Influence on Noise Performance of GaN HEMTs With In Situ and Low-Pressure-Chemical-Vapor-Deposition SiNxPassivation
IEEE Transactions on Electron Devices
Wireless technology for future communication systems has been continuously evolving to meet society's increasing demand on network capacity. The millimetre-wave frequency band has a large amount of bandwidth available, which is a key factor in enabling the capability of carrying higher data rates. However, a challenge with wideband systems is that the capacity of these systems is limited by the noise floor of the local oscillator (LO). The LO in today's communication systems is traditionally
... erated at low frequency and subsequently multiplied using frequency multipliers, leading to a significant degradation of the LO noise floor at millimetre-wave frequencies. For this reason, the thesis considers low phase noise millimetrewave signal source design optimised for future wideband millimetre-wave communications. In an oscillator, low frequency noise (LFN) is up-converted into phase noise around the microwave signal. Thus, aiming for low phase noise oscillator design, LFN characterisations and comparisons of several common III-V transistor technologies, e.g. GaAs-InGaP HBTs, GaAs pHEMTs, and GaN HEMTs, are carried out. It is shown that GaN HEMTs have good potential for oscillator applications where far-carrier phase noise performance is critical, e.g. wideband millimetre-wave communications. Since GaN HEMT is identified as an attractive technology for low noise floor oscillator applications, an in-depth study of some factors which affects LFN characteristics of III-N GaN HEMTs such as surface passivation methods and variations in transistor geometry are also investigated. It is found that the best surface passivation and deposition method can improve the LFN level of GaN HEMT devices significantly, resulting in a lower oscillator phase noise. Several MMIC GaN HEMT based oscillators including X-band Colpitts voltage-controlledoscillators (VCOs) and Ka-band reflection type oscillators are demonstrated. It is verified that GaN HEMT based oscillators can reach a low noise floor. For instance, X-band GaN HEMT VCOs and a Ka-band GaN HEMT reflection type oscillator with 1 MHz phase noise performance of -135 dBc/Hz and -129 dBc/Hz, respectively, are demonstrated. These results are not only state-of-the-art for GaN HEMT oscillators, but also in-line with the best performance reported for GaAs-InGaP HBT based oscillators. Further, the MMIC oscillator designs are combined with accurate phase noise calculations based on a cyclostationary method and experimental LFN data. It has been seen that the measured and calculated phase noise agree well. The final part of this thesis covers low phase noise millimetre-wave signal source design and a comparison of different architectures and technological approaches. Specifically, a fundamental frequency 220 GHz oscillator is designed in advanced 130 nm InP DHBT process and a D-band signal source is based on the Ka-band GaN HEMT oscillator presented above and followed by a SiGe BiCMOS MMIC including a sixtupler and an amplifier. The Ka-band GaN HEMT oscillator is used to reach the critical low noise floor. The 220 GHz signal source presents an output power around 5 dBm, phase noise of -110 dBc/Hz at 10 MHz offset and a dc-to-RF efficiency in excess of 10% which is the highest number reported in open literature for a fundamental frequency signal source beyond 200 GHz. The D-band signal source, on the other hand, presents an output power of 5 dBm and phase noise of -128 dBc/Hz at 10 MHz offset from a 135 GHz carrier signal. Commenting on the performance of these two different millimetre-wave signal sources, the GaN HEMT/SiGe HBT source presents the best normalized phase noise at 10 MHz, while the integrated InP HBT oscillator demonstrates significantly better conversion efficiency and still a decent phase noise.