believed to be a more efficient carrier transport mechanism than tunneling through the AlGaN barrier.

Oxygen Plasma Treated Aluminum as a Gate Dielectric for AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN high electron mobility transistors (HEMTs) grown on 4 in. silicon substrates were demonstrated using different treatments at the Schottky gate terminal, namely (i) a regular Schottky contact made of Pd/Ti/Au (HEMT), (ii) an Al-based metal-oxide-semiconductor high electron mobility transistor (MOS-HEMT) using 3 nm Al/Pd/Ti/Au, and (iii) a plasma oxidized 3 nm Al/Pd/Ti/Au (plasma treated MOS-HEMT). A high drain current density of 1086 mA/mm and a transconductance of 209 mS/mm were obtained for Al-based MOS-HEMTs. In the process of plasma oxidized Al, the aluminum-based oxide formed was identified as by X-ray photoelectron spectrum measurements. For HEMTs with regular Schottky contact, a gate bias beyond leads to excess gate leakage limiting the gate bias application. However, for the MOS-HEMTs, a gate bias as high as could be applied without any gate leakage. Further, a low two-terminal gate leakage was observed for our MOS-HEMTs. A two-dimensional electron gas channel at an increased depth was observed for the MOS-HEMTs because of an insulating oxide layer at the Schottky contact. The out-diffusion of oxide states at the AlGaN barrier layer into thin Al facilitates the formation of as a gate dielectric for AlGaN/GaN HEMTs.

InAlN-barrier HFETs with GaN and InGaN channels

Advantages and drawbacks of InN-containing GaN-based heterostructure field-effect transistors (HFETs) are reviewed. The main attention is paid to fundamental processes and experimental observations associated with hot-electron transport in two-dimensional electron gas (2DEG) channels subjected to high electric fields. Complementary information is obtained from fluctuations. When interface roughness and alloy scattering are minimized, an important transport limitation originates from additional scattering of hot electrons by non-equilibrium longitudinal optical (LO) phonons (hot phonons) governed by the hot-phonon temperature and the hot-phonon decay into acoustic phonons. The ultrafast decay is observed for high supplied power where the hot-phonon lifetime of ∼60 fs is found. The optimal 2DEG density for the fastest decay of hot phonons is estimated and associated with LO-phonon–plasmon resonance. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of Fringing Effect on Resonant Plasma Frequency in Plasma Wave Devices

The effect of fringing electric field due to the nonideality of the gate two-dimensional electron gas (2DEG) channel capacitance on the fundamental resonant frequency of plasma waves in a high-electron-mobility transistor (HEMT) channel was investigated. The spatial distribution of sheet electron density in the fringed region of the 2DEG channel and the expression for the fundamental resonant frequency of plasma oscillation were obtained. A cascaded transmission line (TL) equivalent circuit model was developed to represent the gated and ungated fringed regions of the HEMT 2DEG channel. Results of calculation and IsSpice simulation show that fringing effects can be the cause of the fundamental plasma frequency reduction. Thus, the developed fringing effect model improves the deviation between theoretical and experimental data.

High breakdown voltage AlGaN/GaN HEMTs employing fluoride plasma treatment

We proposed channel-modulated AlGaN/GaN high electron mobility transistors (HEMTs) employing fluoride plasma treatment and carried out a detailed numerical simulation of device operation using ISE TCAD software. A reduction of peak electric field is required for achieving the high breakdown voltage of AlGaN/GaN HEMTs. The proposed channel-modulated AlGaN/GaN HEMTs created the fluoride plasma-treated region between the gate and the drain. The fluoride plasma-treated region modulated the two-dimensional electron gas (2-DEG) channel concentration, and effectively reduced the peak electric field without any field plates. The reduction of peak electric field in the gate-to-drain region caused the breakdown voltage to increase by 206%. The dc characteristics of channel-modulated AlGaN/GaN HEMTs remained practically the same as those of the conventional AlGaN/GaN HEMTs. The breakdown voltage could be increased without a significant degradation of dc characteristics due to the SiO2 passivation layer. The deposition of the SiO2 passivation layer combined with fluoride plasma treatment is a powerful process for channel-modulated AlGaN/GaN HEMTs.

