Additive phase noise measurements of AlGaN/GaN HEMTs using a large signal network analyzer and a tunable monochromatic light source

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 unpassivated 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.