- Post Doctoral
MIT Unit Affiliation:
- Electrical Engineering & Computer Science
Post Doc Sponsor / Advisor:
Date PhD Completed:
Top 3 Areas of Expertise:
Expected End Date of Post Doctoral Position:
Gallium Nitride (GaN)-based high-electron-mobility transistor (HEMT) has been an emerging technology for the use in next-generation wireless communication systems. The combination of high electron mobility and critical electric field allows unprecedented power level, power-added-efficiency (PAE) and breakdown voltage in GaN power amplifiers (PAs). Despite the advances in GaN RF technology, the non-linearity characteristics of GaN PAs and the impact on the circuit performance have not been thoroughly investigated.
The goal of this project is to understand the key device design parameters and their impact on the amplifier linearity, to improve the gm and fT linearity characteristics of GaN PAs by means of device-level and thermal design without compromising the aforementioned figures of merits (FOMs).
From the results based on the MIT Virtual Source GaNFET-RF (MVSG) model, we understand that the primary non-linear content in gm occurs in the region of transition from weak to strong accumulation. This causes significant higher order harmonic components and intermodulation distortion (IMD) in power amplification. To solve this problem, we are proposing several new approaches to engineer the transition between weak and strong accumulation in GaN multi-finger power amplifiers. In this way, we are able to tailor the overall transconductance characteristics to attenuate the higher order transconductance and minimize the IMD.
Power electronics are used in advanced technology for generating and using sustainable energy. Examples are solar converters, motor drives, hybrid electrical vehicles or switch mode power supplies. In this technology, they play a key role in any form of power conversion. Gallium Nitride (GaN), next to its use in light emitting diodes (LEDs), also enables high-voltage, high-power, and high-temperature electronic circuits. It has an electrical breakdown field that is an order of magnitude higher than silicon. The AlGaN/GaN materials system offers significant advantages over Si for power devices, as it allows fabricating High-Electron-Mobility Transistors (HEMTs) with fast switching properties, high ratio of breakdown voltage over on-resistance, and high temperature operation. In power convertors, the circuits usually require a high voltage power diode next to the HEMT transistor.
The purpose of this PhD is to look into the fundamental aspects of the design of GaN diodes, and the process architectures for co-integration with the HEMT. This thesis shows that the AlGaN/GaN Schottky barrier diode with a gated edge termination (GET-SBD) proves to be a promising architecture. Combined with AlGaN barrier recess process in GET-SBD, low leakage current and low forward voltage can be simultaneously achieved in GaN diodes demonstrating state-of-the-art performance. This has been experimentally realized in small GaN diodes and 10-mm power diodes, fabricated on 8-in silicon wafers with Au-free CMOS-compatible process flows. However, the stability of the forward characteristics can be severely influenced by electron trapping, when the GaN diode is subjected to off-state stress.
Significant improvement in diode stability has been achieved by applying plasma cleaning steps, using an in-situ SiN passivation layer and low-dispersive buffer layers, etc. Initial reliability tests show that the GET-SBD has a good on-state reliability. Sequential time-dependent dielectric breakdown characteristics have been observed when GET-SBDs were stressed in high temperature reverse bias (HTRB) reliability tests. In summary, the low-cost recessed GET-SBD architecture has demonstrated
competitive performance over Si and SiC power diodes for 200-V application platform, further buffer design and better choice of the edge termination materials are required to allow for stable and reliable GaN diodes towards higher voltage applications.
Top 5 Awards and honors (name of award, date received):
5 Recent Papers:
J. Hu, S. Stoffels, S. Lenci, N. Ronchi, B. De Jaeger, G. Groeseneken, and S. Decoutere, “<a href="http://ieeexplore.ieee.org/document/7517385/">Statistical analysis of the impact of AlGaN barrier recess on the electrical characteristics of AlGaN/GaN Schottky diodes with gated edge termination</a>,” <cite>IEEE Transactions on Electron Devices</cite>, vol. 63, no. 9, pp. 3451–3458, Sep. 2016.