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Journal ArticleDOI

Characterization of AlGaN/GaN HEMTs Using Gate Resistance Thermometry

TL;DR: In this article, gate resistance thermometry (GRT) was used to determine the channel temperature of AlGaN/GaN high electron-mobility transistors under various bias conditions.
Abstract: In this paper, gate resistance thermometry (GRT) was used to determine the channel temperature of AlGaN/GaN high electron-mobility transistors. Raman thermometry has been used to verify GRT by comparing the channel temperatures measured by both techniques under various bias conditions. To further validate this technique, a thermal finite-element model has been developed to model the heat dissipation throughout the devices. Comparisons show that the GRT method averages the temperature over the gate width, yielding a slightly lower peak temperature than Raman thermography. Overall, this method provides a fast and simple technique to determine the average temperature under both steady-state and pulsed bias conditions.
Citations
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Journal ArticleDOI
TL;DR: In this article, the authors examined self-heating in GaN-on-Si HEMTs via measurements of channel temperature using above-bandgap UV thermoreflectance imaging in combination with fully coupled electrothermal modeling.
Abstract: Self-heating in AlGaN/GaN high electron mobility transistors (HEMTs) negatively impacts device performance and reliability. Under nominal operating conditions, a hot-spot in the device channel develops under the drain side corner of the gate due to a concentration of volumetric heat generation leading to nonequilibrium carrier interactions and non-Fourier heat conduction. These subcontinuum effects obscure identification of the most salient processes impacting heating. In response, we examine self-heating in GaN-on-Si HEMTs via measurements of channel temperature using above-bandgap UV thermoreflectance imaging in combination with fully coupled electrothermal modeling. The methods together highlight the interplay of heat concentration and subcontinuum thermal transport showing that channel temperature cannot be determined solely by continuum scale heat transfer principles. Under conditions of equal power dissipation (PDISS = VDS × IDS = 250 mW), for example, a higher VDS bias (∼23 V) resulted in an ∼44% larger rise in peak junction temperature compared to that for a lower VDS (∼7.5 V) condition. The difference arises primarily due to reduction in the heat generating volume when operating under partially pinched-off (i.e., high VDS) conditions. Self-heating amplifies with this reduction as heating now takes place primarily over length scales less than the mean free path of the phonons tasked with energy dissipation. Being less efficient, the subcontinuum transport restricts thermal transport away from the device hot-spot causing a net increase in channel temperature. Taken together, even purely thermally driven device mean-time-to-failure is not, therefore, based on power dissipation alone as both bias dependence and subcontinuum thermal transport influence device lifetime.

52 citations

Journal ArticleDOI
TL;DR: In this paper, the authors demonstrate the simultaneous measurement of temperature rise, inverse piezoelectric stress, thermoelastic stress, and vertical electric field via micro-Raman spectroscopy from the shifts of the E2 (high), A1 longitudinal optical (LO), and E 2 (low) optical phonon frequencies in wurtzite GaN.
Abstract: As semiconductor devices based on silicon reach their intrinsic material limits, compound semiconductors, such as gallium nitride (GaN), are gaining increasing interest for high performance, solid-state transistor applications. Unfortunately, higher voltage, current, and/or power levels in GaN high electron mobility transistors (HEMTs) often result in elevated device temperatures, degraded performance, and shorter lifetimes. Although micro-Raman spectroscopy has become one of the most popular techniques for measuring localized temperature rise in GaN HEMTs for reliability assessment, decoupling the effects of temperature, mechanical stress, and electric field on the optical phonon frequencies measured by micro-Raman spectroscopy is challenging. In this work, we demonstrate the simultaneous measurement of temperature rise, inverse piezoelectric stress, thermoelastic stress, and vertical electric field via micro-Raman spectroscopy from the shifts of the E2 (high), A1 longitudinal optical (LO), and E2 (low) optical phonon frequencies in wurtzite GaN. We also validate experimentally that the pinched OFF state as the unpowered reference accurately measures the temperature rise by removing the effect of the vertical electric field on the Raman spectrum and that the vertical electric field is approximately the same whether the channel is open or closed. Our experimental results are in good quantitative agreement with a 3D electro-thermo-mechanical model of the HEMT we tested and indicate that the GaN buffer acts as a semi-insulating, p-type material due to the presence of deep acceptors in the lower half of the bandgap. This implementation of micro-Raman spectroscopy offers an exciting opportunity to simultaneously probe thermal, mechanical, and electrical phenomena in semiconductor devices under bias, providing unique insight into the complex physics that describes device behavior and reliability. Although GaN HEMTs have been specifically used in this study to demonstrate its viability, this technique is applicable to any solid-state material with a suitable Raman response and will likely enable new measurement capabilities in a wide variety of scientific and engineering applications.

