Publications

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This paper describes a process for forming a buried field shield in GaN by an etch-and-regrowth process, which is intended to protect the gate dielectric from high fields in the blocking state. GaN trench MOSFETs made at Sandia serve as the baseline to show the limitations in making a trench gated device without a method to protect the gate dielectric. Device data coupled with simulations show device failure at 30% of theoretical breakdown for devices made without a field shield. Implementation of a field shield reduces the simulated electric field in the dielectric to below 4 MV/cm at breakdown, which eliminates the requirement to derate the device in order to protect the dielectric. For realistic lithography tolerances, however, a shield-to-channel distance of 0.4 μm limits the field in the gate dielectric to 5 MV/cm and requires a small margin of device derating to safeguard a long-term reliability and lifetime of the dielectric.

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Vertical gallium nitride (GaN) p-n diodes have garnered significant interest for use in power electronics where high-voltage blocking and high-power efficiency are of concern. In this article, we detail the growth and fabrication methods used to develop a large area (1 mm2) vertical GaN p-n diode capable of a 6.0-kV breakdown. We also demonstrate a large area diode with a forward pulsed current of 3.5 A, an 8.3-mΩ·cm2 differential specific ON-resistance, and a 5.3-kV reverse breakdown. In addition, we report on a smaller area diode (0.063 mm2) that is capable of 6.4-kV breakdown with a differential specific ON-resistance of 10.2 m·Ω·cm2, when accounting for current spreading through the drift region at a 45° angle. Finally, the demonstration of avalanche breakdown is shown for a 0.063-mm2 diode with a room temperature breakdown of 5.6 kV. These results were achieved via epitaxial growth of a 50-μm drift region with a very low carrier concentration of < 1×1015 cm-3 and a carefully designed four-zone junction termination extension.

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Ultrasounds have been investigated for data communication to transmit data across enclosed metallic structures affected by Faraday shielding. A typical channel consists in two piezoelectric transducers bonded across the structure, communicating through elastic mechanical waves. The rate of data communication is proportional to the transmission bandwidth, which can be widened by reducing the thickness of the transducers. However, thin transducers become brittle, difficult to bond and have a high capacitance that would draw a high electric current from function generators. This work focuses on investigating novel transducer shapes that would allow to provide a constant transmission across a large bandwidth while maintaining large-enough thickness to avoid brittleness and electrical impedance constraints. The transducers are shaped according to a staircase thickness distribution, whose geometry has been designed through an analytical model describing its electro-mechanical behavior formulated for this purpose.

Ultrasonic waves can be used to transfer power and data to electronic devices in sealed metallic enclosures. Two piezoelectric transducers are used to transmit and receive elastic waves that propagate through the metal. For an efficient power transfer, both transducers are typically bonded to the metal or coupled with a gel which limits the device portability. We present an ultrasonic power transfer system with a detachable transmitter that uses a dry elastic layer and a magnetic joint for efficient coupling. We show that the system can deliver more than 2 W of power to an electric load with 50% efficiency.

Several applications, such as underwater vehicles or waste containers, require the ability to transfer data from transducers enclosed by metallic structures. In these cases, Faraday shielding makes electromagnetic transmission highly inefficient, and suggests the employment of ultrasonic transmission as a promising alternative. While ultrasonic data transmission by piezoelectric transduction provides a practical solution, the amplitude of the transmitted signal strongly depends on acoustic resonances of the transmission line, which limits the bandwidth over which signals are sent and the rate of data transmission. The objective of this work is to investigate piezoelectric acoustic transducer configurations that enable data transmission at a relatively constant amplitude over large frequency bands. This is achieved through structural modifications of the transmission line, which includes layering of the transducers, as well as the introduction of electric circuits connected to both transmitting and receiving transducers. Both strategies lead to strong enhancements in the available bandwidth and show promising directions for the design of effective acoustic transmission across metallic barriers.