GaN High Electron Mobility Transistors Research Articles

A GaN HEMT Amplifier Design for Phased Array Radars and 5G New Radios.

Power amplifiers applied in modern active electronically scanned array (AESA) radars and 5G radios should have similar features, especially in terms of phase distortion, which dramatically affects the spectral regrowth and, moreover, they are difficult to be compensated by predistortion algorithms. This paper presents a GaN-based power amplifier design with a reduced level of transmittance distortions, varying in time, without significantly worsening other key features such as output power, efficiency and gain. The test amplifier with GaN-on-Si high electron mobility transistors (HEMT) NPT2018 from MACOM provides more than 17 W of output power at the 62% PAE over a 1.0 GHz to 1.1 GHz frequency range. By applying a proposed design approach, it was possible to decrease phase changes on test pulses from 0.5° to 0.2° and amplitude variation from 0.8 dB to 0.2 dB during the pulse width of 40 µs and 40% duty cycle.

Guidelines for Reduced-Order Thermal Modeling of Multifinger GaN HEMTs

Abstract The increasing demand for tightly integrated gallium nitride high electron mobility transistors (HEMT) into electronics systems requires accurate thermal evaluation. While these devices exhibit favorable electrical characteristics, the performance and reliability suffer from elevated operating temperatures. Localized device self-heating, with peak channel and die level heat fluxes of the order of 1 MW cm−2 and 1 kW cm−2, respectively, presents a need for thermal management that is reliant on accurate channel temperature predictions. In this publication, a high-fidelity multiphysics modeling approach employing one-way electrothermal coupling is validated against experimental results from Raman thermometry of a 60-finger gallium nitride (GaN) HEMT power amplifier under a set of direct current (DC)-bias conditions. A survey of commonly assumed reduced-order approximations, in the form of numerical and analytical models, are systematically evaluated with comparisons to the peak channel temperature rise of the coupled multiphysics model. Recommendations of modeling assumptions are made relating to heat generation, material properties, and composite layer discretization for numerical and analytical models. The importance of electrothermal coupling is emphasized given the structural and bias condition effect on the heat generation profile. Discretization of the composite layers, with temperature-dependent thermal properties that are physically representative, are also recommended.

Estimation of losses of GaN HEMT in power switching applications based on experimental characterization

An experimental study based on pulsed I-V characterization is conducted at various temperatures to estimate the losses of GaN High-Electron-Mobility Transistors (HEMTs) for switching circuit applications. The estimation of the GaN HEMT power losses is performed by a SPICE simulation using a non-segmented Electro-thermal model. The parameters of this model are extracted using Levenberg–Marquardt Algorithm. The proposed modeling methodology is compared to literature and shows good convergence of static characteristics. The temperature dependency of device parameters is also taken into consideration. Furthermore, the modelled device is verified in a real switching application using a developed efficient switching bench. The verification of the GaN HEMT model shows a good convergence to measurements in term of conduction power losses. Finally, the evolution of the GaN HEMT power losses in switching applications is modelled as a function of the temperature and output current.

Delay analysis of high-electron mobility transistors under high drain bias

In this work, we present a new delay analysis of high-electron-mobility transistors (HEMTs), in which we show that the total intrinsic delay can be separated into six distinct contributions. We distinguish between delays caused by the capacitive coupling between electrodes, the quasi-static charge distribution within the device and a dynamic delay originating from on-state charge flow. An analytical expression for the dynamic delay is established in the form of a delay density using a position dependent image charge analysis. This delay density allows us to show graphically the position dependence of the delay contributions of mobile charge throughout the depletion region. Furthermore, the dynamic delay is separated into a component dependent on the length of the gate electrode and a drain delay component dependent on the drain voltage bias. The influence of high drain bias on the delay components is investigated with the aid of GaN HEMT device simulations and measurements, showing the relative importance of scaling the gate and the drain regions. The RF delay analysis reported here is based upon the definition of small-signal current gain and elucidates the effect of position dependent mobile charge within a HEMT, clarifying the requirements for device scaling.