48 citations

Journal ArticleDOI
TL;DR: In this article, the authors used transient thermoreflectance imaging (TTI) to measure the temperature rise of the passivated gate metal measured by TTI and the averaged gate temperature monitored by gate resistance thermometry (GRT).
Abstract: The development of steady-state thermal characterization techniques for AlGaN/GaN high-electron mobility transistors (HEMTs) has been used to measure the device’s peak temperature under DC conditions. Despite these methods enabling the accurate quantification of the device’s effective thermal resistance and power density dependence, transient thermometry techniques are necessary to understand the nanoscale thermal transport within the active GaN layer where the highly localized joule heating occurs. One technique that has shown the ability to achieve this is transient thermoreflectance imaging (TTI). The accuracy of TTI is based on using the correct thermoreflectance coefficient. In the past, alternative techniques have been used to adjust the thermoreflectance coefficient to match the correct temperature rise in the device. This paper provides a new method to accurately determine the thermoreflectance coefficient of a given surface and is validated via an electrical method: gate resistance thermometry (GRT). Close agreement is shown between the temperature rise of the passivated gate metal measured by TTI and the averaged gate temperature monitored by GRT. Overall, TTI can now be used to thermally map GaN HEMTs under pulsed conditions providing simultaneously a submicrosecond temporal resolution and a submicrometer spatial resolution.

38 citations


Cites background or methods from "Characterization of AlGaN/GaN HEMTs..."

  • ...As shown in [21], the averaged GRT temperature (40 °C) does not capture this gradient showing the necessity of a mapping technique such as TTI to capture the peak temperature and gradient....

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  • ...1, the devices tested were six fingered devices with a 370-μm gate width and were identical to the devices measured via GRT in [21]....

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  • ...Similar to the setup described in [21], the devices were biased under pulsed conditions using an AMCAD pulsed IV...

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  • ...6 to the GRT measurements previously conducted under DC biasing [21]....

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  • ...The temperature profiles are validated via gate resistance thermometry (GRT) [21], a technique that uses four-point sensing to monitor the average temperature change in the gate metal....

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Journal ArticleDOI
Mei Wu1, Xiaohua Ma1, Ling Yang1, Qing Zhu1, Meng Zhang1, Lin-An Yang1, Yue Hao1 
TL;DR: In this paper, the authors proposed a novel electrical method for the determination of channel temperature in AlGaN/GaN high-electron mobility transistors, which has advantages of high accuracy, easy positioning, and immunity to the gate head geometry.
Abstract: This paper proposes a novel electrical method for the determination of channel temperature in AlGaN/GaN high-electron mobility transistors. A test structure combining various device geometries has been utilized to achieve the temperature dependence of the channel resistance and then applied to detect the channel temperature under the gate instead of that at the gate edge on the drain side. A 2-D electrothermal model has been built, and the model demonstrates excellent agreement with the experimental data. Compared with gate resistance thermometry and micro-Raman spectroscopy, our method has advantages of high accuracy, easy positioning, and immunity to the gate head geometry. This facilitates its potential for temperature extraction, especially for short channel devices. Finally, we apply this method to different device structures to demonstrate its scalability, and the uncertainty caused by current collapse is also discussed.

25 citations


Cites methods from "Characterization of AlGaN/GaN HEMTs..."

  • ...Recently, gate resistance thermometry (GRT) has been applied in GaN-based FETs, which can provide the best accuracy among all the available electrical methods by means of designed test structures [18], [19]....

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  • ...The GRT approach used in this paper is based on the AlGaN/GaN HEMTs featuring a double-ended gate terminal [18], [20], [30]–[32]....

