Overcurrent Test for GaN HEMT with Cryogenic Cooling

The self-heating effect has long been a persistent issue for high electron mobility transistors based on gallium nitride due to their inherently poor heat dissipation capability. Although a wide variety of thermal management solutions has to date been proposed, the problem of the extremely non-uniform heat dissipation at the micrometer scale is still challenging. It has recently been demonstrated, however, that the performance of gallium nitride high electron mobility transistors can be substantially improved by the introduction of various heat-spreading elements based on graphene, boron nitride or diamond. In this paper, using numerical simulation, we carried out a design optimization procedure for a normally-off gallium nitride high electron mobility transistor containing both graphene and cubic boron nitride layers. First, a screening experiment based on a very economical Plackett-Burman design was performed in order to find the most critical geometric parameters that influence the dc characteristics. After that, a full two-level factorial experiment consisting of three factors was implemented and an optimized parameter set was yielded. By applying this set, the output power was increased by 11.35%. The combination of the most significant parameters does not include any factors related to the heat-spreading layers. Keywords: gallium nitride, high electron mobility transistor, optimization, Plackett-Burman design, screening experiment, self-heating.

Gallium nitride high electron mobility transistors (HEMTs) have shown the potential to be highly resistant to radiation damage, making them ideal for use in microwave power amplifiers and DC/DC converters in space-based applications. To investigate the mechanisms of radiation-induced degradation in AlGaN/GaN HEMTs, 2MeV protons were used to simulate the space radiation environment, and to probe the second order effect of inherent material defects on the response the initial material quality was intentionally varied by studying HEMTs on sapphire, Si, and SiC substrates. Irradiation was performed incrementally up to a fluence of 6x1014 cm-2. The Hall mobility and 2DEG density was measured on Van der Pauw structures before irradiation and at each dose. The decrease in mobility can be attributed to increased carrier scattering in the 2DEG as a result of radiation-induced defects while the decrease in sheet carrier density is attributed to screening of the 2DEG from charged trap formation [1]. The magnitude of change in the 2DEG density is the same for all substrate materials (1x1012 cm-2), though the percent change varies due to different initial values. Note that mobility begins to degrade at lower fluence than sheet carrier density. FET I-V analysis indicated increased ON-resistance and decreased saturation current, consistent with the reduced mobility and sheet carrier density observed in the Hall measurements. The off-state leakage current decreased and threshold voltage was shifted slightly positive, again due to the reduced 2DEG density. The trapping mechanisms were further probed using pulsed I-V measurements at varying quiescent bias conditions. While the dynamic ON-resistance degradation under quasistatic conditions was consistent with DC measurements, the degradation under off-state stress conditions initiated at a lower fluence, and a culminated with a 10X higher degradation at the final fluence. This implies that dynamic ON-resistance, a parameter related to charge trapping, is much more sensitive to radiation than DC characteristics and must be considered when evaluating the radiation hardness of HEMT devices. TEM imaging was employed to directly probe the mechanisms suggested by the electrical measurements. Process-induced defects were visible in both reference and irradiated samples, notably an overflow of Au in the metal stack at the gate edge, possibly related to PECVD deposition of passivation layers. In addition a new, radiation-induced void at both edges of the gate in the Ni region of the Ni/Au gate metallization was revealed [2]. EDS line scans indicate that, after radiation, the 20 nm Ni layer diffused up into the Au, and to a lesser extent, into the AlGaN, leaving voids protruding ~300 nm under the gate edges. The mechanism of the void formation shows all features of Ni/Au inter-diffusion through vacancy exchange, known as the Kirkendall effect. The exact process is still under investigation, but the voids result in reduced gate area, which is responsible for the degraded off-state performance. It is particularly rare that such pronounced diffusion occurs at room temperature. Since the voids only occur at the gate edges, an additional electrochemical or strain-induced driving force is likely present. Therefore the role of the SiNx passivation layer, as well as strain, at the gate edge will be factor in this ongoing research. 1. T.J. Anderson, et. Al. IEEE Electron Dev. Lett. 35 (8), 826-828 (2014) 2. A.D. Koehler, et. Al. IEEE Electron Dev. Lett. 35 (10) (2014)

