Operation Of High Electron Mobility Transistors Research Articles

Study of Acoustic Emission from the Gate of Gallium Nitride High Electron Mobility Transistors

Nitrides are the leading semiconductor material used for the fabrication of high electron mobility transistors (HEMTs). They exhibit piezoelectric properties, which, coupled with their high mechanical stiffness, expand their versatile applications into the fabrication of piezoelectric devices. Today, due to advances in device technology that result in a reduction in the size of individual transistor elements and due to increased structural complexity (e.g., multi-gate transistors), the integration of piezoelectric materials into HEMTs leads to an interesting occurrence, namely acoustic emission from the transistor gate due to piezoelectric effects. This could affect the device’s performance, reliability, and durability. However, this phenomenon has not yet been comprehensively described. This paper aims to examine this overlooked aspect of AlGaN/GaN HEMT operation, that is, the acoustic emission from the gate region of the device induced by piezoelectric effects. For this purpose, dedicated test structures were designed, consisting of two narrow 1.7 μm-wide metallization strips placed at distances ranging from 5 μm to 200 μm fabricated in AlGaN/GaN heterostructures to simulate and examine the gate behavior of the HEMT transistor. For comparison, the test device structures were also fabricated on sapphire, which is not a piezoelectric material. Measurements of acoustic and electrical interactions in the microwave range were carried out using the “on wafer” method with Picoprobe’s signal–ground–signal (SGS)-type microwave probes. The dependence of reflectance |S11| and transmittance |S21| vs. frequency was investigated, and the coupling capacitance was determined. An equivalent circuit model of the test structure was developed, and finite element method simulation was performed to study the distribution of the acoustic wave in the nitride layers and substrate for different frequencies using Comsol Multiphysics software. At frequencies up to 2–3 GHz, the formation of volume waves and a surface wave, capable of propagating over long distances (in the order of tens of micrometers) was observed. At higher frequencies, the resulting distribution of displacements as a result of numerous reflections and interferences was more complicated. However, there was always the possibility of a surface wave occurrence, even at large distances from the excitation source. At small gate distances, electrical interactions dominate. Above 100 µm, electrical interactions are comparable to acoustic ones. With further increases in distance, weakly attenuated surface waves will dominate.

Theoretical Prediction of Mobility Improvement in GaN-Based HEMTs at High Carrier Densities

In this article, we use a semiclassical low-field mobility modeling framework, which includes all relevant scattering mechanisms, to examine the dependence of electron mobility on 2-D electron gas (2DEG) density in previously characterized high Al-content AlGaN/gallium nitride (GaN) high electron mobility transistors (HEMTs) with and without an AlN spacer layer. Polar optical phonon (POP) scattering has previously been identified as the dominant scattering mechanism in GaN-based HEMTs at room temperature, so the analysis of POP scattering rates is of particular importance. We find that the POP-limited mobility in both structures decreases with increasing 2DEG density up to a critical density of around <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${10}^{{13}}$ </tex-math></inline-formula> cm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-{2}}$ </tex-math></inline-formula> , above which we observe a significant improvement of up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${46}\%$ </tex-math></inline-formula> , contrary to the behavior typically reported for GaN HEMT structures. The POP-limited mobility increase is explained by the interplay of ground-state energy and quasi-Fermi energy within the channel, by which intrasubband POP emission within the ground state is suppressed at high 2DEG densities. This provides a universal guideline for the design and operation of GaN-based HEMTs to achieve a minimum ON-resistance in switching applications at room temperature and above.

Extraction of Dynamic Threshold Voltage in Resistive Load Hard Switching Operation of Schottky-Type p-GaN Gate HEMT

Threshold voltage (VTh) drift of Schottky-type p-GaN gate high-electron mobility transistor (HEMT) was analyzed under high-voltage and hard switching conditions with a resistive load (R-Load). To overcome the distortion in the drain current (ID) – gate voltage (VG) curve caused by the internal drain capacitance (IDisp) charging current during the switching transients, a method is presented to accurately measure dynamic VTh through appropriate correction of IDisp. The dynamic variation of VTh was tracked with a gate-drive rise/fall times of 200 ns. During the switching operation, VTh increases rapidly at the beginning and then saturates at a stable value. This is attributed to the gate forward current during turn-on that increases the hole depletion of the p-GaN gate through the AlGaN barrier. The dynamic VTh increases linearly at a rate of 40% with the increase of gate turn-on voltage. VTh is independent of duty cycle but minorly increases with frequency at high voltages, suggesting that channel hot electrons be injected into the AlGaN barrier and p-GaN.

