Development Of Gallium Nitride Based Ballistic Electron Acceleration Negative Differential Conductivity Diodes For Terahertz Power Generation: Thermal And Electrical Modeling, Simulation, Processing And Characterization

Abstract

The portion of the electromagnetic spectrum that lacks any viable devices is that in the region near one terahertz. Neither lasers nor electronic devices have reached this important frequency range to date. Compact, continuous wave (CW) and room temperature devices that will generate much greater than one miliwatts of power are being sought for medical, security and many other important applications. Today, the only viable solid state device seems to be the quantum cascade laser which has only recently been improved to work around room temperature, and is capable of only a couple of hundred nanowatts THz energy output. This dissertation presents a comprehensive treatment of the processing, modeling, electrical/thermal simulations, and characterization of a revolutionary device concept - the ballistic electron acceleration negative differential conductivity (BEAN) diode and its potential for electronic THz power generation. Wurtzite GaN has been the material of choice for two reasons: (i) its strong inflection point (to harness the negative effective mass quantum states) and (ii) its capability of withstanding a few MV/cm electric fields due to it’s 3.4 eV band gap. Two different device concepts based on electron acceleration in GaN were investigated as part of this research effort. These are ballistic negative effective mass (vertical) diodes and Quasi-ballistic (horizontal) diodes. In the first half of this dissertation, the quantum theoretic foundations of the negative effective mass diode will be explained and subsequent experimental results will be discussed. The details of the process development for different structures will follow. In the second half, similarly, both the theoretical foundations and experimental results on the quasi-ballistic horizontal device will be presented. Results on large-signal circuit and transient thermal simulations will be presented with discussions. The proposed diode operation in this section is in accumulation-layer transit-time mode and conversion efficiencies up to ~3.4 % at ~1.5 THz are shown to be possible. This is followed by a detailed account of the process development on this particular device. We conclude the dissertation with a chapter devoted to the ohmic contact studies on n++ doped GaN (1x1020 cm-3). The need for such studies arose from destructive joule heating at poor contacts and the intolerable voltage that drops across them.