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Journal ArticleDOI
TL;DR: In this article, the thermal response of GaN/GaN HEMTs on high conductivity substrates is assessed using a high-resolution thermoreflectance (TR) imaging technique.
Abstract: Gallium nitride (GaN) high electron-mobility transistors (HEMTs) offer considerable high-power operation but suffer in reliability due to potentially damaging self-heating. In this study, self-heating in AlGaN/GaN HEMTs on high conductivity substrates is assessed using a high-resolution thermoreflectance (TR) imaging technique, to compare the thermal response between GaN-on-Si, GaN-on-Diamond, and GaN-on-4H-SiC. The TR method accuracy at high-power density is verified using a nonlinear coefficient of TR ( ${C}_{\text {TR}}$ ) as a function of temperature. The acquired steady-state thermal maps give a thermal resistance of $11.5~\text {mm} \cdot \text {K/W}$ for GaN-on-Si (based on peak channel temperature), compared to 2.7 and $3.3~\text {mm} \cdot \text {K/W}$ for GaN-on-SiC and GaN-on-diamond substrates, respectively. The tested GaN-on-diamond HEMT exhibits similar heating rates to those seen on a SiC substrate, with a slightly higher peak temperature, which indicates a higher thermal boundary resistance that could offset the benefits of using a higher conductivity substrate and lead to faster thermally enhanced degradation. The analysis reveals the importance using high-resolution imaging to detect hotspots and areas of peak temperature that largely affect failure initiation and device reliability and which may not be otherwise observable.

22 citations


Cites background from "Characterization of AlGaN/GaN HEMTs..."

  • ...In comparison, μ-Raman reports a thickness-average temperature within the GaN, and visible TR [13], [23] illumination reflects from multiple interfaces within the stack structure, with both effects leading to an...

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References
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Journal ArticleDOI
Raymond S. Pengelly1, Simon Wood1, J.W. Milligan1, Scott T. Sheppard1, W. Pribble1 
TL;DR: Examples of broadband amplifiers, as well as several of the main areas of high-efficiency amplifier design-notably Class-D, Class-E, class-F, and Class-J approaches, Doherty PAs, envelope-tracking techniques, and Chireix outphasing are described.
Abstract: Gallium-nitride power transistor (GaN HEMT) and integrated circuit technologies have matured dramatically over the last few years, and many hundreds of thousands of devices have been manufactured and fielded in applications ranging from pulsed radars and counter-IED jammers to CATV modules and fourth-generation infrastructure base-stations. GaN HEMT devices, exhibiting high power densities coupled with high breakdown voltages, have opened up the possibilities for highly efficient power amplifiers (PAs) exploiting the principles of waveform engineered designs. This paper summarizes the unique advantages of GaN HEMTs compared to other power transistor technologies, with examples of where such features have been exploited. Since RF power densities of GaN HEMTs are many times higher than other technologies, much attention has also been given to thermal management-examples of both commercial “off-the-shelf” packaging as well as custom heat-sinks are described. The very desirable feature of having accurate large-signal models for both discrete transistors and monolithic microwave integrated circuit foundry are emphasized with a number of circuit design examples. GaN HEMT technology has been a major enabler for both very broadband high-PAs and very high-efficiency designs. This paper describes examples of broadband amplifiers, as well as several of the main areas of high-efficiency amplifier design-notably Class-D, Class-E, Class-F, and Class-J approaches, Doherty PAs, envelope-tracking techniques, and Chireix outphasing.

840 citations


"Characterization of AlGaN/GaN HEMTs..." refers background in this paper

  • ...The development and fabrication of aluminum gallium nitride/gallium nitride (AlGaN/GaN) HEMTs on silicon carbide (SiC) substrates has sufficient reliability to enable the production of high-quality devices [1]....

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Journal ArticleDOI
TL;DR: In this paper, the authors report on the noninvasive measurement of temperature, i.e., self-heating effects, in active AlGaN/GaN HFETs grown on sapphire and SiC substrates.
Abstract: We report on the noninvasive measurement of temperature, i.e., self-heating effects, in active AlGaN/GaN HFETs grown on sapphire and SiC substrates. Micro-Raman spectroscopy was used to produce temperature maps with /spl ap/1 /spl mu/m spatial resolution and a temperature accuracy of better than 10/spl deg/C. Significant temperature rises up to 180/spl deg/C were measured in the device gate-drain opening. Results from a three-dimensional (3-D) heat dissipation model are in reasonably good agreement with the experimental data. Comparison of devices fabricated on sapphire and SiC substrates indicated that the SiC substrate devices had /spl sim/5 times lower thermal resistance.

