2DEG Channel Research Articles

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.

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.

Substrate-Coupled Cross-Talk Effects on an AlGaN/GaN-on-Si Smart Power IC Platform

Cross talks between low-voltage peripheral devices and high-voltage (HV) power devices on an AlGaN/GaN-on-Si smart power IC platform are investigated by monitoring the transfer, output, and transient characteristics of a low-voltage AlGaN/GaN high-electron mobility transistor (HEMT) with an HV bias stress applied to an adjacent isolated electrode. The electric measurement was conducted under two types of substrate termination, i.e., floating and grounded. The substrate termination is shown to be the critical factor in the cross talk. The low-voltage HEMT exhibits significant drain-current degradation with a floating substrate, but remains stable with a grounded substrate. It is determined that a coupling path of the switched HV stress is formed from the HV node to the floating low-resistivity Si substrate, and then from the substrate to the HEMT. The negative net charges in the GaN buffer are caused by the electrons injected from the 2DEG channel. It is found that a grounded substrate is helpful for eliminating the cross-talk effect.

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.

Performance of AlGaN/GaN MISHFET using dual-purpose thin Al2O3 layer for surface protection and gate insulator

In this work, we have investigated a role of a thin Al2O3 layer in AlGaN/GaN MISHFET by characterizing the variation of the sheet resistance of the 2DEG channel layer. The Al2O3 layer, varying the thickness from 0 to 10nm, was utilized as the gate insulator of the device as well as the surface protection layer during RTP for ohmic contact formation. After RTP, the 2DEG channel layer without the Al2O3 layer was rapidly degraded by increasing the sheet resistance of the layer to 1360Ω/□ from the sheet resistance of 400Ω/□ of the as-grown sample. The degradation was still observed even when 1.5nm-thick Al2O3 layer was used. However, the sheet resistances of the devices remained constant with slightly decreased value from that of the as-grown sample when the thickness is larger than 3nm, which indicates that the 3nm-thick Al2O3 layer well protects the AlGaN surface above the 2DEG channel during RTP. The slight decrease in sheet resistance is probably because some acceptor-like states existing at AlGaN surface become neutralized and hence the 2DEG density increases. The Al2O3 layer was not removed for proceeding the fabrication of AlGaN/GaN MISHFET, but rather used as a gate dielectric, which simplifies the device fabrication eliminating the additional deposition steps for the gate dielectric. The threshold voltage of the device, investigated in this work, was increased to the negative direction with increasing the thickness of Al2O3 layers while the transconductance was decreased. The best performances were obtained from the device with 8nm-thick Al2O3 layer, exhibiting very low gate leakage current of 10−9A/mm with subthreshold swing (SS) of 80mV/dec and very high Ion/Ioff ratio (>9 orders).

Modeling of GaN-Based Normally-Off FinFET

In this letter, a macromodel for normally-off (enhancement mode) AlGaN/GaN-based FinFET (2-DEG channel at top with two MOS like sidewall channels) is proposed. AlGaN/GaN-based FinFET devices have improved gate control on the channel due to additional sidewall gates compared with planar structures, but device characteristics exhibit strong nonlinear dependence on fin-width. The proposed model captures both 2-DEG and sidewall channel conduction as well as the fin-width dependency on device characteristics. Model shows excellent agreement with state-of-the-art experimental data.

Two-dimensional electron gases in strained quantum wells for AlN/GaN/AlN double heterostructure field-effect transistors on AlN

Double heterostructures of strained GaN quantum wells (QWs) sandwiched between relaxed AlN layers provide a platform to investigate the quantum-confined electronic and optical properties of the wells. The growth of AlN/GaN/AlN heterostructures with varying GaN quantum well thicknesses on AlN by plasma molecular beam epitaxy (MBE) is reported. Photoluminescence spectra provide the optical signature of the thin GaN QWs. Reciprocal space mapping in X-ray diffraction shows that a GaN layer as thick as ∼28 nm is compressively strained to the AlN layer underneath. The density of the polarization-induced two-dimensional electron gas (2DEG) in the undoped heterostructures increases with the GaN QW thickness, reaching ∼2.5 × 1013/cm2. This provides a way to tune the 2DEG channel density without changing the thickness of the top barrier layer. Electron mobilities less than ∼400 cm2/Vs are observed, leaving ample room for improvement. Nevertheless, owing to the high 2DEG density, strained GaN QW field-effect transistors with MBE regrown ohmic contacts exhibit an on-current density ∼1.4 A/mm, a transconductance ∼280 mS/mm, and a cut off frequency fT∼104 GHz for a 100-nm-gate-length device. These observations indicate high potential for high-speed radio frequency and high voltage applications that stand to benefit from the extreme-bandgap and high thermal conductivity of AlN.

