The role of spontaneous and piezoelectric polarization in III-V nitride heterostructure devices is discussed. Problems as well as opportunities in incorporating polarization in abrupt and graded heterojunctions composed of binary, ternary, and quaternary nitrides are outlined.
In this work, high field carrier transport in two dimensional (2D) graphene is investigated. Analytical models are applied to estimate the saturation currents in graphene, based on the high scattering rate of optical phonon emission. Non-equilibrium
(hot) phonon effect was studied by Monte Carlo (MC) simulations. MC simulation confirms that hot phonon effects play a dominant role in current saturation in graphene. Current degradation due to elastic scattering events is much smaller compared to the hot phonon effect. Transient phenomenon as such as velocity overshoot was also studied using MC simulation. The simulation results shows promising potential for graphene to be used in high speed electronic devices by shrinking the channel length below 100nm if electrostatic control can be exercised in the absence of a band gap.
The large electronic polarization in III-V nitrides allow for novel physics not possible in other semiconductor families. In this work, interband Zener tunneling in wide-bandgap GaN heterojunctions is demonstrated by using polarization-induced electr
ic fields. The resulting tunnel diodes are more conductive under reverse bias, which has applications for zero-bias rectification and mm-wave imaging. Since interband tunneling is traditionally prohibitive in wide-bandgap semiconductors, these polarization-induced structures and their variants can enable a number of devices such as multijunction solar cells that can operate under elevated temperatures and high fields.
The transport properties of carriers in semiconducting graphene nanoribbons are studied by comparing the effects of phonon, impurity, and line-edge roughness scattering. It is found that scattering from impurities located at the surface of nanoribbon
s, and from acoustic phonons are as important as line edge roughness scattering. The relative importance of these scattering mechanisms varies with the temperature, Fermi level location, and the width of the ribbons. Based on the analysis, strategies for improvement of low-field mobility are described.
A theory is developed for interband tunneling in semiconducting carbon nanotube and graphene nanoribbon p-n junction diodes. Characteristic length and energy scales that dictate the tunneling probabilities and currents are evaluated. By comparing the
Zener tunneling processes in these structures to traditional group IV and III-V semiconductors, it is proved that for identical bandgaps, carbon based 1D structures have higher tunneling probabilities. The high tunneling current magnitudes for 1D carbon structures suggest the distinct feasibility of high-performance tunneling-based field-effect transistors.
