Showing papers by "Eric R. Heller published in 2017"

Flexible Gallium Nitride for High‐Performance, Strainable Radio‐Frequency Devices

Abstract: Flexible gallium nitride (GaN) thin films can enable future strainable and conformal devices for transmission of radio-frequency (RF) signals over large distances for more efficient wireless communication. For the first time, strainable high-frequency RF GaN devices are demonstrated, whose exceptional performance is enabled by epitaxial growth on 2D boron nitride for chemical-free transfer to a soft, flexible substrate. The AlGaN/GaN heterostructures transferred to flexible substrates are uniaxially strained up to 0.85% and reveal near state-of-the-art values for electrical performance, with electron mobility exceeding 2000 cm2 V-1 s-1 and sheet carrier density above 1.07 × 1013 cm-2 . The influence of strain on the RF performance of flexible GaN high-electron-mobility transistor (HEMT) devices is evaluated, demonstrating cutoff frequencies and maximum oscillation frequencies greater than 42 and 74 GHz, respectively, at up to 0.43% strain, representing a significant advancement toward conformal, highly integrated electronic materials for RF applications.

High pulsed current density β-Ga2O3 MOSFETs verified by an analytical model corrected for interface charge

Abstract: We report on Sn-doped β-Ga2O3 MOSFETs grown by molecular beam epitaxy with as-grown carrier concentrations from 0.7 × 1018 to 1.6 × 1018 cm−3 and a fixed channel thickness of 200 nm. A pulsed current density of >450 mA/mm was achieved on the sample with the lowest sheet resistance and a gate length of 2 μm. Our results are explained using a simple analytical model with a measured gate voltage correction factor based on interface charges that accurately predict the electrical performance for all doping variations.

Characterization of AlGaN/GaN HEMTs Using Gate Resistance Thermometry

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.

Toward realization of Ga 2 O 3 for power electronics applications

Abstract: As a transparent conducting oxide with a large bandgap of ∼4.9 eV and associated large estimated critical electric field (E c ) strength of 8 MV/cm, β-Ga 2 O 3 (BGO) has been touted for its tremendous potential as a power switch. Power switch metrics such as Baliga's figure of merit (BFOM) estimating dc conduction losses and Huang's material figure of merit (HMFOM) incorporating dynamic switching losses are functions of E C 3 and E C respectively [1, 2]. BFOM for BGO is expected to exceed that of GaN by 400% and HMFOM for BGO is expected to be comparable to GaN. It can also be shown that for a given power loss during switching, the switch frequency (f) varies as E C 2 suggesting the potential of BGO power conversion in the GHz regime [1, 2]. The manufacturability and cost value proposition of BGO based on large area native substrate availability as shown by Huang's chip area manufacturing FOM (HCAFOM) indicate a disruptive cost advantage over GaN (330%). Additionally, the Johnson figure of merit (JFOM) representing the power-frequency product for RF amplification for BGO is similar to that of GaN indicating potential for integration of power conversion and RF applications in the same platform. Finally, Huang's high temperature figure of merit (HTFOM) shows that BGO has the lowest metric for all semiconductors compared here due to low thermal conductivity and high field strength. However, all wide bandgap power semiconductors, including diamond, face significant thermal engineering challenges relative to Si because of the inherent high energy densities of materials with large E c values. A summary of unipolar FET figure-of-merit comparisons for power semiconductors is shown in Table 1.

In-situ TEM study of domain switching in GaN thin films

Abstract: Microstructural response of gallium nitride (GaN) films, grown by metal-organic chemical vapor deposition, was studied as a function of applied electrical field. In-situ transmission electron microscopy showed sudden change in the electron diffraction pattern reflecting domain switching at around 20 V bias, applied perpendicular to the polarization direction. No such switching was observed for thicker films or for the field applied along the polarization direction. This anomalous behavior is explained by the nanoscale size effects on the piezoelectric coefficients of GaN, which can be 2–3 times larger than the bulk value. As a result, a large amount of internal energy can be imparted in 100 nm thick films to induce domain switching at relatively lower voltages to induce such events at the bulk scale.

Recent Progress of B-Ga2O3 MOSFETs for Power Electronic Applications

Abstract: We review AFRL’s major device results in the fabrication of β-Ga2O3 MOSFETs over the past 2 years. This includes: (1) AFRL’s standard fabrication process, (2) improvement of current density, (3) improvement of contact resistance, (4) review of the critical field measurement and (5) review of enhancement mode operation of a β-Ga2O3 finFET.

Damage Characterization for Electronic Components Under Impact Loading

Abstract: A new experimental method has been develop to characterize critical interfacial damage parameters of solder interconnects subjected to high strain-rate mechanical loading, simulating shock or impact loading. A test apparatus and a test specimen were devised to experimentally characterize such critical damage parameters, particularly interfacial shear strength and fracture toughness. The test fixture was designed to easily mount and unmount test specimens, and accommodate various sizes of electronic components and solder layer thicknesses. Test specimens were fabricated with both metallic and polymeric solder materials, and tests were conducted under various shear load rates. It was found that both strength and fracture toughness exhibit significant rate-dependency.