Defense Date

2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Engineering

First Advisor

Hadis Morkoç

Abstract

New power applications for managing increasingly higher power levels require that more heat be removed from the power transistor channel. Conventional treatments for heat dissipation do not take into account the conversion of excess electron energy into longitudinal optical (LO) phonons, whose associated heat is stored in the channel unless such LO phonons decay into longitudinal acoustic (LA) phonons via a Ridley path. A two dimensional electron gas (2DEG) density of ~5×1012cm-2 in the channel results in a strong plasmon–LO phonon coupling (resonance) and a minimum LO phonon lifetime is experimentally observed, implying fast heat removal from the channel. Therefore, it is desirable to shift the resonance condition to higher 2DEG densities, and thereby higher power levels. The more convenient way to attain the latter is by widening the 2DEG density profile via heterostructure engineering, i.e. by using multiple channel heterostructures. A single channel heterostructure (GaN/AlN/AlGaN), a basic heterostructure used to obtain a 2DEG, exhibits a resonance condition at low 2DEG densities (~0.65×1012 cm-2). Successful widening of the 2DEG density xv profile was predicted by simulation results for two types of multiple (Al)GaN channel heterostructures, i.e. coupled channel GaN/AlN/GaN/AlN/AlGaN and dual channel GaN/AlGaN/AlN/AlGaN. Because of a reduction of carrier confinement, it is experimentally observed that control of the channel is moderate in the case of dual channel heterostructures. On the other hand, carrier confinement provides a better control of the channel in coupled channel heterostructures. Furthermore, unlike in a dual channel heterostructure, alloy scattering does not affect carrier transport properties, which results in a higher cut-off frequency. It was found experimentally that the coupled channel heterostructure successfully reaches resonance condition at a 2DEG density that is 23% higher than in a single channel heterostructure. Multiple channel heterostructures therefore provide a convenient way to shift the plasmon-LO phonon resonance to higher 2DEG densities. However, in our grown heterostructures, high power levels under optimal channel working conditions and minimum heat accumulation, all desirable benefits for the development of high power transistors, were only observed in coupled channel heterostructures.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

12-11-2014

Available for download on Tuesday, December 10, 2019

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