GaN HEMT Low Frequency Noise Characterization for Low Phase Noise Oscillator Design

Abstract: The thesis presents low frequency noise (LFN) characterization of Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) for low phase noise oscillator design. First, GaN HEMT technology is benchmarked versus other transistor technologies, e.g., GaAs-InGaP Heterojunction Bipolar Transistor (HBT) and GaAs pHEMT, in terms of noise and power. In the comparison, LFN at given frequency normalized to DC power is used as a benchmark parameter. It is verified that InGaP HBT technology provides better performance in terms of both absolute noise level and normalized values compared to other technologies. However, at higher frequencies where flicker noise is less critical, GaN HEMT has an advantage of higher power. For this reason, GaN HEMT is considered to have good potential for design of oscillators for communication systems with large channel bandwidth. Then, some factors which influence the LFN of two types of GaN HEMTs: AlGaN/GaN based HEMT and AlInN/AlN/GaN based HEMT such as surface passivation methods and variations in transistor geometry are studied. It is seen that the surface passivation has a major impact on the noise level while the effect of transistor geometry (e.g. gate length, gate width and source-drain distance) is insignificant. The best surface passivation, with respect to LFN, is Al2O3 deposited with thermal Atomic Layer Deposition (ALD). Finally, two monolithic integrated circuit (MMIC) oscillators based on GaN HEMT technology are demonstrated. A fixed frequency GaN HEMT oscillator is designed at about 10 GHz with the best achieved phase noise of -100 dBc/Hz @ 100 kHz offset. Another GaN HEMT voltage controlled oscillator (VCO) is also designed with medium (15%) tuning range between 6.45-7.55 GHz, high tuning linearity, average output power about 1 dBm and low phase noise. For a bias of Vd /Id = = 6 V/33 mA, the measured phase noise is -98 dBc/Hz @ 100 kHz and -132 dBc/Hz @ 1 MHz offset frequencies, respectively. This is the lowest phase noise reported for a GaN HEMT based VCO with comparable tuning range and oscillation frequency. Its 1 MHz phase noise performance is comparable to state-of-the-art VCOs based on InGaP-HBT technology with similar tuning range.