Evaluation of thermal performance of all-GaN power module in parallel operation

This work presents an extensive thermal characterization of a single discrete GaN high-electron-mobility transistor (HEMT) device when operated in parallel at temperatures of 25 °C–175 °C. The maximum drain current (ID max), on-resistance (RON), pinch-off voltage (VP) and peak transconductance (gm) at various chamber temperatures are measured and correlations among these parameters studied. Understanding the dependence of key transistor parameters on temperature is crucial to inhibiting the generation of hot spots and the equalization of currents in the parallel operation of HEMTs. A detailed analysis of the current imbalance between two parallel HEMT cells and its consequential effect on the junction temperature are also presented. The results from variations in the characteristics of the parallel-connected devices further verify that the thermal stability and switching behavior of these cells are balanced. Two parallel HEMT cells are operated at a safe working distance from thermal runaway to prevent destruction of the hottest cell.

A study on multiple defect states in low-carbon doped GaN layers and its correlation with AlGaN/GaN high electron mobility transistor operation

A study on defect states in relatively low-carbon doped GaN is presented. A large current collapse was observed in AlGaN/GaN high electron mobility transistor (HEMT) operation when the device channel was doped with carbon of 1×1017cm−3. Deep level transient spectroscopy measurements showed a positive and even negative correlation between the densities of carbon and those of shallow trap states. Along with their small concentrations, shallow traps could not be associated with the collapse of the HEMT. Photo capacitance measurements yielded large signal at very deep levels of 1.6 and 2.4eV in carbon doped GaN. Especially, the 2.4eV deep trap was estimated to be acceptor type and related to some indirect states that the minority carrier transient spectroscopy could not characterize. A 20% of doped carbon was allocated to the very deep traps, and the large current collapse was attributed to these carbon-related states.

Threshold Voltage Shift Due to Charge Trapping in Dielectric-Gated AlGaN/GaN High Electron Mobility Transistors Examined in Au-Free Technology

We report on the investigation of the charge trapping characteristics of dielectric-gated AlGaN/GaN high electron mobility transistors (HEMTs) with atomic layer deposited HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (Tetrakis-(ethylmethylamino)hafnium and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O precursors). The impact of process development and tool contamination in an Au-free 200-mm silicon CMOS line is discussed. The interfacial GaO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> layer is proposed to be the primary location of long time constant traps. We examine the impact of these trap states on threshold voltage engineering of the gate stack. Enhancement mode operation of HEMTs is demonstrated, and the stability of enhancement mode is discussed.

AlGaN Channel HEMT With Extremely High Breakdown Voltage

Enhanced performance of RF power modules is required in a next-generation information society. To satisfy these requirements, we designed a novel high-electron mobility transistor (HEMT) structure employing wider bandgap AlGaN for a channel layer, which we called AlGaN channel HEMT, and investigated it. The wider bandgap is more effective for higher voltage operation of HEMTs and contributes to the increase of output power in RF power modules. As a result, fabricated AlGaN channel HEMTs had much higher breakdown voltages than those of conventional GaN channel HEMTs with good pinchoff operation and sufficiently high drain current density without noticeable current collapse. Furthermore, specific on-state resistances of fabricated AlGaN channel HEMTs were competitive with the best values of reported GaN- and SiC-based devices with similar breakdown voltages. These results indicate that the proposed AlGaN channel HEMTs are very promising not only for an information-communication society but also in the power electronics field.

Polarization-Engineered Enhancement-Mode High-Electron-Mobility Transistors Using Quaternary AlInGaN Barrier Layers

Group III nitride heterostructures with low polarization difference recently moved into the focus of research for realization of enhancement-mode (e-mode) transistors. Quaternary AlInGaN layers as barriers in GaN-based high-electron-mobility transistors (HEMTs) offer the possibility to perform polarization engineering, which allows control of the threshold voltage over a wide range from negative to positive values by changing the composition and strain state of the barrier. Tensile-strained AlInGaN layers with high Al contents generate high two-dimensional electron gas (2DEG) densities, due to the large spontaneous polarization and the contributing piezoelectric polarization. To lower the 2DEG density for e-mode HEMT operation, the polarization difference between the barrier and the GaN buffer has to be reduced. Here, two different concepts are discussed. The first is to generate compressive strain with layers having high In contents in order to induce a positive piezoelectric polarization compensating the large negative spontaneous polarization. Another novel approach is a lattice-matched Ga-rich AlInGaN/GaN heterostructure with low spontaneous polarization and improved crystal quality as strain-related effects are eliminated. Both concepts for e-mode HEMTs are presented and compared in terms of electrical performance and structural properties.

