Date of Award

5-1-2011

Degree Name

Master of Science

Department

Electrical and Computer Engineering

First Advisor

Ahmed, Shaikh

Abstract

AlN/GaN high-electron mobility transistors (HEMT) are subject to internal structural and electrostatic fields originating mainly from: (i) the fundamental crystal atomicity and the interface discontinuity between dissimilar materials, (ii) atomistic strain, (iii) piezoelectricity, and (iv) spontaneous polarization (pyroelectricity). In this thesis, through numerical simulations, we have studied the origin and effects of these competing internal fields on the electrostatics and the I-V characteristic of scaled nitride HEMT structures. It is shown that strain in these devices is asymmetric and long-ranged (demanding simulations using millions of atoms). The resulting piezoelectric polarization is arge and atomistic in nature. However, the pyroelectric potential is significantly larger than the piezoelectric counterpart and opposes the latter at the InN/GaN interface as opposed to AlGas which only produces a piezoelectric potential. The polarization induced charge density is computed using a three-dimensional Poisson solver and shown to be strongly dependent on the thickness of the AlN barrier layer. This finding has been validated using available experimental data. We have also demonstrated that the olarization fields alone can induce channel carriers at zero external bias and lead to a significant increase in the ON current.

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