PhD Defense: "Establishing a Design Space and Technology RF N-polar AlGaN/GaN HEMTs"

Seshadri Kolluri

August 15th (Monday), 2:00pm
Elings Hall (CNSI Building) 1601

As an alternative to traditional Ga-polar GaN high electron mobility transistors (HEMTs), N-polar GaN HEMTs have witnessed a significant interest lately, motivated their potential advantages such as a lower contact resistance and better electron confinement. With the progress in growth and fabrication of N-polar devices, ultra-low contact resistances and excellent small signal performance have already been achieved. This dissertation focuses on improving the RF power performance N-polar GaN HEMTs for microwave and mm-wave applications, using the devices grown by MOCVD.

After the initial demonstration of RF power performance from MOCVD grown N-polar HEMTs, several optimizations were introduced in the epitaxial structure design and processing to improve the device performance and yield. An AlN interlayer and high composition AlGaN back barriers were introduced to improve the electron mobility and sheet charge density in the channel. A high RF output power density of 12.1 W/mm at 4 GHz was achieved for N-polar devies grown on sapphire substrate with a Silicon Nitride passivation. Self-heating was identified as one of the important factors limiting the power performance of these devices grown on sapphire substrates. With the devices grown on semi-insulating SiC substrates, using an AlN interlayer, and a higher composition AlGaN back barrier, excellent RF output power densities of 20.7 W/mm at 4 GHz, and 16.7 W/mm at 10 GHz, with good associated power added efficiencies, were obtained. An Al2O3 based etch-stop technology was developed for the gate recess in these devices to significantly improve the yield compared to the previous devices.

To enable the scaling of these N-polar devices to deep sub-micron gate lengths, a process flow using regrown n+ GaN access regions was established, for reducing the parasitic resistances. T-gate devices with a gate length of 150 nm, defined by e-beam lithography and deposited using a ZEP spacer technology, yielded an extrinsic fT and fmax of 62 GHz and 156 GHz respectively. The devices exhibited a good RF output power density of 6.3 W/mm with an associated PAE of 19% at 30 GHz. Some of the factors limiting the power density and efficiency of these devies at mm-wave frequencies have been identified and possible solutions are discussed.

Hosted by: Professor Umesh Mishra