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.

Au-Free Normally-Off AlGaN/GaN-on-Si MIS-HEMTs Using Combined Partially Recessed and Fluorinated Trap-Charge Gate Structures

In this letter, partially recessed gate structures in conjunction with negative trap charges by F <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> plasma treatments both at AlGaN barrier and on gate dielectric surface are employed to realize the normally-OFF operation for AlGaN/GaN Metal-Insulator-Semiconductor High Electron Mobility Transistors in Au-free scheme. A partial gate recessed trench is designed to effectively reduce the 2-D electron gas (2DEG) density and achieve positive threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ) without severe degradation in 2-DEG channel mobility. Furthermore, the fixed trap charges are innovatively placed at the gate AlGaN and Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> layers by a two-stage F <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> plasma treatment to further increase the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> , without mobility degradation. A high V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> of 1.9 V and a drain current ~200 mA/mm are achieved in the fabricated device, which also has a lower leakage current and the higher breakdown voltage of 580 V.

Effects of Gate Field Plates on the Surface State Related Current Collapse in AlGaN/GaN HEMTs

During off-state, the influence of surface-trapped electron charges induced by high-field stress near the gate electrode of AlGaN/GaN power high-electron mobility transistor devices causes a reduction in two-dimensional electron gas (2DEG) carrier density at the heterointerface. In a pulse turn-on operation, the weakened 2DEG channel results in a higher on-state conduction resistance during the transient, known as the current collapse phenomenon. The phenomenon increases the switching loss by a higher on-state resistance and prolonged turn-on transition time, thus limits the device operating frequency range. In this paper, such a phenomenon is modeled, analyzed by Sentaurus TCAD simulation, and verified by the laboratory measurement data, with the emphasis on the influence of field plates toward the current collapse. The spatial distributions of trapped electrons and excess free electrons along the AlGaN surface are modeled and analyzed to arrive at the quantitative relationships among the trapped electron density, on-resistance increase, and the electric field distribution which provide a reliable criterion for current collapse reduction. It was found that, with a proper field plate design, it is possible to achieve an improvement on transient on-state resistance and the current recovery time.

Silicon Nitride Thickness Dependent Electrical Properties of InAlN/GaN Heterostructures