A simple parallel conduction extraction method (SPCEM) for MODFETs and undoped GaN-based HEMTs

We report a simple method to extract the mobility and sheet carrier densities of conduction channels in conventional modulation doped field-effect transistor (MODFET) structures and unintentionally doped GaN-based high-electron mobility transistor (HEMT) structures for a special case. Extraction of the conduction channels from the magnetic field-dependent data can present number of problems; even the most recent methods encounter great difficulties. For the GaN-based HEMT structures which have lower mobilities and larger effective masses than that of GaAs-based counterparts, these difficulties become more prominent. In this study, we describe a simple method for magnetotransport analysis to extract conduction channels successfully for a special case that is commonly encountered: one bulk channel and one two-dimensional electron gas (2DEG) channel. Advantage of this method is mainly its simplicity. The analysis can be done with only two magnetic field-dependent measurements per temperature step. The method is applied to the magnetotransport results of an unintentionally doped AlGaN/AlN/GaN/AlN heterostructure over a temperature range of 29–350K and in a magnetic field range of 0–1.5T (μB<1). The results are then compared with those obtained using a commercial package for these calculations namely: quantitative mobility spectrum analysis (QMSA).

High-performance AlGaN∕GaN lateral field-effect rectifiers compatible with high electron mobility transistors

A high electron mobility transistor (HEMT)-compatible power lateral field-effect rectifier (L-FER) with low turn-on voltage is demonstrated using the same fabrication process as that for normally off AlGaN∕GaN HEMT, providing a low-cost solution for GaN power integrated circuits. The power rectifier features a Schottky-gate-controlled two-dimensional electron gas channel between the cathode and anode. By tying up the Schottky gate and anode together, the forward turn-on voltage of the rectifier is determined by the threshold voltage of the channel instead of the Schottky barrier. The L-FER with a drift length of 10μm features a forward turn-on voltage of 0.63V at a current density of 100A∕cm2. This device also exhibits a reverse breakdown voltage (BV) of 390V at a current level of 1mA∕mm and a specific on resistance (RON,sp) of 1.4mΩcm2, yielding a figure of merit (BV2∕RON,sp) of 108MW∕cm2. The excellent device performance, coupled with the lateral device structure and process compatibility with AlGaN∕GaN HEMT, make the proposed L-FER a promising candidate for GaN power integrated circuits.

Hot-electron energy relaxation time in AlInN/AlN/GaN 2DEG channels

A microwave noise technique has been used for experimental investigation, at room temperature, of power dissipation in the voltage-biased two-dimensional electron gas channel located in the GaN layer of a lattice-matched Al0.82In0.18N/AlN/GaN heterostructure. No saturation of the relaxation time is found in the investigated electron temperature range up to ∼2800 K: the hot-electron energy relaxation time decreases from ∼6 ps at near equilibrium to 75 ± 20 fs at ∼200 nW/electron. The electron drift velocity reaches ∼1.8 × 107 cm s−1 at 65 kV cm−1 electric field. The hot-phonon effect on power dissipation is discussed.

AlGaN/GaN High Electron Mobility Transistors Irradiated with 17 MeV Protons

AlGaN/GaN high electron mobility transistors (HEMTs) with GaN cap layers were irradiated with high-energy protons to doses up to . There was no significant degradation in dc electrical parameters such as drain–source current and extrinsic transconductance of the HEMTs up to a fluency of . At the highest dose of there was a decrease of in and a decrease in . The data are consistent with the introduction of defect centers in the HEMT structure by the high-energy protons leading to a reduction in carrier concentration and mobility in the two-dimensional electron gas channel. These results show that AlGaN/GaN HEMTs are attractive for space-based applications where high-energy proton fluxes are present.

Plasma mechanisms of resonant terahertz detection in a two-dimensional electron channel with split gates

We analyze the operation of a resonant detector of terahertz (THz) radiation based on a two-dimensional electron gas (2DEG) channel with split gates. The side gates are used for the excitation of plasma oscillations by incoming THz radiation and control of the resonant plasma frequencies. The central gate provides the potential barrier separating the source and drain portions of the 2DEG channel. Two possible mechanisms of the detection are considered: (1) modulation of the ac potential drop across the barrier and (2) heating of the 2DEG due to the resonant plasma-assisted absorption of THz radiation followed by an increase in thermionic dc current through the barrier. Using the device model, we calculate the frequency and temperature dependences of the detector responsivity associated with both dynamic and heating (bolometric) mechanisms. It is shown that the dynamic mechanism dominates at elevated temperatures, whereas the heating mechanism provides larger contribution at low temperatures, T≲35−40 K.

