We present an experimental and theoretical study of current instability in an Al 0.23 Ga 0.77 As/In 0.23 Ga 77 As n-channel delta-doped pseudomorphic high electron-mobility transistor (HEMT). Monte Carlo simulations of the device indicated that it was vulnerable to the formation of unusual 'transverse' Gunn dipoles which caused sudden reductions in the drain current. These dipoles were also responsible for a significant number of impact ionization events at higher drain potentials causing a subsequent upturn in the drain current. Our experimental observations of a similar device are in excellent qualitative and good quantitative agreement with our theoretical predictions and so provide compelling evidence for the existence of Gunn instabilities in HEMTs.

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We investigate the quantum size effect in the phase coherent mesoscopic ring. A quantum dot is embedded in one arm and it is connected to one lead via tunnel barrier. Both Aharonov-Casher and Aharonov-Bohm effects are studied. A spin-dependent conductance has been deduced and it depends on the intrinsic parameters. Our results show that the strength of spin-orbit coupling depends on the size of the present device. This investigation is valuable for fabricating such spintronics devices.

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