| In the last five years, the commercial outlook for GaN optoelectronic and electronic devices has grown considerably. This dissertation is focused on efforts to develop growth and fabrication technology for the GaN HBT. The development of the HBT is motivated by a demand for high power, high frequency electronics. From the demonstration of the first AlGaN/GaN HBT in 1998, and the first HBT on LEO substrates, to the first small signal rf measurements of a GaN HBT in 2000, progress has been made in identifying the major issues in the DC and RF performance of the AlGaN/GaN HBT. Devices have been fabricated on MOCVD and MBE active layers of varying structure including compositionally graded bases, and base layers of varying thickness and doping concentration. The cause of the parasitic offset voltage in the common emitter output characteristics has been investigated, and addressed in the extrinsic regrown base HBT as well as in process improvements associated with the emitter mesa etch and the base metalization process. The effects of dislocations were investigated by fabricating devices on LEO substrates and measuring the collector-emitter leakage as a function of dislocation density and base doping profile. Finally, a process was developed for fabricating CPW compatible devices, and devices were tested and shown to have an current gain cut-off frequency of 2GHz. The gain/frequency characteristic was un-ideal, however, and finite element simulations confirm that this may be due to the low conductivity in the base. |
