Daniel Piedra

A Technology Overview of the PowerChip Development Program

The PowerChip research program is developing technologies to radically improve the size, integration, and performance of power electronics operating at up to grid-scale voltages (e.g., up to 200V) and low-to-moderate power levels (e.g., up to 50W) and demonstrating the technologies in a high-efficiency light-emitting diode driver, as an example application. This paper presents an overview of the program and of the progress toward meeting the program goals. Key program aspects and progress in advanced nitride power devices and device reliability, integrated high-frequency magnetics and magnetic materials, and high-frequency converter architectures are summarized.

GaN-on-Si Vertical Schottky and p-n Diodes

This letter demonstrates GaN vertical Schottky and p-n diodes on Si substrates for the first time. With a total GaN drift layer of only 1.5-μm thick, a breakdown voltage (BV) of 205 V was achieved for GaN-on-Si Schottky diodes, and a soft BV higher than 300 V was achieved for GaN-on-Si p-n diodes with a peak electric field of 2.9 MV/cm in GaN. A trap-assisted space-charge-limited conduction mechanism determined the reverse leakage and breakdown mechanism for GaN-on-Si vertical p-n diodes. The ON-resistance was 6 and 10 mQ · cm2 for the vertical Schottky and p-n diode, respectively. These results show the promising performance of GaN-on-Si vertical devices for future power applications.

3000-V 4.3-

This letter reports the fabrication of InAlN/GaN high-electron mobility transistors (HEMTs) with a three-terminal off-state breakdown voltage (BV) of 3000 V and a low specific on-resistance of 4.25 mΩ·cm². To reduce the drain-to-source leakage current in these devices, an AlGaN back barrier has been used. The gate leakage current in these devices is in the ~10^-10 A/mm range owing to the use of a SiO₂ gate dielectric. This current level is more than six orders of magnitude lower than in Schottky-barrier HEMTs. The combination of an AlGaN back barrier, the high charge sheet density of InAlN/GaN HEMTs, and the low leakage due to the gate-dielectric layer allows for a figure-of-merit BV²/<i>R</i>_ON,SP of ~2.1 × 10⁹ V²·Ω^-1·cm^-2.

\hbox{m}\Omega \cdot \hbox{cm}^{2}

Conventional GaN vertical devices, though promising for high-power applications, need expensive GaN substrates. Recently, low-cost GaN-on-Si vertical diodes have been demonstrated for the first time. This paper presents a systematic study to understand and control the OFF-state leakage current in the GaN-on-Si vertical diodes. Various leakage sources were investigated and separated, including leakage through the bulk drift region, passivation layer, etch sidewall, and transition layers. To suppress the leakage along the etch sidewall, an advanced edge termination technology has been developed by combining plasma treatment, tetramethylammonium hydroxide wet etching, and ion implantation. With this advanced edge termination technology, an OFF-state leakage current similar to Si, SiC, and GaN lateral devices has been achieved in the GaN-on-Si vertical diodes with over 300 V breakdown voltage and 2.9-MV/cm peak electric field. The origin of the remaining OFF-state leakage current can be explained by a combination of electron tunneling at the p-GaN/drift-layer interface and carrier hopping between dislocation traps. The low leakage current achieved in these devices demonstrates the great potential of the GaN-on-Si vertical device as a new low-cost candidate for high-performance power electronics.

InAlN/GaN MOSHEMTs With AlGaN Back Barrier

In this paper, we present self-consistent electrothermal simulations of single-finger and multifinger GaN vertical metal-oxide-semiconductor field-effect transistors (MOSFETs) and lateral AlGaN/GaN high-electron-mobility transistors (HEMTs) and compare their thermal performance. The models are first validated by comparison with experimental dc characteristics, and then used to study the maximum achievable power density of the device without the peak temperature exceeding a safe operation limit of 150°C (P150°C). It is found that the vertical MOSFETs have the potential to achieve a higher P150°C than the lateral HEMTs, especially for higher breakdown voltages and higher scaling level designs.

