Publications

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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.

Impact ionization coefficients play a critical role in semiconductors. In addition to silicon, silicon carbide and gallium nitride are important semiconductors that are being seen more as mainstream semiconductor technologies. As a reflection of the maturity of these semiconductors, predictive modeling has become essential to device and circuit designers, and impact ionization coefficients play a key role here. Recently, several studies have measured impact ionization coefficients. We dedicated the first part of our study to comparing three experimental methods to estimate impact ionization coefficients in GaN, which are all based on photomultiplication but feature characteristic differences. The first method inserts an InGaN hole-injection layer, the accuracy of which is challenged by the dominance of ionization in InGaN, leading to possible overestimation of the coefficients. The second method utilizes the Franz-Keldysh effect for hole injection but not for electrons, where the mixed injection of induced carriers would require a margin of error. The third method uses complementary p-n and n-p structures that have been at the basis of this estimation in Si and SiC and leans on the assumption of a constant electric field, and any deviation would require a margin of error. In the second part of our study, we evaluated the models using recent experimental data from diodes demonstrating avalanche breakdown.

Understanding of semiconductor breakdown under high electric fields is an important aspect of materials’ properties, particularly for the design of power devices. For decades, a power-law has been used to describe the dependence of material-specific critical electrical field (Ecrit) at which the material breaks down and bandgap (Eg). The relationship is often used to gauge tradeoffs of emerging materials whose properties haven’t yet been determined. Unfortunately, the reported dependencies of Ecrit on Eg cover a surprisingly wide range in the literature. Moreover, Ecrit is a function of material doping. Further, discrepancies arise in Ecrit values owing to differences between punch-through and non-punch-through device structures. We report a new normalization procedure that enables comparison of critical electric field values across materials, doping, and different device types. An extensive examination of numerous references reveals that the dependence Ecrit ∝ Eg1.83 best fits the most reliable and newest data for both direct and indirect semiconductors. Graphical abstract: [Figure not available: see fulltext.].

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This work investigates both avalanche behavior and failure mechanism of 3 kV GaN-on-GaN vertical P-N diodes, that were fabricated and later tested under unclamped inductive switching (UIS) stress. The goal of this study is to use the particular avalanche characteristics and the failure mechanism to identify issues with the field termination and then provide feedback to improve the device design. DC breakdown is measured at the different temperatures to confirm the avalanche breakdown. Diode's avalanche robustness is measured on-wafer using a UIS test set-up which was integrated with a wafer chuck and CCD camera. Post failure analysis of the diode is done using SEM and optical microscopy to gain insight into the device failure physics.

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This work reports an on-wafer study of avalanche behavior and failure analysis of in-house fabricated 1.3 kV GaN-on-GaN P-N diodes. DC breakdown is measured at different temperatures to confirm avalanche behavior. Diode's avalanche ruggedness is measured directly on-wafer using a modified unclamped inductive switching (UIS) test set-up with an integrated thermal chuck and high-speed CCD for real-time imaging during the test. The avalanche ruggedness of the GaN P-N diode is evaluated and compared with a commercial SiC Schottky diode of similar voltage and current rating. Failure analysis is done using SEM and optical microscopy to gain insight into the diode's failure mechanism during avalanche operation.

This work provides the first demonstration of vertical GaN Junction Barrier Schottky (JBS) rectifiers fabricated by etch and regrowth of p-GaN. A reverse blocking voltage near 1500 V was achieved at 1 mA reverse leakage, with a sub 1 V turn-on and a specific on-resistance of 10 mΩ-cm2. This result is compared to other reported JBS devices in the literature and our device demonstrates the lowest leakage slope at high reverse bias. A large initial leakage current is present near zero-bias which is attributed to a combination of inadequate etch-damage removal and passivation induced leakage current.

This paper describes the development of vertical GaN PN diodes for high-voltage applications. A centerpiece of this work is the creation of a foundry effort that incorporates epitaxial growth, wafer metrology, device design, processing, and characterization, and reliability evaluation and failure analysis. A parallel effort aims to develop very high voltage (up to 20 kV) GaN PN diodes for use as devices to protect the electric grid against electromagnetic pulses.

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