Publications

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Al-rich AlGaN-channel high electron mobility transistors with 80-nm long gates and 85% (70%) Al in the barrier (channel) were evaluated for RF performance. The dc characteristics include a maximum current of 160 mA/mm with a transconductance of 24 mS/mm, limited by source and drain contacts, and an on/off current ratio of 109. fT of 28.4 GHz and fMAX of 18.5 GHz were determined from small-signal S-parameter measurements. Output power density of 0.38 W/mm was realized at 3 GHz in a power sweep using on-wafer load pull techniques.

Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gallium nitride (GaN) with an aim to accomplish an improvement in size, weight, and power (SWaP) of power electronics beyond current devices based on silicon (Si). However, the increased operating power densities and reduced areal footprints of WBG device technologies result in significant levels of self-heating that can ultimately restrict device operation through performance degradation, reliability issues, and failure. Typically, self-heating in WBG devices is studied using a single measurement technique while operating the device under steady-state direct current (DC) measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since high power dissipation occurs during fast transient switching events. Therefore, it can be useful to probe the WBG devices under transient measurement conditions in order to better understand the thermal dynamics of these systems in practical applications. In this work, the transient thermal dynamics of an AlGaN/GaN high electron mobility transistor (HEMT) were studied using thermoreflectance thermal imaging and Raman thermometry. Also, the proper use of iterative pulsed measurement schemes such as thermoreflectance thermal imaging to determine the steady-state operating temperature of devices is discussed. These studies are followed with subsequent transient thermal characterization to accurately probe the self-heating from steady-state down to sub-microsecond pulse conditions using both thermoreflectance thermal imaging and Raman thermometry with temporal resolutions down to 15 ns.

This chapter discusses the motivation for the use of Ultra-Wide-Bandgap Aluminum Gallium Nitride semiconductors for power switching and radio-frequency applications. A review of the relevant figures of merit for both vertical and lateral power switching devices, as well as lateral radio-frequency devices, is presented, demonstrating the potential superior performance of these devices relative to Gallium Nitride. Additionally, representative results from the literature for each device type are reviewed, highlighting recent progress as well as areas for further research.

We present heavy ion and proton data on AlGaN high-voltage HEMTs showing single event burnout (SEB), total ionizing dose, and displacement damage responses. These are the first such data for materials of this type. Two different designs of the epitaxial structure were tested for SEB. The default layout design showed burnout voltages that decreased rapidly with increasing LET, falling to about 25% of nominal breakdown voltage for ions with LET of about 34 MeV · cm2/mg for both structures. Samples of the device structure with lower AlN content were tested with varying gate-drain spacing and revealed an improved robustness to heavy ions, resulting in burnout voltages that did not decrease up to at least 33.9 MeV · cm2/mg. Failure analysis showed that there was consistently a point, location random, where gate and drain had been shorted. Oscilloscope traces of terminal voltages and currents during burnout events lend support to the hypothesis that burnout events begin with a heavy ion strike in the vulnerable region between gate and drain. This subsequently initiates a cascade of events resulting in damage that is largely manifested elsewhere in the device. This hypothesis also suggests a path for greatly improving the susceptibility to SEB as development of this technology goes forward. Testing with 2.5-MeV protons showed only minor changes in device characteristics.

In this paper, we present heavy ion and proton data on AlGaN highvoltage HEMTs showing Single Event Burnout, Total Ionizing Dose, and Displacement Damage responses. These are the first such data for materials of this type. Two different designs of the epitaxial structure were tested for Single Event Burnout (SEB). The default layout design showed burnout voltages that decreased rapidly with increasing LET, falling to about 25% of nominal breakdown voltage for ions with LET of about 34 MeV·cm2/mg for both structures. Samples of the device structure with lower AlN content were tested with varying gate-drain spacing and revealed an improved robustness to heavy ions, resulting in burnout voltages that did not decrease up to at least 33.9 MeV·cm2/mg. Failure analysis showed there was consistently a point, location random, where gate and drain had been shorted. Oscilloscope traces of terminal voltages and currents during burnout events lend support to the hypothesis that burnout events begin with a heavy ion strike in the vulnerable region between gate and drain. This subsequently initiates a cascade of events resulting in damage that is largely manifested elsewhere in the device. This hypothesis also suggests a path for greatly improving the susceptibility to SEB as development of this technology goes forward. Lastly, testing with 2.5 MeV protons showed only minor changes in device characteristics.

