Electron Gas Channel Research Articles

Silicon Substrate Removal of GaN DHFETs for Enhanced (<1100 V) Breakdown Voltage

In this letter, we present a novel approach to enhance the breakdown voltage <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$(V_{\rm BD})$</tex></formula> for AlGaN/GaN/AlGaN double-heterostructure FETs (DHFETs), grown by metal–organic chemical vapor deposition on Si (111) substrates through a silicon-substrate-removal and a layer-transfer process. Before removing the Si substrate, both buffer isolation test structures and DHFET devices showed a saturation of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$V_{\rm BD}$</tex></formula> due to the electrical breakdown through the Si substrate. We observed a <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$V_{\rm BD}$</tex></formula> saturation of 500 V for isolation gaps larger than 6 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> . After Si removal, we measured a <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$V_{\rm BD}$</tex></formula> enhancement of the AlGaN buffer to 1100 V for buffer isolation structures with an isolation gap of 12 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$\mu\hbox{m}$</tex></formula> . The DHFET devices with a gate–drain <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX"> $(L_{\rm GD})$</tex></formula> distance of 15 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> have a <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$V_{\rm BD} &gt; \hbox{1100}\ \hbox{V}$</tex></formula> compared with <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\sim$</tex></formula> 300 V for devices with Si substrate. Moreover, from Hall measurements, we conclude that the substrate-removal and layer-transfer processes have no impact on the 2-D electron gas channel properties.

Characteristics in AlN/AlGaN/GaN Multilayer-Structured High-Electron-Mobility Transistors

A new multilayer-structured AlN/AlGaN/GaN heterostructure high-electron-mobility transistor (HEMT) is demonstrated. The AlN/AlGaN/GaN HEMT exhibits the maximum drain current density of 800 mA/mm and the maximum extrinsic transconductance of 170 mS/mm. Due to the increase of the distance between the gate and the two-dimensional electron-gas channel, the threshold voltage shifts slightly to the negative. The reduced drain current collapse and higher breakdown voltage are observed on this AlN/AlGaN/GaN HEMT. The current gain cut-off frequency and the maximum frequency of oscillation are 18.5 GHz and 29.0 GHz, respectively.

Photocurrent characteristics of two-dimensional-electron-gas-based AlGaN/GaN metal-semiconductor-metal photodetectors

Photocurrent response characteristics of two-dimensional-electron-gas-based AlGaN/GaN metal-semiconductor-metal (MSM) photodetectors were investigated by changing the frequency of incident light signals. At low chopper frequencies, the contribution to the photocurrent is mainly from the GaN bulk layer at 5 V bias, as expected. With increasing chopper frequency, a peak response at 361 nm wavelength becomes more and more pronounced and dominates gradually the photocurrent spectrum, indicating that this peak response has a much less frequency dependence and a faster response rate than the spectral response from the GaN bulk layer. This peak response is attributed to the contribution of photoelectrons generated in the two-dimensional electron gas (2DEG) channel according to simulation results of electrical field distribution and analysis of carrier transport in the 2DEG-based AlGaN/GaN heterostructure. This characteristic makes the 2DEG-based MSM photodetectors a large potential to develop high-speed ultraviolet optoelectronic integrated devices with AlGaN/GaN high electron mobility transistors.

The Roles of Threading Dislocations on Electrical Properties of AlGaN/GaN Heterostructure Grown by MBE

The role played by different types of threading dislocations (TDs) on the electrical properties of AlGaN/GaN heterostructure grown by plasma-assisted molecular beam epitaxy (MBE) was investigated. Samples with different defect structures and densities were prepared and measurements were taken from the same sample to study the correlative behavior of various TDs. From the Hall measurement, the electron mobility in two-dimensional electron gas channel was mainly controlled by the edge dislocation, which has a dominant amount in the material. The edge TDs acted as Coulomb scattering centers inside the channel and reduces the carrier mobility and increased its resistance. Screw TDs played a much significant role than edge TDs in determining the reverse-bias leakage current of Schottky barrier diodes. Leakage current is affected slightly by the reduction of free carrier density in the channel for samples with a higher edge TD density, but screw TD, which acted as the current leakage path, was more deleterious to the reverse-bias leakage current of AlGaN/GaN structure.

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.

Well parameters of two‐dimensional electron gas in Al0.88In0.12N/AlN/GaN/AlN heterostructures grown by MOCVD

AbstractResistivity and Hall effect measurements were carried out as a function of magnetic field (0‐1.5 T) and temperature (30‐300 K) for Al0.88In0.12N/AlN/GaN/AlN heterostructures grown by Metal Organic Chemical Vapor Deposition (MOCVD). Magnetic field dependent Hall data were analyzed by using the quantitative mobility spectrum analysis (QMSA). A two‐dimensional electron gas (2DEG) channel located at the Al0.88In0.12N/GaN interface with an AlN interlayer and a two‐dimensional hole gas (2DHG) channel located at the GaN/AlN interface were determined for Al0.88In0.12N/AlN/GaN/AlN heterostructures. The interface parameters, such as quantum well width, the deformation potential constant and correlation length as well as the dominant scattering mechanisms for the Al0.88In0.12N/GaN interface with an AlN interlayer were determined from scattering analyses based on the exact 2DEG carrier density and mobility obtained with QMSA. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

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.

