Date of Award

Winter 2001

Document Type


Degree Name

Doctor of Philosophy (PhD)


Electrical & Computer Engineering


Electrical Engineering

Committee Director

Ravindra P. Joshi

Committee Member

Duc T. Nguyen

Committee Member

Linda L. Vahala


The high electron mobility transistors (HEMTs) fabricated using wide-bandgap semiconductors show promise as high-gain, low-noise devices with superior frequency response. The structure and operation principle of HEMT are first briefly discussed. The distinguishing and unique properties of GaN are reviewed and compared with those of GaAs. Calculations of the electronic mobility and drift velocity have been carried out for bulk GaN based on a Monte Carlo approach, which serves as a validity check for the simulation model.

By taking account of polarization effects, degeneracy and interface roughness scattering, important microwave performance measures such as the dynamic range, harmonic distortion and inter-modulation characteristics are fully studied. Monte Carlo based calculations of the large-signal nonlinear response characteristics of GaN-AlGaN HEMTs with particular emphasis on intermodulation distortion (IMD) have been performed. The nonlinear electrical transport is treated on first principles, including all scattering mechanisms. Both memory and distributed effects are built into the model. The results demonstrate an optimal operating point for low intermodulation distortion (IMD) at reasonably large output power due to the exist of a minima in the IMD curve. Dependence of the nonlinear characteristics on the barrier mole fraction “x” is also demonstrated and analyzed. High-temperature predictions of the IMD have also been made by carrying out the simulations at 600 K. Due to a relative suppression of interface roughness scattering, an increase in dynamic range with temperature is predicted.

Finally, towards the end of the research, real-space transfer (RST) phenomena are included in the Monte Carlo simulator to accurately describe the electron transport behavior in HEMTs. The RST is shown to affect the velocity overshoot and inter-modulation distortion behavior and to lead to enhanced substrate leakage current as well as lowered overall performance speed. The potential for drain current compression has also been examined through simulations. Comparisons with and without RST have been performed based on Monte Carlo simulations. Results show that the velocity, IMD and dynamic range are all affected by the applied bias, temperature, internal electric field and gate length characteristics.