Numerical simulation of GaN-based heterojunction field effect transistors

As the dimensions of semiconductor devices shrink further, and as newer structures based on materials other than Si that utilize the properties of hetero-junctions are envisaged, conventional device models fail to treat adequately, the local nonlinearities specific to the system. There is therefore, a necessity for accurate computational tools that can effectively characterize charge transport in sub-micron devices of complex structures. This dissertation work presents a 2-D Ensemble Monte Carlo method to analyze high electron mobility transistors (HEMTs) based on the direct wide band-gap semiconductor, GaN.; First, a hybrid Monte Carlo simulation has been performed to analyze electron transport in bulk {dollar}Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}N{dollar} and the two-dimensional electron gas at the hetero interface composed of {dollar}Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}N/GaN{dollar}. Degeneracy was found to enhance broadening of the tail in the distribution functions in both the low and the high field regions. Degenerate statistics also causes a pronounced negative differential mobility in the high field regions. In this work, a self-consistent solution of the Schrodinger's and the Poisson's equations at the hetero-interface is obtained through a Rayleigh-Ritz method, that results in more accurate electron wave functions for the two-dimensional electron gas at the hetero-interface. The inclusion of degenerate statistics resulted in increased inter-band occupancy besides an increase in the electron energy in the respective sub-bands.; Second, an efficient and rigorous 2-D Ensemble Monte Carlo simulation has been developed for simulating the performance of a conventional HEMT. A robust iterative numerical scheme is used for the solution of the Poisson equation, which is solved in conjunction with the Monte Carlo method. It makes use of coupling the Cloud-In-Cell formulation the charge assignment with a smoothened, successively-under-relaxed electrostatic potential. The smoothing technique dissipates noisy fluctuations in the mobile charge concentrations that usually arise while performing an ensemble Monte Carlo simulation of submicronic devices. This numerical scheme was found to be robust in handling potential discontinuities at the hetero-junctions. The simulation is performed on a conventional {dollar}Alsb{lcub}x{rcub}Gasb{lcub}1-x{rcub}N/GaN{dollar} HFET. The effect of degeneracy of a heavily doped donor on the device performance has been demonstrated. Degeneracy in electron states is found to accelerate real- and k-space transfer that degrades the performance of the device. The phenomena of velocity-overshoot, real-space transfer of electrons from the channel to the surrounding areas and gate leakage currents in GaN-HEMTs are effectively addressed. The modeled results of the current-voltage characteristics are compared with existing experimental results in the literature for comparable device structures. Excellent agreement is obtained.