Date of Graduation


Document Type


Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level



Electrical Engineering


Samir M. El-Ghazaly

Committee Member

Gregory J. Salamo

Second Committee Member

Morgan E. Ware

Third Committee Member

Shui-Qing Yu


Distributed modeling, GaN HEMT, mm-wave transistors, Semiconductor device modeling, Thermal analysis, wave propagations


The research conducted in this dissertation is focused on developing modeling approaches for analyzing high-frequency transistors and present solutions for optimizing the device output power and gain. First, a literature review of different transistor types utilized in high-frequency regions is conducted and gallium nitride high electron mobility transistor is identified as the promising device for these bands. Different structural configurations and operating modes of these transistors are explained, and their applications are discussed. Equivalent circuit models and physics-based models are also introduced and their limitations for analyzing the small-signal and large-signal behavior of these devices are explained. Next, a model is developed to investigate the thermal properties of different semiconductor substrates. Heat dissipation issues associated with some substrate materials, such as sapphire, silicon, and silicon carbide are identified, and thinning the substrates is proposed as a preliminary solution for addressing them. This leads to a comprehensive and universal approach to increase the heat dissipation capabilities of any substrate material and 2X-3X improvement is achieved according to this novel technique.

Moreover, for analyzing the electrical behavior of these devices, a small-signal model is developed to examine the operation of transistors in the linear regions. This model is obtained based on an equivalent circuit which includes the distributed effects of the device at higher frequency bands. In other words, the wave propagation effects and phase velocity mismatches are considered when developing the model. The obtained results from the developed simulation tool are then compared with the measurements and excellent agreement is achieved between the two cases, which serves as the proof for validation. Additionally, this model is extended to predict and analyze the nonlinear behavior of these transistors and the developed tool is validated according to the obtained large-signal analysis results from measurement. Based on the developed modeling approach, a novel fabrication technique is also proposed which ensures the high-frequency operability of current devices with the available fabrication technologies, without forfeiting the gain and output power. The technical details regarding this approach and a sample configuration of the electrode model for the transistor based on the proposed design are also provided.