Origin and Control of OFF-State Leakage Current in GaN-on-Si Vertical Diodes

A BCl3 surface plasma treatment technique to reduce the resistance and to increase the uniformity of ohmic contacts in AlGaN/GaN high electron mobility transistors with a GaN cap layer has been established. This BCl3 plasma treatment was performed by an inductively coupled plasma reactive ion etching system under conditions that prevented any recess etching. The average contact resistances without plasma treatment, with SiCl4, and with BCl3 plasma treatment were 0.34, 0.41, and 0.17 Ω mm, respectively. Also, the standard deviation of the ohmic contact resistance with BCl3 plasma treatment was decreased. This decrease in the standard deviation of contact resistance can be explained by analyzing the surface condition of GaN with x-ray photoelectron spectroscopy and positron annihilation spectroscopy. We found that the proposed BCl3 plasma treatment technique can not only remove surface oxide but also introduce surface donor states that contribute to lower the ohmic contact resistance.

Electrothermal Simulation and Thermal Performance Study of GaN Vertical and Lateral Power Transistors

This letter reports a comparison of AlGaN/GaN high-electron-mobility transistors (HEMTs) fabricated on the same wafer using mesa etch or ion implantation as the isolation technology. The devices fabricated with ion implantation show similar DC performance (i.e., current density and on-resistance) to the devices with mesa isolation but have a higher breakdown voltage for the same gate-to-drain distance. AlGaN/GaN HEMTs with a breakdown voltage of 1800 V and specific on-resistance of 1.9 mΩ cm2 have been realized through ion implantation isolation. The planarity of the device structure and field termination profile are keys to explain this increase in the breakdown voltage.

Formation of low resistance ohmic contacts in GaN-based high electron mobility transistors with BCl3 surface plasma treatment

Passivation films are used in III-nitride (III-N) based devices to suppress current collapse and improve frequency performance. Several passivation films and deposition methods have the added effects of increasing the dc ON- and OFF-state currents in devices. In this paper, the physical mechanisms behind this current increase have been studied in both nanoribbon and planar devices with atomic-layer deposited Al2O3 passivation. Increased tensile stress in the AlGaN layer due to passivation leads to an increase in the charge density in nanoribbon devices. Simultaneously, the mobility in nanoribbons increases after Al2O3 passivation. These effects lead to a large (~118%) increase in the saturation drain current in nanoribbon devices. In contrast, fixed positive charge at the Al2O3-AlGaN interface leads to a small (~6%) saturation drain current increase in planar devices. In addition, the mechanisms behind the increase in the OFF-state drain current in the passivated devices are investigated. Schottky barrier lowering and the increase in surface and buffer conduction are found to be the major causes for the OFF-state current increase with passivation.

Comparative Breakdown Study of Mesa- and Ion-Implantation-Isolated AlGaN/GaN High-Electron-Mobility Transistors on Si Substrate

We demonstrate a novel GaN vertical Schottky rectifier with trench MIS structures and trench field rings. The new structure greatly enhanced the reverse blocking characteristics while maintaining a Schottky-like good forward conduction. The reverse leakage current improved beyond 104-fold and the breakdown voltage increased from 400 V to 700 V, while the low turn-on voltage (0.8 V) and on-resistance (2 mΩ·cm2) were retained. High-temperature operation up to 250 oC and fast switching performance were also demonstrated. This new device shows great potential for high-power and high-frequency applications.

Impact of Al 2 O 3 Passivation on AlGaN/GaN Nanoribbon High-Electron-Mobility Transistors

Vacancy-type defects near interfaces between metal contacts and GaN grown on Si substrates by metal organic chemical vapor deposition have been studied using a monoenergetic positron beam. Measurements of Doppler broadening spectra of the annihilation radiation for Ti-deposited GaN showed that optically active vacancy-type defects were introduced below the Ti/GaN interface after annealing at 800 °C. Charge transition of those defects due to electron capture was observed and was found to correlate with a yellow band in the photoluminescence spectrum. The major defect species was identified as vacancy clusters such as three to five Ga-vacancies coupled with multiple nitrogen-vacancies. The annealing behaviors of vacancy-type defects in Ti-, Ni-, and Pt-deposited GaN were also examined.