Epitaxial growth of full range of compositions of (1 1 1) PbZr1-xTixO3 on GaN

Integrating functional complex oxides with conventional (“non-oxide”) semiconductors emerges to be an important research field and has been attracting great interest. Because of their superior intrinsic material properties, such as a relatively high dielectric constant and polarization, the utilization of PbZr1-xTixO3 (PZT) materials as a dielectric layer is expected to greatly improve the performance of the GaN high electron mobility transistor. The functional PbZr1-xTixO3 exhibits quite different crystal structures and consequently physical properties depending on the composition. In this work we report the growth of full range of compositions of PZT films on MgO buffered GaN substrates. Besides revealing the temperature effect on phase formation and surface morphology, we demonstrated the strong effect of composition on the growth: pure (1 1 1) phase is formed in Ti-rich PZT (x > 0.48) while pyrochlore impurity phase is found in Zr-rich PZT (x < 0.48). By introducing an ultrathin Ti-rich PZT seed layer, we are able to achieve epitaxial growth of Zr-rich PZT. The epitaxial PZT films of different composition all exhibit good ferroelectric properties, showing great promise for future GaN device applications.

Guest Editorial for the special issue on devices and circuits for millimeter‐wave and THz applications

Guest Editorial for the special issue on devices and circuits for millimeter‐wave and THz applications

Characteristics of AlGaN/GaN high electron mobility transistors on metallic substrate**Project supported by the National Natural Science Foundation of China (Grant Nos. 61704008 and 11574362).

We have successfully prepared GaN based high electron mobility transistors (HEMTs) on metallic substrates transferred from silicon substrates by electroplating technique. GaN HEMTs on Cu substrates are demonstrated to basically have the same good electric characteristics as the chips on Si substrates. Furthermore, the better heat dissipation of HEMTs on Cu substrates compared to HEMTs on Si substrates is clearly observed by thermoreflectance imaging, showing the promising potential for very high-power and high-temperature operation. This work shows the outstanding ability of HEMT chips on Cu substrates for solving the self-heating effect with the advantages of process simplicity, high yield, and low production requirement.

GaN power switches on the rise: Demonstrated benefits and unrealized potentials

As a wide bandgap semiconductor with high breakdown field, GaN is expected to outperform the incumbent Si technology for power switching applications. Advances in GaN epitaxial growth, device technology, and circuit implementations have resulted in high-performing power switches based on the GaN high electron mobility transistor (HEMT) structure. Demonstrated system benefits have validated the real value of GaN power switching technology. However, the full potential of GaN power switching technology is still far from being exploited. Various factors, including the size of electrodes and wiring, non-optimal E-field shaping, and substrate capacitive coupling, are limiting the performance of GaN HEMT power switches. Emerging device structures, such as, vertical transistors and multichannel superjunction transistors, have the potential to overcome some of those limitations, thereby bringing the performance benefits of the GaN power switching technology to a new level. Understanding the underlying physics is important to the success of the emerging device structures.

Study of a Fast Over-Current Protection Based on Bus Voltage Drop for Gallium Nitride Power Device

Due to the low threshold voltage, gallium nitride (GaN) high electron mobility transistor (HEMT) is prone to mis-conduction. Meanwhile, compared with silicon (Si) insulated gate bipolar transistors (IGBT) and silicon carbide (SiC) metal- oxide- semiconductor field effect transistor(MOSFET) devices, GaN HEMT have a shorter short-circuit withstand time at highvoltages, only a few hundred nanoseconds, which leads to a serious challenge to overcurrent protection. Traditional inductor detection and desaturation overcurrent detection have a long execution time, which are no longer suitable for over-current protection of GaN. Therefore, this paper proposes a new fast over-current protection method, which is based on the bus voltage drop. Through simulation and experimental verification, the result proves that the proposed protection method can detect the over-current signal within 100ns and turn off the GaN device within 400 ns, achieving fast and reliable over-current protection.