Introduction Lattice-matched InAlN/GaN/Si based high electron mobility transistors (HEMTs) have recently attracted much attention as a candidate for next generation high-power, high-temperature and high-frequency devices, due to their higher charge density than the conventional AlGaN/GaN heterostructure [1]. However, HEMT performance, reliability, integrity and robustness are determined by the contact materials employed and passivation. Various dielectrics, such as SiO2, Si3N4, AlN, MgO, Al2O3, HfO2, Sc2O3, etc have been investigated for passivation. Among them, Si3N4 is the most widely used. Extensive improvement in electrical properties of the AlGaN/GaN/Si heterostructure achieved by the SiN x passivation process has been reported [2]. Such improvements by SiN x passivation were believed to be due to the reduction of the surface states, change in the AlGaN potential barrier height or increase of the 2DEG density at the channel due to tensile stress [3-5]. In spite of various reports on SiN x pasivation of InAlN/GaN heterostructures, SiN x thickness dependent studies have not been extensively explored to date. In the present study, we report SiN x thickness dependent electrical properties and its effect on the device performance of InAlN/GaN heterostructures. Experiment Nearly lattice-matched InAlN/GaN/Si epi-wafer, with a sheet resistance of 348.9 Ω/sq, was used in the present study. Prior to ohmic contact formation, InAlN/GaN/Si substrate surface was ultrasonically cleaned with acetone, IPA and DI water. Hf (15)/Al (200)/Ta (20) nm ohmic contacts were deposited by sputtering followed by rapid thermal annealing in vacuum at 600 oC for 1 min. No surface treatment was performed prior to passivation. Passivation SiN x layer of varying thickness ranging from 25 to 200 nm was deposited at 300°C by PECVD using silane and ammonia as precursor gases. SiN x layer on the ohmic electrodes was removed by etching in buffered hydrofluoric acid. To characterize our samples, room-temperature Hall measurements using van der Pauw geometry and ultraviolet (UV) Photoluminescence (PL) spectra excited using 266 nm laser source were performed. Results and Discussion Figure 1 shows the room-temperature Hall measurements of InAlN/GaN/Si as a function of the passivation SiN x layer thickness. Increase in sheet carrier concentration (ns), whereas decrease in mobility (μ) is observed with increasing SiN x passivation layer thickness. A thickness of ~100 nm can be viewed as optimum as it yields a minimum sheet resistance (Rs), as shown in Fig. 1. The corresponding increase in ns is substantial of almost ~30%, with respect to sample without SiN x passivation. Although the reason for the significant increase in ns is not apparent, yet our preliminary studies by means of UV PL reveal a shift in the PL peak position of the GaN channel layer (Fig. 2), which is most likely due to band bending under stress. However, possibility of reduction of surface states of the passivated InAlN and/or an increase in positive charge at the SiN x /InAlN interface cannot be ruled out. Additionally, the decrease in μ is possibly due to increased electron-electron scattering and/or interface roughness scattering with such a high carrier concentration in 2DEG channel. Interestingly, the trend of increase in 2DEG density is similar to that observed for AlGaN/GaN heterostructure under different SiN x passivation layer thickness [6]. This observation indicates that the intrinsic property of SiN x film may be playing a critical role in the enhancement of 2DEG density rather than the interface property. Further studies to establish the mechanisms responsible for the enhancement in 2DEG density as well as to evaluate the influence of passivation layer thickness on the device performance are in progress. Conclusion In conclusion, surface passivation of InAlN/GaN heterostructures with PECVD SiN x of varying thickness has been investigated. A thickness of ~ 100 nm is found to be optimum and has yielded a minimum sheet resistance. The corresponding 2DEG density change is substantially higher (by ~30 %) than sample without SiN x passivation, and this enhancement is attributed possibly to band bending under stress.

Enhancement of AlGaN/GaN High Electron Mobility Transistor Off-State Drain Breakdown Voltage via Backside Proton Irradiation

The demand for radiation-resistant electronic devices spans multiple disciplines such as radar and communication systems, aerospace, nuclear design. For space applications, the most important type of particles is protons with energies up to hundreds of MeV. Early experiments performed that the radiation tolerance of AlGaN/GaN is much higher than for their AlGaAs/GaAs counterparts. There are many rad-hardness studies have been carried out using protons of various energies on AlGaN/GaN HEMTs. Typically, irradiated HEMTs exhibited degradations on drain saturation current (IDS) and transconductance (gm). Recently, it was reported that the off-state breakdown voltage (VBR) and the critical voltage during the off-state drain-voltage step-stress improved for the proton irradiated HEMTs. The defects created by the high energy proton irradiation were evenly distributed throughout the entire HEMT structure. Thus, it would be very difficult to identify the mechanism of drain breakdown voltage and step-stress critical voltage improvement. In this work, protons were irradiated from the backside of the HEMT structure through via holes fabricated on Si substrate, which was used as the substrate to grow the HEMT structure. DC performance before and after irradiation was performed and electric field distribution around the gate electrode was simulated with FLOODs and TCAD. The AlGaN/GaN HEMT structure was grown by MOCVD on top of Si wafers. The layer structure included an AlN nucleation layer, followed with a series of composition-graded AlxGa1-xN transition layers to reduce the strain, an 800 nm GaN buffer layer and capped with a 16 nm unintentionally-doped Al0.26Ga0.74N barrier layer. Device fabrication started with Ohmic contacts formation by lifting-off e-beam evaporated Ti (200 Å)/Al (1000 Å)/Ni (400 Å)/Au (800 Å). The contacts were annealed at 850°C for 45 s under N2. Device isolation was achieved with multiple-energy and multiple-dose of N+implantations. 70 nm silicon nitride deposited by PECVD was used for device passivation. 1-µm gates were defined by lift-off of e-beam deposited Ni/Au metallization. The wafer front-side process was finished with e-beam evaporated Ti/Pt/Au (300 Å/300 Å /2500 Å) interconnection contacts.The ranges of proton irradiation energy and dose were 225 to 340 keV and 1×1012 cm-2 to 5×1012 cm-2, respectively. Prior to the proton implantation, rectangular via holes were formed to remove the Si under GaN HEMT structure by etching through the Si substrate from the back side of the sample with a standard BOSCH process in a STS deep reactive ion etching system using AZ9260 photoresist as the etching mask. Figure 1 shows the SEM picture of the rectangular via holes created on Si substrate. Those via holes were formed directly beneath HEMT active area.Stopping and Range of Ions in Matter (SRIM) was used to simulate proton penetration depths and vacancies generated by the implantations. Figure 2 illustrates the vacancy distribution. The tail of the vacancy distribution for the back side implantation is sharper, thus defects, mainly gallium vacancies, can be precisely put in the AlGaN transistor layer, GaN buffer layer or GaN/AlGaN 2 dimension electron gas (2DEG) channel layer. The effect of proton irradiation on Ids, gm, VBR is listed in Table I. The VBR improvement was only found at the condition of proton energy equals to 340keV, in which the defects were created at 2DEG region. Moreover, VBR improvement irradiation with 340keV and 1×1012 /cm2was not observed and indicating a certain amounts of defects at 2DEG region needed to form virtual gate to have the improvement. FLOODS and TCAD simulation tools were also used to model the electric field distribution around the gate region. These simulation results will be presented.

