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The high-power performance of a D-band (110–170 GHz) traveling wave amplifier (TWA) is reported. The amplifier was designed and fabricated using a GaN-on-SiC high-electron mobility transistor (HEMT) technology integrated with a substrate integrated waveguide (SIW) structure for low-loss on-chip power combining. Active injection load-pull measurements of both discrete HEMTs as well as the completed MMIC TWA were performed. The discrete HEMT measurements at D-band supplement the available design data for these scaled GaN HEMTs. The TWA achieved a peak power-added efficiency (PAE) of 9.1% at 145 GHz. The available output power exceeded 23.5 dBm from 135-145 GHz, with a maximum output power of 24.7 dBm (295 mW) at 140 GHz. Keywords—millimeter 

This article presents an approach for modeling polarization-graded gallium nitride (GaN) high-electron-mobility transistors (HEMTs). Unlike conventional GaN HEMTs, where a 2-D electron gas (2DEG) forms at the barrier–channel interface, graded structures feature a 3-D electron distribution. TCAD simulations are used to extract carrier density and energy band diagrams, which form the basis for model development. The derivation uses refined approximations for the Fermi–Dirac integral solu- tion, ensuring differentiability while accurately correlating carrier density with the applied gate bias through the use of potential balance. A surface-potential-based approach is subsequently used to model terminal currents and charges. Validation of the model is done through comparison with on-wafer measurements and published data, including dc transfer and output characteristics and measured S-parameters over the frequency range of 10 MHz–110 GHz. Furthermore, model accuracy in representing linearity is verified by comparing to large signal and intermodulation measurements at 10 GHz. 

Gallium nitride (GaN)‐based high‐electron‐mobility transistors (HEMTs) are advantageous for power RF applications due to their dense 2DEG and high mobility. However, at mm‐wave frequencies, the lateral scaling required to achieve high‐frequency operation reduces breakdown voltage, limiting output power. Polarization grading of the barrier offers a solution by enhancing both RF performance and breakdown voltage. This study evaluates nonlinear grading profiles, which improve power handling without the need for additional field plates. Simulations using Synopsys Sentaurus technology computer‐aided design (TCAD) reveal that polarization‐graded HEMTs, featuring a graded AlGaN barrier layer (6–18 nm), exhibit more negative threshold voltage, increased drain current, and flatter transconductance compared to conventional abrupt‐interface HEMTs. The graded layer reduces peak lateral electric fields by as much as 39%, resulting in breakdown voltages that are 2.2 times higher than reference HEMT structures without grading. Polarization‐graded HEMTs with an 18 nm graded layer achieve a maximum output power of 8.8 W mm⁻¹, nearly three times that of conventional HEMTs. These results demonstrate the potential of polarization‐graded structures for mm‐wave applications, enhancing intrinsic field and power‐handling capabilities.