Thesis Title: "Design, Fabrication, Characterization & Packaging of Large Area RF GaN HEMTs"

Name of the Student: Mr. SHONKHO SHUVRO

Degree Registered: Ph.D. Engineering 

Advisors: Prof. Digbijoy Nath & Prof. Prosenjit Sen, CeNSE

Date: 9th April 2026, (Thursday), 4 PM

Venue : CeNSE Seminar Hall (Hybrid)

Abstract:

Wide-bandgap Gallium Nitride (GaN)-based High Electron Mobility Transistors (HEMTs)
 are strong candidates for RF power amplifiers due to their high electron saturation 
velocity, wide bandgap, and large critical electric field, enabling high power handling, 
and high-frequency operation. This work begins with the fabrication and characterization
of baseline 2×50 µm RF GaN HEMTs on SiC substrates, where performance is established through 
DC, RF, and load-pull measurements.

Building on this baseline, large-area scaling is enabled through a simple and robust 
air-bridge fabrication methodology for reliable source interconnects across wide device 
peripheries while reducing process complexity. To address current collapse caused by trapping 
effects, a passivation-first approach along with post gate metallization anneal is implemented, 
reducing drain lag from 21% to 10.4% for GaN-on-SiC with negligible knee walkout, while on a 
novel compensation free GaN-on-Si shows 15% collapse at knee. With reduced current collapse and 
optimized air-bridges, output powers of 12 W (for GaN/SiC) and 8 W (for GaN/Si) are achieved at X-band (10 GHz).

Linearity is then investigated using two-tone intermodulation measurements at 6 GHz on 2×125 
µm devices. An OIP3/PDC of 15.06 dB is achieved for GaN-on-Si—the highest reported for C and 
X bands—while GaN-on-SiC shows ~10.2 dB, comparable to literature.

Subsequently, a novel IR laser-assisted etching technique is introduced for substrates with 
low etch rates, or which are difficult to etch such as SiC, Sapphire, Diamond, Ga₂O₃, and Quartz.
 As a proof-of-concept, embedded microfluidic channels has been etched in Ga₂O₃ which demonstrated 
48% temperature reduction with liquid cooling, highlighting its potential for thermal management in Ga₂O₃ based devices.

Finally, to address self-heating in large-area devices, microfluidic cooling is implemented 
using channels on a bonded copper flange with DI water with a flow rate of 200 mL/min at room 
temperature, resulting in 46% RF power improvement (from 5.6 W to 8.1 W at 6 GHz) for GaN-on-Si.
 This approach is further extended to commercial GaN devices in a commercial RF package. 
By etching the backside of the package, the coolant is brought closer to the die hotspots, 
enabling direct thermal management beneath the active region. Importantly, this is achieved 
without altering the standard packaging process flow or having grounding issues, making it a 
practical solution for integrating liquid cooling in packaged RF GaN devices.

In summary, this work demonstrates a simple and robust air-bridge fabrication enabling 
large-area scaling, along with reduced current collapse, improved linearity, and near-junction 
cooling, collectively enabling high-performance RF GaN HEMTs.