Enhancement of AlGaN/GaN High Electron Mobility Transistor Off-State Drain Breakdown Voltage via Backside Proton Irradiation

Proton irradiations from the backside of the AlGaN/GaN high electron mobility transistors (HEMTs) were employed to enhance its off-state drain breakdown voltage. Via holes were fabricated directly under the active area of the HEMTs by etching through the Si substrate for the backside irradiations. The steep drop at the end of proton energy loss profile allows radiation-induced defects to be placed at precise locations within the AlGaN/GaN HEMT structure. Neither degradation of drain current nor enhancement of off-state drain voltage breakdown voltage was observed for the HEMTs irradiated with the proton energy of 225 or 275 keV, for which the defects were put in the GaN buffer. HEMTs with defects placed in the two dimensional electron gas (2DEG) channel region and the AlGaN barrier, using 330 or 340 keV irradiations, not only showed degradations on drain current and extrinsic transconductance but also demonstrated improvements of the off-state drain breakdown voltage.

Enhancement of AlGaN/GaN high electron mobility transistors off-state drain breakdown voltage via backside proton irradiation

Proton irradiation from the backside of the samples were employed to enhance off-state drain breakdown voltage of AlGaN/GaN high electron mobility transistors (HEMTs) grown on Si substrates. Via holes were fabricated directly under the active area of the HEMTs by etching through the Si substrate for subsequent backside proton irradiation. By taking the advantage of the steep drop at the end of proton energy loss profile, the defects created by the proton irradiation from the backside of the sample could be precisely placed at specific locations inside the AlGaN/GaN HEMT structure. There were no degradation of drain current nor enhancement of off-state drain voltage breakdown voltage observed for the irradiated AlGaN/GaN HEMTs with the proton energy of 225 or 275 keV, for which the defects created by the proton irradiations were intentionally placed in the GaN buffer. HEMTs with defects placed in the two dimensional electron gas (2DEG) channel region and AlGaN barrier using 330 or 340 keV protons not only showed degradation of both drain current and extrinsic transconductance but also exhibited improvement of the off-state drain breakdown voltage. The Florida Object Oriented Device and Process Simulator Technology Computer Aided Design finite-element simulations were performed to confirm the hypothesis of a virtual gate formed around the 2DEG region to reduce the peak electric field around the gate edges and increase the off-state drain breakdown voltage.

Enhancement-Mode GaN-Based High-Electron Mobility Transistors on the Si Substrate With a P-Type GaN Cap Layer

An enhancement-mode (E-mode) high-electron mobility transistor (HEMT) was demonstrated by inserting a p-type GaN layer underneath the gate electrode. The effects of process flows and device structures on the electrical properties are investigated in this paper. We demonstrated a threshold voltage (Vth) of 4.3 V by adjusting the built-in voltage of the diode formed between the p-GaN and channel by the alloy temperature. Next, we found the existence of parallel conduction paths of the p-GaN layer and 2-D electron gas (2DEG) channel in such a HEMT structure. By removing p-GaN above the gate-source and gate-drain regions, current conduction migrates from p-GaN to 2DEG channel. The process window of the p-GaN residual thickness to ensure a steady forward current-voltage operation was estimated to be 10±5 nm in our case. Finally, with the p-GaN underneath the gate contact to deplete surface leakage current, an E-mode HEMT with a breakdown voltage (VBD) of 1630 V is achieved.

