Gallium Nitride Devices and Microwave Power Amplifiers

There is an increasing demand for high power, broadband, high efficiency solid-state microwave power amplifiers for commercial and military applications in wireless communication. The advantages of semiconductor power transistors, if available, are high reliability, small size, low cost, and compatibility with integrated circuit technology. Wide bandgap gallium nitride (GaN) transistor has been extensively studied recently due to its capability of high speed, high power operation with good thermal management relative to the conventional gallium arsenide or silicon-based transistors. The goal of our research in the past and present is to develop high efficiency, broadband power amplifiers using GaN transistors for high power microwave application and to study the intrinsic noise characteristics of these devices for low noise device and circuit design.

Using GaN transistors, we have designed and fabricated broadband (3 dB bandwidth of DC-8 GHz with 13 dB small signal gain), high output power (3-6 Watts), high efficiency (31 % maximum power added efficiency) monolithic microwave amplifier (chip size: 2.5 mm x 1.4 mm) in collaboration with Cornell University. The concept of nonuniform distributed amplification (NDA) was developed for high efficiency and broadband design, and fabricated amplifiers demonstrated the multi-octave high power operation in a GaN monolithic microwave integrated circuit (MMIC). Using GaN transistors, we have further investigated the power-bandwidth performance limitation in the NDA, and our study suggested the possibility of a GaN MMIC NDA with 10 Watts output power (40 % maximum power added efficiency) and DC-20 GHz bandwidth.

We are currently investigating the intrinsic noise characteristics of GaN transistors. Low frequency and microwave noise measurements were performed on GaN transistors, and a noise de-embedding technique was applied to extract the intrinsic noise sources. The emphasis of this work is to determine the precise intrinsic noise sources and their origin for GaN transistors, thus enabling the development of low noise device and circuit design concepts.

The Microwave laboratory (EE316) has extensive characterization equipment in the 0-40 GHz range, along with sources, detectors and other waveguide components in the millimeter-wave range. This equipment includes: HP8510B network analyzer with an HP8516A S-parameter test set, HP8341B synthesized sweeper, and HP83420A lightwave test set for general scattering parameter measurement; HP8970B noise figure meter with an HP8971C noise figure test set and HP8673G synthesized CW generator for noise measurement; HP8350B sweep oscillator, 438A power-meter for RF power measurement; and HP8757E scalar network analyzer (or spectrum analyzer) with an indoor anechoic chamber for antenna characterization.

Chip photo of the fabricated NDA. Three cascode-connected AlGaN/GaN HEMT cells were employed to design and fabricate a very compact, broadband, high power MMIC. On-wafer measurements yielded 6.1 W maximum output power (31 % power added efficiency) at 3 GHz in Class A operation and 3-6 W output power over DC-8 GHz.

Measured and modeled minimum noise figure and optimum source reflection coefficient of 0.25 mm AlGaN/GaN HEMT. The dotted line is with 50 % of the estimated gate leakage current and the dashed line is the case with 10 % of the leakage current. Developed noise model correctly predicts the frequency dependent noise for AlGaN/GaN HEMTs.