Date of Award

Fall 2024

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Asif Khan

Abstract

Due to high critical electric field and saturation velocity, wide bandgap (WBG) III-Nitride semiconductor materials (AlxGa1-xN) and their heterostructures have gone through extensive research in academia and industry for applications requiring high-voltage, high-current, high-frequency, and high-temperature transistor operation. AlGaN/GaN high electron mobility transistor (HEMT) has been commercialized for 650-V power converters, smartphones, PCs, USB wall power outlets, on-board and off-board EV chargers, and numerous other applications. However, AlGaN/GaN HEMTs grown on low-cost sapphire or Si substrates face significant self-heating issues due to the poor thermal conductivity of the substrate. The high thermal resistance (RTH) and heat capacity of the substrates lead to excessively high channel temperatures leading to potential device failure under extreme operating conditions. Potential approaches to mitigate this heating issue are to use high thermal conductivity substrates such as AlN or SiC, removing the low-thermal conductivity substrate and replacing it with high thermally conductivity metallic substrate such as Copper; or deposition of high thermal conductive heat removal layers such as diamond. However, all these approaches require either significant processing optimization or high cost. In this dissertation, we describe the development of a low-cost approach namely growing high thermal conductive thick aluminum nitride (AlN) buffer layer as a template for the growth of GaN channel HEMT for better thermal management. This was accomplished using a modified metal-organic chemical vapor deposition (MOCVD) approach that produced AlN templates (over sapphire) with a thickness of 16.0 μm, and a record thermal conductivity of 321 Wm-1K-1. GaN channel HEMTs grown on 3.0, 6.0, and 12.0 μm AlN templates showed the thermal resistance RTH of 22.2, 19.5, and 15.4 K-mm/W respectively, which are substantially lower than ~ 30-50 RTH for HEMTs with thinner AlN or GaN buffer layers. In this thesis, we also focused on developing ultrawide-bandgap (UWBG) AlxGa1-xN (x > 0.4, EG > 4.5 eV) and extreme bandgap (EBG) AlxGa1-xN (x > 0.6, EG > 5.0 eV) channel HEMTs which are promising due to their potential for high-temperature, high-voltage, and high-power applications. Finally, combining EBG AlxGa1-xN (x > 0.6, EG > 5.0 eV) channel HEMTs with growth on bulk AlN substrates, devices with a record two-terminal critical breakdown field, EC > 11 MV/cm were achieved. This translates to a Baliga Figure of Merit (BFOM) of 2.27 GW/cm2. The thermal impedance for these devices decreased to 10 K-mm/W, a value which is 1/3 of that for devices on sapphire substrates.

Rights

© 2025, Md Abdullah-Al Mamun

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