Design and fabrication of InGaN/GaN heterojunction bipolar transistors for microwave power amplifiers
- Author(s): Keogh, David Martin
- et al.
The GaN material system is widely recognized for its opto- electronic properties, with the recent commercialization of blue, green, and violet light emitting devices, but also has enormous potential for high power applications across a range of frequencies. The combination of high breakdown field, high electron saturation velocity, and high thermal conductivity, make it especially useful for delivering high power at high frequencies for wireless base stations, emerging WiMAX technology, and satellite communications. Though HEMTs have shown impressive performance, HBTs have many advantages as compared to HEMTs, and therefore represent an important technology. Bipolar technology, however, has not achieved the same level of success as HEMTs, as a result of some important technological obstacles. For example, the main issue with GaN-based HBTs is the issue of acceptor impurity activation, which is typically less than 1% for GaN, limiting free hole concentrations to less than 1x1018 cm- 3. Through the use of InGaN alloys in the base of an HBT, however, it is possible to achieve doping levels greater than 1x1019 cm-3, with higher mobilities and less lattice damage, enabling a high performance RF device. This dissertation embodies the design, fabrication, and characterization of InGaN/GaN HBTs under DC and RF conditions. Design of the epitaxial layer structure accounts for the piezo-electric and polarization effects present in the nitrides, which is critical for proper device operation. Furthermore, the DC and RF performance is simulated using physically based TCAD device design software to estimate the performance of an InGaN/GaN HBT. In addition, the performance of a fully-matched Class-B power amplifier is simulated at 1 GHz. Processing of InGaN /GaN HBTs was a significant portion of this thesis, and as such, a robust scheme for their fabrication was developed. Dry-etching was accomplished using Inductively Coupled Plasma (ICP), and the effects of etch conditions on the characteristics of the device explored. Also, boiling KOH solutions were found to be useful for improving the surface quality after dry-etching, and as part of a digital etching process. The final process enabled the successful fabrication of InGaN/GaN HBTs with excellent DC performance, and a maximum cut-off frequency of 0.8 GHz