Additive Phase Noise Measurements of AlGaN/GaN HEMTs Using a Large Signal Network Analyzer and a Tunable Monochromatic Light Source Inwon Suh * , Patrick Roblin * , Youngseo Ko * , Chieh-Kai Yang * , Andrew Malonis * , Aaron Arehart * , Steven Ringel * , Christiane Poblenz † , Yi Pei † , James Speck † and Umesh Mishra † * Electrical and Computer Engineering, The Ohio State University, Columbus, OH 43210, USA Email: roblin@ece.osu.edu, Telephone: (614) 292-0998 † Materials Science and Electrical Engineering departments, University of California, Santa Barbara, CA 93106, USA Abstract— An additive phase noise measurement system is integrated with a large signal network analyzer (LSNA) and a tunable monochromatic light source. This system is used to measure the additive phase noise characteristics of an unpas- sivated AlGaN/GaN high electron mobility transistor (HEMT) at 2 GHz under various operating conditions. Illumination with different photon energies, below the AlGaN bandgap, is applied to probe the dependence of the RF additive phase noise on the trap and 2DEG population. Different drain voltages are also used to investigate the bias dependence of the phase noise. From 1 Hz to 10 KHz, an 1/f region is identified in the additive phase noise at 2 GHz, which is indicative of the presence of uniformly distributed traps. Further a decrease in additive phase noise is clearly observed with increasing photon energies below the GaN bandgap. This is due to the decrease of the trap population induced by photon assisted emission of electrons from the trap levels to the conduction band. Further it is found that the additive phase noise at 2 GHz increases at higher drain voltages. Various RF load impedances are also used to further characterize the noise performance of both passivated and unpassivated AlGaN/GaN HEMTs. The larger the drain voltage swing introduced, the more additive phase noise is observed. A degration of additive phase noise is also observed with the unpassivated device compared to the passivated device. Some preliminary results from a physical cyclostationary model are also presented. The observed 1/f noise increase at RF occuring at large bias or in large signal RF operation are attributed to the increase efficiency of the RF upconversion of the trap 1/f occupation fluctuation when the drain resistance increases. This work also demonstrates that the new combined additive phase noise/LSNA testbed developed is a useful tool for characterizing the additive phase noise in transistors/amplifiers under large signal operation. Index Terms— Additive phase noise, AlGaN/GaN HEMTs, 1/f noise, large signal network analyzer (LSNA). I. I NTRODUCTION Additive phase noise measurements are useful for accurately characterizing the phase noise contributed by a device under test (DUT) in a system. An additive phase noise measure- ment system differs from a conventional absolute phase noise measurement system in that it ideally cancels the noise of the external RF source in the system to achieve a lower noise floor [1]. This system has been widely used to analyze and model the noise characteristics of various devices and microwave components such as GaAs heterojunction bipolar transistor (HBT), AlGaN/GaN HEMT, Silicon (Si) bipolar junction transistor (BJT), GaAs field effect transistor (FET), frequency dividers and multipliers [1]-[4]. The low-frequency noise of both unpassivated and passi- vated AlGaN/GaN HEMT was reported in [2]. A reduced phase noise was observed at low-frequency in passivated devices compared to unpassivated devices. In this work, an additive phase noise measurement system integrated with (1) an LSNA and (2) a tunable monochromatic light source is presented to investigate the noise characteristic of both unpassivated and passivated AlGaN/GaN HEMTs under various operating conditions. A physical analysis on the origin of 1/f noise in AlGaN/GaN HEMTs is also presented in support of the various measurement results. In section II, the measurement set-up for this work is described. In section III, experimental results and a detailed analysis are presented. Finally, a summary of the achievements in this work is given in section V. II. MEASUREMENT SET- UP DESCRIPTION A. Additive Phase Noise Measurement System An additive phase noise measurement system integrated with an LSNA and a tunable monochromatic light source is presented in Fig. 1. The additive phase noise system consists of a power divider, two attenuators, a phase shifter, a low-noise mixer and a phase noise analyzer. A signal source providing 16.5 dBm at 2 GHz is used to power the additive phase noise testbed. A constant power of 12 dBm is provided to the LO port of the mixer. Two step attenuators are used to control the input and output power levels at the DUT and adjust the output power level to the mixer. The phase shifter is also used to put the signals in the two branches in quadrature. By maintaining this quadrature condition, the mixer can be used as a phase detector to measure the additive phase noise of the DUT [5].