Ultrafast decay of non‐equilibrium (hot) phonons in GaN‐based 2DEG channels

AbstractFast and ultrafast decay of non‐equilibrium longitudinal optical phonons (hot phonons) is discussed on examples of two‐dimensional electron gas (2DEG) channels confined in nominally‐undoped nitride and selectivelydoped arsenide heterostructures subjected to pulsed dc electric power. At a low–moderate power, a nonmonotonous dependence of the hot‐phonon lifetime on the 2DEG density is found. The fastest decay takes place at ∼2.5 × 1012 cm–2 in GaInAs 2DEG channel and at ∼7 × 1012 cm–2 in GaN 2DEG channel. The result is discussed in terms of plasmon–LO‐phonon resonance. The hot‐phonon lifetime decreases when the supplied power is increased if the 2DEG density exceeds the resonance density. The shortest hot‐phonon lifetime of 60 fs is obtained at/above 50 nW/electron for the lattice‐matched AlInN/AlN/GaN 2DEG channel (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Strain dependent electron drift velocity in Al1–xInxN/AlN/GaN

AbstractWe have measured the room‐temperature velocity–field characteristics in Al1–x Inx N/AlN/GaN strained structures (x = 0.12, 0.15, 0.22) with two‐dimensional electron gas channels biased up to 200 kV/cm using nanosecond‐pulsed current–voltage technique. The drift velocity was estimated under assumptions of uniform electric field and no change in sheet electron density. No velocity saturation was observed. The lowest velocity was obtained for the heterostructure with an In content of x = 0.22. The experimental results are compared with those for lattice‐matched Al1–x Inx N/AlN/GaN, AlGaN/GaN, AlGaN/GaN/AlN/GaN, and silicon‐doped GaN. The conduction band profile, electron wave functions were calculated for the investigated nitride heterostructures through a self‐consistent solution of Schrödinger–Poisson equations. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

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)

HEMT‐compatible lateral field‐effect rectifier using CF4 plasma treatment

AbstractA HEMT‐compatible power lateral field effect rectifier (L‐FER) is reported and fabricated using the CF4 plasma treatment on a GaN‐on‐Si wafer. The power rectifier features a Schottky‐gate‐controlled two‐dimensional electron gas (2DEG) channel between the cathode and anode. By tying up the Schottky gate and anode together, the forward turn‐on voltage (VF, ON) 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 15 μm features a VF, ON of 0.78 V at a current density of 100 A/cm2. This device also exhibits a reverse breakdown voltage (BV) of 460 V at a current level of 1mA/mm and a specific on‐resistance (RON, sp) of 2.3 mΩ·cm2, yielding a figure‐of‐merit (BV2/RON, sp) of 92 MW/cm2. The high temperature characteristics of the L‐FER are also investigated. The measurement results show a robust high temperature operation up to 250 °C. The excellent device performances, together with the lateral device structure and process compatibility with AlGaN/GaN HEMT, provide a low‐cost solution for GaN power integrated circuits. (© 2009 WILEY‐VCH Verlag GmbH &amp; 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.

Increase of Breakdown Voltage on AlGaN/GaN HEMTs by Employing Proton Implantation

The breakdown voltage of new AlGaN/GaN high electron mobility transistors (HEMTs) was increased considerably without sacrificing any other electrical characteristics by proton implantation. The breakdown voltage of proton-implanted AlGaN/GaN HEMTs with 150 KeV 1 times 1014 -cm-2 fluence after thermal annealing at 400 degC for 5 min under N2 ambient was 719 V, while that of conventional device was 416 V. The increase of the breakdown voltage is attributed to the expansion of the depletion region under the 2-D electron gas (2-DEG) channel. The depletion region expanded downward into the GaN buffer layer because implanted protons acted as positive ions and attracted electrons in the 2-DEG channel.

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).

Magneto‐capacitance study of an n‐AlGaAs/GaAs heterojunction supporting a sizable dc current

AbstractThe magneto‐capacitance between the gate and the two‐dimensional electron gas (2DEG) channel has been investigated in a selectively doped n‐AlGaAs/GaAs heterojunction with feeding a sizable current to the 2DEG channel. By measuring and analyzing the differential capacitance Cmeas and longitudinal resistance Rxx, we found that Cmeas includes extrinsic oscillatory components resulting from Rxx and showed how the intrinsic component Cint of the capacitance is extracted from Cmeas. The capacitance Cint was examined as a function of the magnetic field B around BL = 3.75 T, which corresponds to the filling factor ν = 4 of the Landau levels. We found that Cint gets very close to the zero‐field capacitance C0 when the deviation ΔB = |B ‐BL | exceeds ∼ 0.1 T, while Rxx remains zero over a wider range of B (between 3.6 and 3.9 T). This suggests that an incompressible region percolates through the channel and forms a path, even when compressible regions spread over the most region of the sample. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)