Physisorption of functionalized gold nanoparticles on AlGaN/GaN high electron mobility transistors for sensing applications

AlGaN/GaN high electron mobility transistors (HEMTs) were used to measure electrical characteristics of physisorbed gold nanoparticles (Au NPs) functionalized with alkanethiols with a terminal methyl, amine, or carboxyl functional group. Additional alkanethiol was physisorbed onto the NP treated devices to distinguish between the effects of the Au NPs and alkanethiols on HEMT operation. Scanning Kelvin probe microscopy and electrical measurements were used to characterize the treatment effects. The HEMTs were operated near threshold voltage due to the greatest sensitivity in this region. The Au NP/HEMT system electrically detected functional group differences on adsorbed NPs which is pertinent to biosensor applications.

Linearity Characterization and Optimization of Millimeter-Wave GaN HEMTs

This paper presents the first comprehensive study of the linearity characteristics of GaN/SiC high-electron mobility transistors (HEMTs) at millimeter-wave (mmW). Similar size devices from three sources are compared using AM-AM, AM-PM, two-tone, and 16-quadrature-amplitude modulation (16QAM) modulated waveforms measurements at 31.5 GHz. Additionally, error vector magnitude (EVM) and spectrum regrowth [adjacent channel power ratio (ACPR)] data are also presented. The results were measured, using a unique digital waveform system integrated with a mmW (50-GHz) load-pull system, for HEMTs at different classes of operation: A, AB, and B. In all devices, it is observed that a “balanced AB” (b-AB) class of device bias condition where linearity and efficiency are simultaneously optimized. A linearity figure of merit FOM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">linearity</sub> , based on single-tone power measurements, is defined and calculated for each class. The FOM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">linearity</sub> closely correlates with linearity performance of the HEMT under two-tone and digitally modulated drive conditions. Compared with class A, at a fixed output power, a b-AB class showed a 5-10-dB improvement in intermodulation distortions, combined with 1.2 × to 1.9 × enhancement in drain efficiency. The characterization methodology and derived FOM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">linearity</sub> should benefit monolithic microwave integrated circuit (MMIC) designers to optimize the operation of GaN HEMTs. The results also indicate that GaN HEMTs can provide linearity characteristics suitable for applications such as satellite communications, while maintaining high efficiency and power density.

Analytical Model for Power Switching GaN-Based HEMT Design

The GaN high-electron mobility transistor (HEMT) structure has been widely investigated, particularly for radio-frequency applications. This structure is suitable for high-frequency switching applications in power electronics because of the high breakdown field of GaN and the high mobility of the channel. In this paper, a physical model for predicting the power-switching operation of GaN-based HEMTs as a function of material and device parameters is proposed. Analytical equations for total losses and power dissipation density are derived and discussed both qualitatively and quantitatively.

Impact of SF 6 plasma treatment on performance of AlGaN/GaN HEMT

Selective plasma treatment of an AlGaN/GaN heterostructure in the RF discharge of the electronegative SF 6 gas was studied. Shallow recess-gate etching of AlGaN (∼5 nm) was performed in CCl 4 plasma through a photoresist mask. Subsequently, recess-gate etching followed in situ by SF 6 plasma. The plasma treatment provides the following advantages in the technology of AlGaN/GaN high-electron mobility transistors (HEMT): It (1) simplifies their technology; (2) ensures sufficient selectivity; and (3) enables the technologist to set the threshold voltage of the HEMTs controllably. At the same time, the treatment can (1) provide the AlGaN/GaN heterostructure with surface passivation; (2) modify the 2DEG in any area of a HEMT channel; and (3) make it possible to convert a HEMT operation from depletion mode to enhancement mode. The treatment also improved significantly the DC and RF parameters of HEMTs studied.

