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   COLLOQUIUM OF THE LABORATORY FOR COMPUTER DESIGN OF MATERIALS        
                   School of Computational Sciences	
                           (CSI 898-Sec 001) 
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           Duality theory of self-oscillations in quantum 
                      tunneling nanostructures

                            F. A. Buot
             Naval Research Laboratory, Washington, D.C.

     What is not widely recognized is that double-barrier resonant 
tunneling structures have the potential to exhibit autonomous oscillation 
at much higher frequencies (THz range) than those of impact ionization 
avalanche transit time (IMPATT) diodes and Gunn effect microwave sources. 
These tunneling devices have nanometric sizes, whereas IMPATT and Gunn 
effect devices are quite large in comparison. As THz sources, there are 
clear advantages in terms of simplicity, compactness, and monolithic 
integration with power combiners, matching guided-wave structures and 
antennas using integrated-circuit fabrication technology. Here, the 
duality theory or pairing of relevant dynamical variables in resonant 
tunneling structures (RTS's) with conventional (type I) and with 
staggered (type II) alignments of the heterostructure energy-band gaps 
is discussed. Two entirely different physical models for type I and 
type II RTS's were introduced and described by a unifying set of coupled 
nonlinear-rate equations. These coupled equations were solved for the 
limit cycle solution. Applying the result to type I RTS, the limit cycle 
predicts a rising average current whereas the non-oscillatory solution 
predicts a falling current as a function of bias in the current ''plateau'', 
after the resonant-current peak. The limit cycle also predicts decreasing 
amplitude of current oscillation as a function of bias in the current 
plateau. The behavior of the fundamental frequency as a function of bias 
in the plateau agrees with experiments and simulations of AlGaAs/GaAs 
type I RTS. Applying the result to type II RTS, the limit-cycle 
oscillation of the barrier-well polarization and trapped charge in the 
barrier induce THz oscillation in the resonant tunneling current across 
the device, before the resonant-current peak. The time-averaged results 
agree with the measured current-voltage (I-V) characteristic of 
AlGaSb/InAs/AlGaSb type II RTS. In particular, the measured smaller 
current offset at forward bias compared to that of reverse bias in the 
I-V hysteresis loop is predicted by the physical model and limit cycle 
analysis. Thus, the duality theory of type I and type II RTS is hereby 
established.

                        Monday , September 18, 2000
                                  4:30 pm
                        Room 206, Science & Tech. I

Refreshments will be served.
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Find the schedule at http://csi.gmu.edu/lcdm/seminar/schedule.html