Growth, Fabrication and Characterization of III-Nitride Hot Electron Transistors

This thesis investigates the device design, fabrication and characterization of III-Nitride Hot Electron Transistors (HET). The hot electron transistor is a vertical device, and uses similar terminology as the heterojunction bipolar transistor (HBT) for the definition of the active regions of the device. Unlike the HBT, the hot electron transistor is unipolar in nature. In a hot electron transistor, a large band-gap material is used as the emitter to inject carriers (electrons). The potential drop at the base-emitter junction largely appears as the excess energy of the electrons being injected, which are thus termed "hot" with respect to the "cold" electrons at the base Fermi level. The base thickness in the HET is nominally low, of the order of the mean free path of the injected electrons, to minimize scattering of the injected electrons. The fraction of the electrons which finally transit the base are collected over the base-collector barrier into the collector contact.;The III-nitride system offers several unique material properties advantageous to the design of the HET. Using Al(Ga)N alloys as the large band-gap material and GaN, large conduction band discontinuities are possible, enabling the efficient design of both base-emitter and base-collector barriers. The Gamma to A, M valley separation in the III-Nitride system is much larger than the other compound semiconductors (for GaN Gamma to A is ∼ 2.1 eV) which offers the device designer higher flexibility in the choice of the injection energy of the hot electrons. Inter-valley scattering events are also minimized due to this reason. The presence of polarization-induced charges is another advantage and can be utilized to reduce base and emitter contact resistances. Low base and emitter resistances are critical for the reduction of parasitic delays to enable high frequency performance of the HET.;Based on the above considerations, we developed the III-nitride HET using GaN as the base layer. High current gain (beta > 120) and near unity transfer ratio (alpha > 0.99) was obtained at room temperature. Insertion of current blocking layers, and emitter and base-contact regrowth were also implemented to improve the device characteristics. Finally the HET was used to determine experimentally the mean free path and the relaxation time of the injected electrons. For injection energies far from the higher valley (A valley in our case) a good fit to the theoretical prediction for relaxation time in GaN, based on the LO-phonon interaction, was obtained.