Combined resonance and resonant detection of modulated terahertz radiation in a micromachined high‐electron mobility transistor

AbstractWe analyze the operation of a resonant detector of modulated terahertz (THz) radiation based on a micromachined high‐electron mobility transistor (HEMT) with a microcantilever (MC) as mechanically floating gate. Using the device model, which describes both MC mechanical motion and plasma effects in the HEMT twodimensional electron channel, we calculate the output gate and source‐drain ac currents and the detector responsivity as functions of the modulation and carrier frequencies in linear and saturation regimes of the HEMT operation. It is shown that the THz detector responsivity exhibit sharp and high maxima under the conditions of both mechanical and plasma resonances. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Pulsed Current-Voltage-Temperature Characteristics of AlGaN/GaN High Electron Mobility Transistor under Isothermal Conditions

AlGaN/GaN high electron mobility transistors (HEMTs) have great potential, because of the material advantages. However, there are still some problems with AlGaN/GaN HEMTs, such as current collapse (power slump), the self-heating effect and the power scaling problem. In this paper, we have studied the self-heating effect using pulsed current-voltage (IV) and current-voltage-temperature (IVT) measurement under `the same dc bias power condition'- the isothermal condition. It was shown how self-heating affects the AlGaN/GaN HEMT's operation. It can be thought that a reason for the power slump problem of the GaN-based HEMT using a sapphire substrate and the power scaling difficulty is a thermal problem caused by the self-heating effect.

Results, Potential and Challenges of High Power GaN-Based Transistors

The fabrication of AlGaN/GaN high electron mobility transistors (HEMTs) employs strong spontaneous and piezoelectric polarization. The material was grown by OMVPE at 1050 °C and 75 Torr. The Hall measurements yielded ≈1 × 1013/cm2 electron sheet density, with 1200 to 1600 cm2/Vs electron mobility for 300 A Al0.3Ga0.7N/GaN on sapphire at 300 K. The process steps for mesa isolation, Ohmic and Schottky contacts are presented. The frequency response and drain–source breakdown voltage have been determined experimentally. The average electron transit velocity, determined from cut-off frequency, is 1.2 × 107 cm/s, and the breakdown electric field is estimated to be 3 MV/cm. On sapphire substrates, large periphery devices yielded 1.8 W/mm at 78% power-added efficiency and small periphery devices yielded 3.3 W/mm at 36% power-added efficiency at 4 GHz. Performance limitations due to electron trapping during HEMT operation are included. Predictions of >20 W/mm microwave output power are presented for these devices on SiC substrates.

Recent Development in InGaAs based Millimeter Wave HEMTs

Modulation doped structures involving GaAIAs-GaAs combination initiated a vigorous activity of materials growth and device fabrication of high electron mobility transistors (HEMTs) for microwave and millimeter wave applications. Choice of materials for higher carrier sheet concentration and enhanced mobility has ultimately zeroed on InGaAs as channel layer with a number of possible donor and Schottky barrier layer combinations. Use of pure InAs sandwiched between two thin layers of InGaAs has resulted in highest mobility measured so far. Recent developments have led to extremely low noise HEMT devices i.e., 0.8 and 1.2dB noise figures at 60 and 94GHz respectively. The present review covers some basic concepts of HEMTs operation, device design, fabrication and the required materials. Highlights of the methods involved in the epitaxial growth of various layers of HEMT structures are also included for comparing the capabilities of the existing techniques and evaluating the potentials of upcoming one i.e., atomic layer epitaxy.

High electric field transport effects on low temperature operation of pseudomorphic HEMTs

High electric field effect in very small pseudomorphic High Electron Mobility Transistor (HEMT) Al 0.22 Ga 0.78 As/In 0.2 Ga 0.8 As/GaAs and their influence at low temperature are investigated for 0.1μm up to 0.4μm gate lengths. The extent of transport improvement at low temperature and performance degradation associated with gate length reduction are underlined. Limitations in performance improvement appear at low temperature due to trapping effects and to an enhancement of the mechanisms responsible of short channel effects. In pulsed drain operation the evolutions of drain current versus time in the 10ns-600μs range illustrate the influence of trapping centers and self heating of the lattice in the device. We analyze the variation of gate current versus temperature at high drain bias (>3V) and the influence of impact ionization

Charge Fluctuations in High-Electron-Mobility Transistors: A Review

The physics of high-electron-mobility transistors (HEMTs) plays a central role in contemporary design for millimetre-wave communications. HEMTs are the early fruits in a harvest of increasingly radical devices whose structural features are measured in nanometres. The operating principles of these devices are richly varied, and almost always far from classical. One of the tasks for device physics is to understand fluctuation phenomena, .or noise: the control of charge fluctuations is basic to high performance, yet the description of these processes remains incomplete if not obscure. This paper reviews some aspects of charge-transport noise that affect HEMT operation.