Working at high RF power leads gallium nitride (GaN) high electron mobility transistors (HEMT) to self-heating that poses a limit to device performances and reliability. Thermal characterization is therefore of great value for proper design of heat dissipation and also for device reliability studies. The peak power dissipation in HEMT devices is located in the channel near the gate edge, which is typically buried under a field plate. This hot spot is thus inaccessible to direct temperature measurement. Therefore, any method used for temperature measurement has to be augmented by a thermal simulation to calculate the actual temperature in the channel from temperature measured elsewhere. In this work, we used thermal imaging to measure temperature during device operation in various pulsed RF conditions. To obtain the hot spot temperature, we developed a thermal simulation of AlGaN/GaN HEMT transistor on SiC substrate. The simulation estimates the channel temperature using 3D finite element method with multi-parameter input. To calibrate the simulation, we compared simulation results with IR images of a 2 mm AlGaN/GaN HEMT transistor operating at various pulsed RF conditions. The simulation is typically slow. In this work, we used the calibrated simulation to study the hot spot temperature as a function of the working conditions and formulated an approximated equation for the thermal behavior of the transistor as a function of power dissipation, base plate temperature, pulse width, and pulse duty cycle that may be used to estimate the channel temperature in real time.

Understanding the cryogenic performance of Gal-lium Nitride (GaN) devices is essential for determining their suitability for use in the electrification of large-scale transport such as aircraft, where the power electronics may need to withstand cryogenic temperatures. In this paper we investigate the temperature relationship of the channel on-resistance and gate threshold voltage of various GaN devices, using liquid nitrogen as an inexpensive and effective cryogenic medium. Two cascode-configuration high electron mobility transistors (Cascode HEMT) and two enhancement mode high electron mobility transistors (E-HEMT) devices, each from different manufacturers were selected to ensure a range of technologies were covered. The on-resistance of GaN devices was found to decrease approximately linearly with reducing temperature, until a knee-point is reached whereupon the on-resistance rises. One E-HEMT device did not reach a knee point, indicating the need for further testing at lower temperatures to determine whether, or where such a knee point exists. The device with the lowest normalised on resistance at -196.6°C exhibited 32.8% of its room temperature value, while the device with the highest normalised on resistance showed 89.3% of its room temperature value. It was also found that the gate thresh-old voltage shows temperature dependency. For both Cascode manufacturers, the threshold voltage increased linearly with decreasing temperature. For one of the E-HEMTs the threshold voltage increased non-linearly with decreasing temperature. For the other E-HEMTs the threshold voltage decreased linearly with decreasing temperature. Initial results indicate that GaN devices are compatible with operation in or near cryogenic environments. Further characteristics need investigation to fully confirm suitability. Output characteristics, as well switching rise and fall times may change in cryogenic conditions, potentially affecting suitability.

A series of switchable resonators are proposed by incorporating the parasitic effects of two gallium nitride (GaN) high electron mobility transistor (HEMT) devices in this letter, based on which a broadband single-pole double-throw (SPDT) switch is presented with a bandpass response. As for on-chip switches, the ideal transistor is desired to act as a capacitor in its off-state but a resistor in its on-state. In conventional switch designs, the inductive effects of transistors are typically suppressed due to their detrimental impact on impedance matching and isolation. In contrast to this conventional approach, this study proposes a resonator-based design strategy that intentionally exploits and amplifies these inductive characteristics to construct two distinct GaN HEMT-integrated resonators. The first resonator employs the enhanced on-state inductance of a switching transistor combined with an MIM capacitor to form a series resonant network, enabling broadband impedance matching. The second resonator utilizes the large off-state capacitance of a power transistor and a short-circuited transmission line to establish a parallel resonant network. Leveraging the unique properties of these resonators, a broadband switch topology is accordingly proposed and experimentally validated. For demonstration, a SPDT switch is designed and fabricated in a 100nm GaN-on-Si process. The proposed switch operates from 16 to 33 GHz based on experimental measurements. Two transmission poles are observed in the passband. This result experimentally validates the GaN HEMT-based resonator design.

: A key component of microwave telecommunication systems is the power amplifier (PA). The design parameters of a communication system, such as link performance, power budget, and thermal design are typically driven by the power amplifier's linearity, output power, and efficiency. These design parameters of PA are the key to ensuring an efficient system because the PA is the most power-consuming circuit in the communication system. In an effort to have students involved in this research area, the International Microwave Symposium (IMS) sponsored by IEEE Microwave Theory and Technique Society (MTT-S) has introduced a High Efficiency Power Amplifier Design Competition. In this report, we present a 2.8-GHz, 5-W, highly efficient PA based on a wide bandgap gallium nitride (GaN) high electron mobility transistor (HEMT) device (CGH 40010f from CREE). The PA design was presented at the 2010 IMS Student Design Competition and achieved the competition goal of output power greater than 5 W with a measured power added efficiency (PAE) of 61% at 2.8 GHz.