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GaN is an attractive material for high-power electronics due to its wide bandgap and large breakdown field. Verticalgeometry devices are of interest due to their high blocking voltage and small form factor. One challenge for realizing complex vertical devices is the regrowth of low-leakage-current p-n junctions within selectively defined regions of the wafer. Presently, regrown p-n junctions exhibit higher leakage current than continuously grown p-n junctions, possibly due to impurity incorporation at the regrowth interfaces, which consist of c-plane and non-basal planes. Here, we study the interfacial impurity incorporation induced by various growth interruptions and regrowth conditions on m-plane p-n junctions on free-standing GaN substrates. The following interruption types were investigated: (1) sample in the main MOCVD chamber for 10 min, (2) sample in the MOCVD load lock for 10 min, (3) sample outside the MOCVD for 10 min, and (4) sample outside the MOCVD for one week. Regrowth after the interruptions was performed on two different samples under n-GaN and p-GaN growth conditions, respectively. Secondary ion mass spectrometry (SIMS) analysis indicated interfacial silicon spikes with concentrations ranging from 5e16 cm^-3 to 2e18 cm^-3 for the n-GaN growth conditions and 2e16 cm^-3 to 5e18 cm-3 for the p-GaN growth conditions. Oxygen spikes with concentrations ~1e17 cm^-3 were observed at the regrowth interfaces. Carbon impurity levels did not spike at the regrowth interfaces under either set of growth conditions. We have correlated the effects of these interfacial impurities with the reverse leakage current and breakdown voltage of regrown m-plane p-n junctions.

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Photodetectors sensitive to the ultra-violet spectrum were demonstrated using an AlGaN high electron mobility transistor with an GaN nanodot optical floating gate. Peak responsivity of 2 x 10⁹ A/W was achieved with a gain-bandwidth product > 1 GHz at a cut-on energy of 4.10 eV. Similar devices exhibited visible-blind rejection ratios > 10⁶. The photodetection mechanism for $β$-Ga₂O₃ was also investigated. It was concluded that Schottky barrier lowering by self-trapped holes enables photodetector gain.

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GaN is an attractive material for high-power electronics due to its wide bandgap and large breakdown field. Verticalgeometry devices are of interest due to their high blocking voltage and small form factor. One challenge for realizing complex vertical devices is the regrowth of low-leakage-current p-n junctions within selectively defined regions of the wafer. Presently, regrown p-n junctions exhibit higher leakage current than continuously grown p-n junctions, possibly due to impurity incorporation at the regrowth interfaces, which consist of c-plane and non-basal planes. Here, we study the interfacial impurity incorporation induced by various growth interruptions and regrowth conditions on m-plane p-n junctions on free-standing GaN substrates. The following interruption types were investigated: (1) sample in the main MOCVD chamber for 10 min, (2) sample in the MOCVD load lock for 10 min, (3) sample outside the MOCVD for 10 min, and (4) sample outside the MOCVD for one week. Regrowth after the interruptions was performed on two different samples under n-GaN and p-GaN growth conditions, respectively. Secondary ion mass spectrometry (SIMS) analysis indicated interfacial silicon spikes with concentrations ranging from 5e16 cm-3 to 2e18 cm-3 for the n-GaN growth conditions and 2e16 cm-3 to 5e18 cm-3 for the p-GaN growth conditions. Oxygen spikes with concentrations ∼1e17 cm-3 were observed at the regrowth interfaces. Carbon impurity levels did not spike at the regrowth interfaces under either set of growth conditions. We have correlated the effects of these interfacial impurities with the reverse leakage current and breakdown voltage of regrown m-plane p-n junctions.

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Inherent advantages of wide bandgap materials make GaN-based devices attractive for power electronics and applications in radiation environments. Recent advances in the availability of wafer-scale, bulk GaN substrates have enabled the production of high quality, low defect density GaN devices, but fundamental studies of carrier transport and radiation hardness in such devices are lacking. Here, we report measurements of the hole diffusion length in low threading dislocation density (TDD), homoepitaxial n-GaN, and high TDD heteroepitaxial n-GaN Schottky diodes before and after irradiation with 2.5 MeV protons at fluences of 4-6 × 1013 protons/cm2. We also characterize the specimens before and after irradiation using electron beam-induced-current (EBIC) imaging, cathodoluminescence, deep level optical spectroscopy (DLOS), steady-state photocapacitance, and lighted capacitance-voltage (LCV) techniques. We observe a substantial reduction in the hole diffusion length following irradiation (50%-55%) and the introduction of electrically active defects which could be attributed to gallium vacancies and associated complexes (VGa-related), carbon impurities (C-related), and gallium interstitials (Gai). EBIC imaging suggests long-range migration and clustering of radiation-induced point defects over distances of ∼500 nm, which suggests mobile Gai. Following irradiation, DLOS and LCV reveal the introduction of a prominent optical energy level at 1.9 eV below the conduction band edge, consistent with the introduction of Gai.

AlGaN-channel high electron mobility transistors (HEMTs) are among a class of ultra wide-bandgap transistors that are promising candidates for RF and power applications. Long-channel Al_xGa_1-xN HEMTs with x = 0.7 in the channel have been built and evaluated across the -50°C to +200°C temperature range. These devices achieved room temperature drain current as high as 46 mA/mm and were absent of gate leakage until the gate diode forward bias turn-on at ~2.8 V, with a modest -2.2 V threshold voltage. A very large I_on/I_off current ratio, of 8 × 10⁹ was demonstrated. A near ideal subthreshold slope that is just 35% higher than the theoretical limit across the temperature range was characterized. The ohmic contact characteristics were rectifying from -50°C to +50°C and became nearly linear at temperatures above 100°C. An activation energy of 0.55 eV dictates the temperature dependence of off-state leakage.

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