Investigation of buffer traps in AlGaN/GaN-on-Si devices by thermally stimulated current spectroscopy and back-gating measurement

Thermally stimulated current (TSC) spectroscopy and high-voltage back-gating measurement are utilized to study GaN buffer traps specific to AlGaN/GaN lateral heterojunction structures grown on a low-resistivity Si substrate. Three dominating deep-level traps in GaN buffer with activation energies of ΔET1 ∼ 0.54 eV, ΔET2 ∼ 0.65 eV, and ΔET3 ∼ 0.75 eV are extracted from TSC spectroscopy in a vertical GaN-on-Si structure. High back-gate bias applied to the Si substrate could influence the drain current in an AlGaN/GaN-on-Si high-electron-mobility transistor in a way that cannot be explained with a simple field-effect model. By correlating the trap states identified in TSC with the back-gating measurement results, it is proposed that the ionization/deionization of both donor and acceptor traps are responsible for the generation of buffer space charges, which impose additional modulation to the 2DEG channel.

500 °C operation of AlGaN/GaN and AlInN/GaN Integrated Circuits

High-temperature technology platform has been developed utilizing planar III-nitride heterostructures approach. The record high electron concentration and mobility in 2DEG channel of III-nitride devices result in very high operation speed and are remarkably stable within a broad temperature range, allowing device operation above 500 °C. The developed IC technology is based on three key elements: (1) exceptional quality III-nitride heterostructure with very high carrier concentration and mobility that enables IC fast operation in a broad temperature range; (2) heterostructure field effect transistor approach that provides fully planar IC structure which is easy to scale and to combine with the other high temperature electronic components; (3) robust design with self-compensating 2DEG load resistors, advance metallization and high-k passivation/gate dielectrics, specially developed for high temperature operation. The feasibility of technology was demonstrated by modeling, design and fabrication of inverter and differential amplifier type of ICs using III-nitride heterostructures. IC's performance was studied using probe station with heating chuck in ambient atmosphere. Temperature stability of structures with various barrier compositions was compared. At temperature exceeding 500 °C the developed ICs show the leakage currents below 10−7A, unit-gain bandwidth above 1 MHz and internal response time 45 ns.

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.

Compact Charge-Based Physical Models for Current and Capacitances in AlGaN/GaN HEMTs

This paper presents physics-based compact models for the C-V and I-V characteristics of AlGaN/GaN HEMT devices. The contribution of only the first energy level in the triangular quantum well at the AlGaN/GaN interface (where most of the charge carriers of the 2-DEG channel reside) is considered, which resulted in an accurate and simple unified charge control model. Based on this, analytical models of the drain current, the gate charge, and the gate capacitances have been developed. The models cover all the different operating regimes of a device. The excellent agreements between the model and measured C-V and I-V characteristics of devices with different gate lengths have demonstrated the validity of the model.

Electric field modulation technique for high-voltage AlGaN/GaN Schottky barrier diodes

A novel structure of AlGaN/GaN Schottky barrier diode (SBD) featuring electric field optimization techniques of anode-connected-field-plate (AFP) and magnesium-doped p-type buried layer under the two-dimensional electron gas (2DEG) channel is proposed. In comparison with conventional AlGaN/GaN SBDs, the magnesium-doped p-type buried layer in the proposed structure can provide holes that can help to deplete the surface 2DEG. As a result, surface field strength around the electrode edges is significantly suppressed and the electric field along the channel is distributed more evenly. Through 2D numerical analysis, the AFP parameters (field plate length, LAFP, and field plate height, TAFP) and p-type buried layer parameters (p-type layer concentration, NP, and p-type layer thickness, TP) are optimized to achieve a three-equal-peak surface channel field distribution under exact charge balance conditions. A novel structure with a total drift region length of 10.5 μm and a magnesium-doped p-type concentration of 1 × 1017 cm−3 achieves a high breakdown voltage (VB) of 1.8 kV, showing 5 times improvement compared with the conventional SBD with the same device dimension.