This paper reports a high-performance microwave receiver protector (RP) based on a single gallium nitride (GaN) high electron mobility transistor (HEMT) at an operation frequency of 30 to 3000 MHz. The HEMT-based RP exhibits multi features: high power compression, constant output power, tunable threshold power level, and insensitivity to frequency variation. With a low drain voltage (Vds) of 3 V, constant output power of 9.9 dBm was acquired for input power over its threshold power of 3.2 dBm. Power compression of 13.3 dB was achieved at the input power of Pin = 20 dBm. In addition, adjustable threshold power level Pth could be obtained by merely tuning drain voltage. Transducer gain measurement results were employed to explain the occurrence of output power saturation. Relatively higher Pth was linked to wider gate voltage swing which extended the linear region of the Pout-Pin characteristic. In addition, the GaN HEMT’s power compression capability shows great immunity to frequency variation, which is promising for protecting sensitive receiver components at both low and high frequencies. Finally, the phase shift of the GaN HEMT RP at high input power was measured and analyzed by the nonlinear behaviors of input capacitance Cgs.

With the increased power density of gallium nitride (GaN) high electron mobility transistors (HEMTs), effective cooling is required to eliminate the self-heating effect. Incorporating diamond into GaN HEMT is an alternative way to dissipate the heat generated from the active region. In this review, the four main approaches for the integration of diamond and GaN are briefly reviewed, including bonding the GaN wafer and diamond wafer together, depositing diamond as a heat-dissipation layer on the GaN epitaxial layer or HEMTs, and the epitaxial growth of GaN on the diamond substrate. Due to the large lattice mismatch and thermal mismatch, as well as the crystal structure differences between diamond and GaN, all above works face some problems and challenges. Moreover, the review is focused on the state-of-art of polycrystalline or nanocrystalline diamond (NCD) passivation layers on the topside of GaN HEMTs, including the nucleation and growth of the diamond on GaN HEMTs, structure and interface analysis, and thermal characterization, as well as electrical performance of GaN HEMTs after diamond film growth. Upon comparing three different nucleation methods of diamond on GaN, electrostatic seeding is the most commonly used pretreatment method to enhance the nucleation density. NCDs are usually grown at lower temperatures (600–800 °C) on GaN HEMTs, and the methods of “gate after growth” and selective area growth are emphasized. The influence of interface quality on the heat dissipation of capped diamond on GaN is analyzed. We consider that effectively reducing the thermal boundary resistance, improving the regional quality at the interface, and optimizing the stress–strain state are needed to improve the heat-spreading performance and stability of GaN HEMTs. NCD-capped GaN HEMTs exhibit more than a 20% lower operating temperature, and the current density is also improved, which shows good application potential. Furthermore, the existing problems and challenges have also been discussed. The nucleation and growth characteristics of diamond itself and the integration of diamond and GaN HEMT are discussed together, which can more completely explain the thermal diffusion effect of diamond for GaN HEMT and the corresponding technical problems.

Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) are thermally limited much below the electrical capability of the devices. The unique challenge of a GaN HEMT is its ultra-high heat flux at the micro-scale gate fingers, which cannot be effectively and adequately addressed by conventional packaging and thermal management systems and lead to a large junction-to-ambient thermal resistance. A novel thermal interface material (TIM) that offers unique heat spreading, CTE compliance and ultra-high thermal performance was developed. The new TIM enables attaching GaN die directly onto a copper carrier. The heat spreading feature of the TIM can effectively dissipate heat near junction and greatly reduce the maximum heat flux, which leads to smaller temperature difference across each layer underneath of the TIM. A preliminary demonstration of the technology on a GaN-on-Silicon device shows 50% higher heat dissipation capability, compared to the state-of-the-art pin fin cold plate liquid cooling, while maintaining the device junction temperature at 150°C.

In this paper, a temperature and gate voltage dependent current-voltage (I-V) model for Gallium nitride (GaN) high electron mobility transistors (HEMTs) devices is studied. In order to help researchers and designers simulate devices in the absence of experimental data and improve parameter extraction efficiency, the temperature and gate voltage dependence of mobility are discussed and incorporated into the proposed model which is revised from the standard advanced SPICE model (ASM) for GaN HEMTs. The mobility variations with gate voltage from -2 V to 6 V and temperature from 10 K to 1000 K are analyzed. The simulation results of our model are compared with the standard ASM-HEMT model, and the output and transfer characteristics of the device from 270 K to 420 K are simulated. Thereby, our model may simulate various applications of GaN HEMTs at different gate voltages and temperatures in the early stages of design and research.