Design of Single-Turn Air-Core Integrated Planar Inductor for Improved Thermal Performance of GaN HEMT-Based Synchronous Buck Converter

Gallium nitride (GaN) high electron mobility transistor (HEMT) has a lateral device structure with wafer-level packaging, which is advantageous in minimizing parasitic parameters and overall device size. This type of packaging also has better thermal performance than conventional packaging due to low junction-to-bottom thermal resistances. It indicates copper layers in printed circuit boards can effectively serve as heat dissipation channels for the switching devices. When these copper traces and pours in the vicinity of the switching devices are carefully designed and shaped, they can also be utilized as an integrated output filter. In this article, a synchronous buck converter with a single-turn air-core integrated planar inductor is proposed to achieve both zero-voltage switching and high thermal performance of the GaN HEMTs. A new planar inductor design flowchart is introduced, and analytically estimated inductance and resistance are verified with simulation and experiment. Three different inductors are designed and fabricated to prove the improved thermal performance of the proposed converters.

Interdependence of Electronic and Thermal Transport in AlxGa1–xN Channel HEMTs

Aluminum gallium nitride (AlGaN) high electron mobility transistors (HEMTs) are candidates for next-generation power conversion and radio frequency (RF) applications. Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N channel HEMT devices (x = 0.3, x = 0.7) were investigated using multiple in-situ thermal characterization methods and electro-thermal simulation. The thermal conductivity, contact resistivity, and channel mobility were characterized as a function of temperature to understand and compare the heat generation profile and electro-thermal transport within these devices. In contrast to GaN-based HEMTs, the electrical output characteristics of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.70</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.30</sub> N channel HEMTs exhibit remarkably lower sensitivity to the ambient temperature rise. Also, during 10kHz pulsed operation, the difference in peak temperature between the AlGaN channel HEMTs and GaN HEMTs reduced significantly.

Thermal Performance of GaN/Si HEMTs Using Near-Bandgap Thermoreflectance Imaging

Superlattice (SL) structures have been used to reduce the stress in the GaN epilayer of high-electron-mobility transistors (HEMTs). This has led to an improvement in their properties such as the breakdown voltage. The increase in thermal resistance associated with these structures, however, causes elevated device temperatures which may outweigh the benefits of this approach. To verify this, the thermal performance of SL structures on HEMTs must, thus, be accurately characterized. Transient thermoreflectance imaging (TTI) is an optical technique that can map the temperature distribution across a surface. For materials, such as gold, TTI shows a high spatial and temporal resolution. Consequently, the technique has been primarily used to monitor the gate metal temperature distribution in HEMTs. The origin of the localized heating in HEMTs, however, is known to be in the active GaN layer and, thus, accurate thermal characterization of the channel is necessary. Using a UV LED excitation source with a wavelength near the bandgap of GaN, TTI of GaN channels in HEMTs is presented and verified by comparing the gate metal temperature. To ensure a strong thermoreflectance signal from the GaN surface, the importance of using an excitation wavelength near the bandgap of the GaN channel is highlighted. The effect of the bandgap on the magnitude and the linearity of the thermoreflectance coefficient is presented and discussed. Overall, the improvements to TTI discussed in this article make the technique an accurate method to measure the temperature distribution in GaN HEMT SL structures.

High-frequency AlGaN/GaN T-gate HEMTs on extreme low resistivity silicon substrates

The T-gate high frequency AlGaN/GaN high electron mobility transistors (HEMTs) are demonstrated on an 8 inch extremely-low resistivity (ELR) silicon substrate with a resistivity of ∼2.5 mΩ cm to investigate the potential of using the ELR Si substrate for RF applications. The devices are also fabricated on the 60 Ω cm substrate for comparison. The 0.1 μm T-gate is realized by e-beam lithography to improve the high frequency characteristics of the devices. The short-circuit current gain cutoff frequency (fT), the maximum oscillation frequency (fmax), and maximum transconductance (gm,max) of 27 GHz, 71 GHz and 247 mS mm−1 can be achieved, respectively. The obtained high frequency performance is among the best reported to date for the GaN HEMTs on such low resistivity silicon substrates.

