GaN-Based Impact-Ionization Avalanche Transit-Time (IMPATT) Diodes and Low-Loss Interconnects for Microwave and Millimeter Wave Applications

GaN based devices are ideal for high frequency and high power applications due to GaN's wide bandgap and high electron saturation velocity, and the ability to take advantage of polarization in heterostructures. To further enhance the capabilities of GaN-based electronics, work in two main directions has been pursued: low-loss interconnects for GaN-on-Si monolithic microwave integrated circuits (MMICs) and GaN IMPATT diodes as novel high power microwave and mm-wave sources.

Low-loss interconnects are important for maximizing circuit performance in amplifiers and oscillators, but have proven challenging to realize in low-cost GaN-on-Si technology. In this work, low-loss coplanar waveguides (CPWs) on AlGaN/GaN HEMT heterostructures grown on high-resistivity Si substrates (GaN-on-Si) are demonstrated. Performance has been measured from 0.1 to 20 GHz, and loss as low as 0.27 dB/mm at 20 GHz have been achieved. In addition, the nonlinear behavior of the CPW lines were experimentally characterized using large-signal measurements. In contrast to the small-signal loss, more significant differences in second- and third-order nonlinearity, and thus intermodulation, are observed between Si and SiC substrates.

IMPATT diodes employ impact ionization and transit time effects to directly generate RF power with high efficiency. However, the impact ionization behavior of GaN is not well understood. In order to measure the impact ionization rates of electrons and holes in GaN, p-i-n avalanche diodes grown on native GaN substrates have been fabricated and characterized. By incorporating a pseudomorphic In0.07Ga0.93N layer below the drift layer, the photomultiplication method was used to experimentally determine the impact ionization coefficients. The impact ionization coefficients of electrons and holes are also measured at elevated temperatures up to 498 K. In addition, the low frequency noise characteristics of these devices have been measured under forward and reverse bias conditions. At reverse biases in the avalanche regime, the multiplication noise overwhelms the 1/f noise, resulting in a white noise spectrum. The impact ionization ratio extracted from multiplication noise is consistent with impact ionization coefficients obtained using the photomultiplication method.

In addition, the DC and RF performance of a GaN IMPATT diode operating at W-band is simulated using TCAD Sentaurus. Numerical simulation results demonstrate that the IMPATT diode is capable of generating RF power larger than 1 MW/cm2 from 80 GHz to 120 GHz with an efficiency larger than 18% over W-band. First and second generation GaN IMPATT diodes grown on bulk GaN substrates have been fabricated and characterized. Two fabrication processes, based on mesa etching and ion implantation for isolation, were developed and their influence on the diode's forward and reverse IV characteristic were compared. The 1st generation IMPATT diodes show poor reverse-bias characteristics due to inadequate edge termination. Compared to the 1st generation IMPATT diode, the second generation IMPATT diodes exhibit promising avalanche behavior with improved edge termination methods. However, the RF characteristics of the second generation IMPATT diodes do not exhibit negative conductance, possibly due to large series resistance.

In terms of future work, the demonstration of functional GaN IMPATT diodes and oscillators based on them is proposed.