Spin interference controlled by electric field: Ensemble average effect on AAS and AB oscillation amplitudes

Time reversal symmetric Al’tshuler–Aronov–Spivak (AAS) oscillations are measured in an array of InGaAs mesoscopic loops. We confirm that gate voltage dependence of h/2 e period oscillations is due to spin interference from the effect of ensemble average on the AAS and Aharonov–Bohm (AB) amplitudes. This spin interference is based on the time reversal Aharonov–Casher (AC) effect. The AC interference oscillations are controlled over several periods. This result shows evidence for electrical manipulation of the spin precession angle in an InGaAs two-dimensional electron gas channel. We control the precession rate in a precise and predictable way with an electrostatic gate.

Characterization of deep levels in high electron mobility transistor by conductance deep level transient spectroscopy

The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN high electron mobility transistor (HEMT) on silicon (111) substrate is profited to investigate traps that are located between the substrate and the two-dimensional electron gas (2DEG) channel. The transient of the drain current after applying a negative substrate voltage is evaluated in the temperature range from 77 to 600 K. With this method, known as Conductance Deep Level Transient Spectroscopy (CDLTS), majority deep levels with activation energy of 61 meV as well as minority carrier traps at 74 meV and capture cross-section respectively 2.56 × 10 − 15 cm 2, 2.1 × 10 − 15 cm 2 are identified. Finally, the correlation between the anomalies observed on the output characteristics and defects is discussed.

Analysis of plasma oscillations in high-electron mobility transistorlike structures: Distributed circuit approach

We develop simple distributed circuit model of the high-electron mobility transistor (HEMT)-like structure for the analysis of the effects associated with plasma oscillations excited in its two-dimensional electron gas (2DEG) channel. Circuit components of the model are related to physical and geometrical parameters of the structure. Developed model accounts for dependence of resistance and inductance of 2DEG channel gated region on gate voltage. Such an approach facilitates and improves understanding of HEMT-like structures’ behavior in the regime of excitation of plasma oscillation and is applicable for their performance evaluation and optimization as well.

Time reversal Aharonov–Casher effect using Rashba spin–orbit interaction

We propose a spin interferometer using Rashba spin–orbit interaction. A spin interference effect is demonstrated in small arrays of mesoscopic InGaAs rings. This spin interference is the time reversal Aharonov–Casher (AC) effect. The AC interference oscillations are controlled over several periods. This result shows evidence for electrical manipulation of the spin precession angle in an InGaAs two-dimensional electron gas channel. We control the precession rate in a precise and predictable way with an electrostatic gate.

N-polar GaN∕AlGaN∕GaN high electron mobility transistors

We describe the development of N-polar GaN-based high electron mobility transistors grown by N2 plasma-assisted molecular beam epitaxy on C-face SiC substrates. High mobility AlGaN∕GaN modulation-doped two-dimensional electron gas channels were grown, and transistors with excellent dc and small-signal performance were fabricated on these wafers. Large-signal dispersion was observed, and the trap states responsible for this were identified, and layer designs to remove the dispersive effects of these traps were demonstrated. Finally, an AlGaN-cap layer was used to reduce gate leakage in these devices, and a low-dispersion high breakdown voltage device was achieved. This detailed study of dispersion and leakage in N-polar GaN-based transistors establishes a technological base for further development of field effect devices based on N-polar III-nitrides.

Characterization of Non-Alloyed Ohmic Contacts to Si-Implanted AlGaN/GaN Heterostructures for High-Electron Mobility Transistors

This paper reports results of a study of non-alloyed ohmic contacts on Si-implanted AlGaN/GaN heterostructures, obtained from current–voltage characteristics of transfer-length method (TLM) test structures. It is shown that the measured contact resistance from the Ti/Au/Ni metal contacts, deposited on Si-implanted regions, to the two-dimensional electron gas channel at the AlGaN/GaN heterointerface of the non-implanted region, is formed by three different components: (i) contact resistance between the metal␣and the semiconductor (0.60 ± 0.16 Ω mm), (ii) resistance of the implanted region (0.62 ± 0.03 Ω mm) and (iii) an additional resistance (0.72 ± 0.24 Ω mm) giving a total value of 1.9 ± 0.3 Ω mm. The specific ohmic contact resistance was determined to be (2.4 ± 0.5) × 10−5 Ω cm2.