Analysis of trap and recovery characteristics based on low-frequency noise for E-mode GaN HEMTs with p-GaN gate under repetitive short-circuit stress

In this letter, the degradation and recovery characteristics of E-mode AlGaN/GaN high-electron mobility transistors (HEMTs) were investigated under repetitive short-circuit (SC) stress. Output, transfer, transconductance and gate-leakage characteristics were analyzed in detail before and after repetitive SC stress. After stress, the electrical characteristics of the devices gradually degraded as the SC pules increased. Low-frequency noise measurements are performed over the frequency range of 1 Hz–10 KHz by increasing SC pulses. Furthermore, the recovery tendency of DC characteristics and trap density is observed between repetitive SC measurements, and this physically confirms that the mechanism of the performance degradation could be attributed to the trapping and releasing processes of electrons in the p-GaN layer and AlGaN barrier layer of AlGaN/GaN HEMTs, which change the electric field distribution under the gate.

A novel model of avalanche current generation in the GaN HEMT equivalent circuit

We consider the large-signal equivalent circuit of a field-effect transistor (FET) with characteristics and parameters calculated using a two-dimensional (2D) quasi-hydrodynamic model while taking into account the electron velocity overshoot. In accordance with this equivalent circuit when applied to an AlGaN/GaN high-electron-mobility transistor (HEMT), the avalanche current (in the feedback branch) is zero at all operating points. However, this contradicts the significant difference seen between the results of time-domain simulations of a radiofrequency (RF) amplifier in which the transistor is simulated using a 2D model that does versus does not include the avalanche multiplication of the charge carriers. We propose a new model for the avalanche current generation, based on such simulations including the avalanche multiplication and on the theory of impact-ionization avalanche transit-time (IMPATT) diodes. This model allows the synthesis of amplifiers with high power-added efficiency (PAE).

Failure analysis of normally-off GaN HEMTs under avalanche conditions

Gallium nitride (GaN) high electron-mobility transistors (HEMTs) are promising devices in the power electronics field owing to their wide bandgap (WBG). However, all the potential advantages provided by their WBG require reliability improvement. In industrial applications, robustness is one of the main factors considered by circuit designers. This study focuses on the observation of the degradation behavior of the main waveforms of unclamped inductive-switching (UIS) test circuits of two different commercial GaN HEMT structures. The relevance of this study lies in the potential applications of these devices to high-voltage applications and automotive systems where they are subjected to many UIS events over their lifetime. This study shows that avalanche does not occur on these devices; therefore, the breakdown is caused by the high voltage. A deeper analysis of the breakdown mechanism is achieved using a curve/tracer analyzer, lock-in thermography, and focused ion beam. These experiments reveal that impact ionization is the main failure mechanism that causes breakdown in both structures.

Deadband Effect and Accurate ZVS Boundaries of GaN-Based Dual-Active-Bridge Converters With Multiple-Phase-Shift Control

Gallium nitride (GaN) high-electron-mobility transistors (HEMTs) are promising for the next-generation power switching applications due to their low on-resistance, high-frequency operation, and small gate capacitances, which are essential for the power density and efficiency improvement. However, with the increase of the switching frequency, the deadband effect will become a challenging issue for the high-efficiency converter design over a wide operation range. In this article, a comprehensive theoretical analysis and experimental verification of the deadband effect in the GaN-based dual-active-bridge (DAB) converters with multiple-phase-shift (MPS) control is presented. First, resonant transitions for various switching conditions with different initial currents by using MPS controls are discussed, and the effect of the stray capacitance of GaN HEMTs is analyzed. On this basis, the accurate boundaries among zero-voltage switching (ZVS), partial ZVS, and hard switching with MPS control can be obtained. A deadband compensation strategy is proposed by considering different switching conditions. Finally, a DAB prototype based on GaN HEMTs was built, and main experimental results were provided to verify the effectiveness of the proposed resonant transition analysis, ZVS boundaries, and the deadband compensation strategy.

