An Exploration of GaN-based Heterojunction Bipolar Transistors

In this work, the exploration of GaN based Heterojunction Bipolar Transistors (HBTs) is presented. The material properties of GaN, specifically, the high critical electric field for breakdown and the high electron saturation velocity combined with the merits of an HBT such as high transconductance, normally-off operation, high power density, uniform threshold voltage, etc. make GaN HBTs attractive for high frequency and high power applications.;AlGaN/GaN HBTs structures grown by MOCVD typically have a problem of Mg diffusion from the base into the emitter layers during growth, resulting in a Mg concentration tail in the emitter layer. To circumvent this problem, Ga-polar AlGaN/GaN HBTs grown by MOCVD with Regrown Emitters were fabricated. The HBTs had a high current density with good current gain. However, the fabrication of these devices revealed a problem of effectively activating the p-type GaN base layer buried underneath the emitter layer. Ammonia Molecular Beam Epitaxy (NH3 MBE) is attractive as an alternate method for the growth of GaN HBTs and other vertical devices. Chief benefits include sharp Mg concentration profiles, absence of Ga-filled dislocations that can cause vertical leakage currents, and active p-type layers. AlGaN/GaN HBTs grown by NH3 MBE are demonstrated. To obtain good ohmic contacts to the base layer, devices were fabricated with two different processes involving regrowths---Regrown base contact and Regrown emitter. HBTs with high current density and current gain were obtained by both processes and the results are discussed. High base resistance is found to limit the performance of the devices.;InGaN layers doped with Mg are a better option compared to GaN as the base layer of the HBT. Due to higher ionization efficiency of Mg in InGaN than GaN, higher hole concentrations can be obtained in p-type InGaN layers. GaN/InGaN/GaN HBTs with Common-Emitter operation and good collector current densities are demonstrated. Recombination of electrons at the emitter sidewalls and extrinsic base surface due to ion etch-damage is found to lower the current gain. A photoelectrochemical (PEC) etch treatment is found to enhance device performance and is discussed.;High hole concentrations can be obtained in graded AlGaN or InGaN layers doped with Mg by the mechanism of polarization-induced doping. N-polar GaN enables design of HBTs with graded base layers that can have high hole concentrations from polarization doping as well as quasi-electric fields that propel electrons from emitter to collector enhancing current gain. N-polar HBTs were constructed using both MOCVD and NH3 MBE growth techniques. The device results are presented. Problems in the device behavior and possible solutions are discussed.;Finally, Collector-up Ga-face HBTs were investigated. Collector-up HBT structure has the advantage of lower Base-Collector capacitance but the disadvantage of undesired injection and recombination of electrons from the emitter to the extrinsic base region. In addition, HBTs with graded base layers can be designed to obtain polarization hole doping as well as quasi electric field in the base similar to the N-polar HBTs while employing the more mature Ga-polar growth to grow the devices. The device results are presented and future challenges are presented.