EXPERIMENTAL STUDIES OF LATERAL ELECTRON TRANSPORT IN GALLIUM-ARSENIDE - ALUMINUM-GALLIUM - ARSENIDE HETEROSTRUCTURES

The electron-transport characteristics of modulation-doped GaAs-Al(,x)Ga(,1-x)As heterostructures have been measured over a wide range of temperatures using a diverse set of device structures. Short voltage pulses were used to apply a broad range of lateral (parallel to the interface) electric fields and the resulting current-field characteristics were determined using a sampling oscilloscope and x-y recorder.;For higher fields (above 2 kV/cm) it was found that the electrons could gain enough energy to be thermionically emitted over the conduction-band discontinuity from the high-mobility GaAs to the low-mobility AlGaAs. This real-space transfer (RST) of electrons resulted in current saturation or various degrees of negative differential resistance (NDR) in the samples being studied. The characteristics of the NDR were found to be adjustable by changing the sample growth parameters such as the AlAs mole fraction, layer widths, and doping concentrations. It was also observed that the NDR could be significantly enhanced in many samples by illuminating the surface of the heterostructure. In a few structures, the increase in conductivity due to the photoconductive effect was found to persist at low temperatures long after the source of illumination was removed.;It was demonstrated that the new real-space transfer mechanism could be used in the creation of fast electron switching and storage devices and also high-frequency oscillators. The frequency of the oscillation was shown to be controlled by an external circuit and was not dependent on the sample length. The NDR occurring at high fields due to RST will also have an important influence on all heterostructure devices such as FETs which depend on high-speed electron transport parallel to the layers.;It was observed that the high electron mobility in these structures initially increased as the electric field was increased from zero. The low-field mobility reached a maximum at fields below 500 V/cm and then dropped quickly at low temperatures for increasingly higher electric fields. At higher temperatures (200 K to 300 K) there was comparatively little change in the mobility for fields up 2 kV/cm.