Real time detection of Hg2+ ions using MoS2 functionalized AlGaN/GaN high electron mobility transistor for water quality monitoring

A sensor for highly sensitive, selective, and rapid determination of the trace amount of toxic Hg2+ ions is developed for the first-time using molybdenum disulfide (MoS2) functionalized AlGaN/GaN high electron mobility transistor (HEMT). The vertically aligned, flower-like MoS2 structures are synthesized through a simple hydrothermal route and applied on the gate region of AlGaN/GaN HEMT. The scanning electron microscopy, Raman spectroscopy, and X-ray diffraction are performed for structural characterization of MoS2. Further, the sensing of Hg2+ ions is performed by electrical characterizations of MoS2 functionalized AlGaN/GaN HEMT. The sensor showed an excellent sensitivity of 0.64 μA/ppb and detection limit of 0.01152 ppb with the rapid response time of 1.8 s. The sensor exhibits the linear range of detection from 0.1 ppb to 100 ppb and highly selective behavior towards Hg2+ ions. The results demonstrated that the MoS2 possess excellent Hg2+ ions capture property, that could be attributed to the complexation of Hg2+ ions with sulfur and the electrostatic interaction between MoS2 and Hg2+ ions alters the drain to source current (IDS) of the HEMT at a constant drain to source voltage (VDS). Therefore, the MoS2 based AlGaN/ GaN HEMT devices have a huge potential for next-generation ion sensing applications.

Temperature‐dependent dynamic R DS,ON under different operating conditions in enhancement‐mode GaN HEMTs

High-voltage enhancement-mode (E-mode) gallium nitride (GaN) high electron mobility transistor (HEMT) is a superior candidate to enable higher efficiency and higher power density when compared with silicon power devices in power converter applications. However, the dynamic R DS , ON problem affects the conduction loss of the converter and remains one of the major issues that must be resolved. In this study, a comprehensive experimental evaluation and analysis method of the temperature-dependent dynamic R DS , ON of GaN HEMT in a circuit level is proposed. A commercial E-mode GaN HEMT (GS66508T) is used as a sample to study the temperature-dependent dynamic R DS , ON using a double-pulse-tester. The temperature-dependent dynamic R DS , ON under different DC-link voltages and different load currents are studied and the results show a non-monotonic temperature-dependence of the dynamic R DS , ON . It is concluded that the temperature dependence of the buffer-induced trapping and de-trapping effect, and the temperature dependence of electron mobility together influence the dynamic R DS , ON of E-mode GaN HEMT device during operation. This finding is important since in converter applications the devices are typically operating at elevated temperatures. The proposed comprehensive experimental method can be used to estimate and analyse the dynamic R DS , ON characteristics of other GaN devices.

Extreme Temperature Modeling of AlGaN/GaN HEMTs

The industry standard advanced SPICE model (ASM)-GaN compact model has been enhanced to model the GaN high electron mobility transistors (HEMTs) at extreme temperature conditions. In particular, the temperature dependence of the trapping behavior has been considered and a simplifying approximation in the temperature modeling of the saturation voltage in the ASM-GaN model has been relaxed. The enhanced model has been validated by comparing the simulation results of the model with the dc ${I}$ – ${V}$ measurement results of a GaN HEMT measured with chuck temperatures ranging from 22 °C to 500 °C. A detailed description of the modeling approach is presented. The new formulation of the ASM-GaN compact model can be used to simulate the circuits designed for extreme temperature environments.