A physics-based analytical compact device model for an enhancement-mode Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT) lateral power device structure is presented. The model was implemented in the Saber® simulator and the physics-based model parameters are specifically crafted so that they can easily be extracted from dc I-V and C-V data typically available in a datasheet. An 80 V, 90 A (420 A, pulse rated), 2.5 mQ commercial EPC device datasheet is used as an example in this paper to demonstrate the parameter extraction procedure for the compact model. The model has been validated against the turn-on and turn-off characteristics of a commercial EPC GaN device. The model is scalable and can be used for a wide range of commercial GaN devices by using the extraction procedure detailed in this work.

본 논문은 GaN(Gallium Nitride) HEMT(High Electron Mobility Transistor) die를 이용하여 S-대역 내부 정합형 전력 증폭기 설계, 제작 그리고 실험 결과에 대해 기술하였다. S-대역 내부 정합형 전력 증폭기를 설계하기 위하여 고유전율을 가지는 기판과 알루미나 기판을 이용하여 입/출력단 정합 회로를 설계 및 제작하였다. 측정 결과로는 펄스 모드로 동작시켰을 때 3 GHz에서 55.4 dBm의 출력 전력, 78 % 드레인 효율 그리고 11 dB의 전력 이득을 얻었다. This paper presents the design, fabrication and measurement results of a S-band internally-matched power amplifier using Gallium Nitride High Electron Mobility Transistor(GaN HEMT) die. In order to fabricate the S-band internally-matched power amplifier, a high dielectric substrate and alumina were used for input/output matching circuits. The measured output power is 55.4 dBm, the drain efficiency is 78 % and the power gain is 11 dB under pulse operation at the frequency of 3 GHz.

The development of high-quality gallium nitride (GaN) high electron mobility transistors (HEMTs) has provided opportunities for the next generation of high-performance radio frequency and power electronics. Operating devices with smaller length scales at higher voltages result in excessively high channel temperatures, which reduce performance and can have detrimental effects on the device's reliability. The thermal characterization of GaN HEMTs has traditionally been captured from either the top or bottom side of the device. Under this configuration, it has been possible to map the lateral temperature distribution across the device with optical methods such as infrared and Raman thermometry. Due to the presence of the gate metal, however, and often also the addition of a metal air bridge and/or field plate, the temperature of the GaN channel under the gate is typically inferred by numerical simulations. Furthermore, measuring the vertical temperature gradient across multiple epitaxial layers has shown to be challenging. This study proposes a new cross-sectional imaging technique to map the vertical temperature distribution in GaN HEMTs. Combining advanced cross-sectioning processing with the recently developed near bandgap transient thermoreflectance imaging technique, the full transient thermal distribution across a GaN HEMT is achieved. The cross-sectional thermal imaging of the GaN channel is used to study the effects of biasing on the Joule heating profile. Overall, the direct measurement of the GaN channel, capturing both the vertical and lateral gradient, will provide deeper insight into the device's degradation physics and supply further experimental data to validate previously developed electrothermal models.

This paper describes a simple, EM-based approach to accurately predict the frequency behavior of a custom common-gate (CG) Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT) starting from a similar device, but in the most frequently available standard common-source (CS) configuration. The proposed approach exploits electromagnetic (EM) simulations and measured scattering parameters of the reference CS device to represent its extrinsic and intrinsic sections, respectively. Then, the frequency behavior of the CG device is obtained by combining the CS intrinsic section data with the information extracted from the EM simulations of the passive parts of the CG layout. In particular, an 8×130 μm CS GaN HEMT has been adopted as reference device to predict the behavior of an 8×150 μm GaN HEMT in a custom CG configuration. Experimental results confirm the ability of the proposed method to accurately predict the behavior of the custom CG device up to 30 GHz.

In this paper, a detailed performance comparative study is performed among AlGaN/GaN, InAlN/GaN and conventional AlGaAs/GaAs based high electron mobility transistor (HEMT) devices. The comparison analysis is executed using a non-linear two-dimensional T-CAD numerical device simulator. With the recent carrier transport advancements, our paper mainly focuses on superior electrical property and high electron velocity of Gallium Nitride (GaN) for HEMT device modeling. GaN-based HEMT devices exhibit maximum drain current as 2.8×10−4A, the threshold voltage of -1V, and unity current gain cut-off frequency of 75.4 GHz. The Silvaco T-CAD simulator greatly highlights the various HEMT device performance reports, targeting microwave frequency-based device applications.