342 citations


"Characterization of AlGaN/GaN HEMTs..." refers background in this paper

  • ...average temperature across the thickness of the GaN layer inside the channel [4] and near the interface with the substrate...

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Journal ArticleDOI
TL;DR: In this article, a self-heating in AlGaN/GaN device structures was probed using integrated micro-Raman/Infrared (IR) thermography.
Abstract: Self-heating in AlGaN/GaN device structures was probed using integrated micro-Raman/Infrared (IR) thermography. IR imaging provided large-area-overview temperature maps of powered devices. Micro-Raman spectroscopy was used to obtain high-spatial-resolution temperature profiles over the active area of the devices. Depth scans were performed to obtain temperature in the heat-sinking SiC substrate. Limitations in temperature and spatial resolution, and relative advantages of both techniques are discussed. Results are compared to three-dimensional finite-difference simulations

241 citations


"Characterization of AlGaN/GaN HEMTs..." refers background in this paper

  • ...This problem can be partially solved by probing the GaN layer from the back side of the device, but induces some challenges in the heat sinking of the device, because part of the heat sink has to be removed to provide optical access [7]....

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Journal ArticleDOI
TL;DR: In this paper, two typically employed electrical methods were assessed to provide a simple means of extracting average channel temperatures in devices, and they found that electrical methods significantly underestimate peak channel temperatures, due to the fact that electrical techniques measure an average temperature over the entire active device area.
Abstract: Self-heating in AlGaN/GaN HFETs was investigated using electrical analysis and micro-Raman thermography. Two typically employed electrical methods were assessed to provide a simple means of extracting average channel temperatures in devices. To quantify the accuracy of these electrical temperature measurements, micro-Raman thermography was used to provide submicron resolution temperature information in the source-drain opening of the devices. We find that electrical methods significantly underestimate peak channel temperatures, due to the fact that electrical techniques measure an average temperature over the entire active device area. These results show that, although electrical techniques can be used to provide qualitative comparisons between different devices, they have challenges for the accurate estimation of peak channel temperatures. This needs to be taken into account for lifetime testing and reliability studies based on electrical temperature measurements.

107 citations


"Characterization of AlGaN/GaN HEMTs..." refers methods in this paper

  • ...Previous attempts to use electrical methods for thermometry show significant differences in temperature rises measured with different techniques [11]....

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Journal ArticleDOI
TL;DR: In this article, thermal conduction phenomena in GaN composite substrates containing Si, SiC, and diamond are reviewed and the potential benefits of the use of diamond on the device thermal performance is assessed.
Abstract: The thermal management challenge posed by gallium nitride (GaN) high-electron-mobility transistor (HEMT) technology has received much attention in the past decade. The peak amplification power density of these devices is limited by heat transfer at the device, substrate, package, and system levels. Thermal resistances within micrometers of the transistor junction can limit efficient heat spreading from active device regions into the substrate and can dominate the overall temperature rise. Gallium nitride composite substrates, which consist of AlGaN/GaN heterostructures with thickness of a few microns on a thicker non-GaN substrate, govern the thermal resistance associated with the “near-junction” region. Silicon and silicon carbide have been widely used as a substrate material, but the performance of GaN devices grown on these substrates is still severely limited by thermal constraints and associated reliability issues. The importance of effective junction-level heat conduction has motivated the development of composite substrates containing high-thermal-conductivity diamond, but these composites require careful attention to thermal resistances between the GaN and the diamond. This chapter reviews thermal conduction phenomena in GaN composite substrates containing Si, SiC, and diamond. The review discusses the governing conduction physics and overviews the relevant measurement techniques. The best available experimental data for GaN composite substrates as well as the relevant thermal modeling are presented. The review concludes with an assessment of the potential benefits of the use of diamond on the device thermal performance.

66 citations


"Characterization of AlGaN/GaN HEMTs..." refers background in this paper

  • ...To obtain the full potential out of these devices, understanding and controlling the device channel temperature will be important to their lifetime and performance [2]....

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