Ballistic electron transport in nanoscale three-branch junctions

Presented here is an experimental study on a novel electron device utilizing ballistic electron transport. This device is a three-terminal structure comprised of lithographically defined Y-shaped two-dimensional electron gas (2DEG) in a compound semiconductor heterostructure. Ballistic electron transport causes a nonlinear input-output transfer curve, which can be exploited for signal rectification, frequency multiplication, and logic gate function. Device fabrication technique using electron beam lithography and a carbon-hard-mask was developed, in order to reliably fabricate ∼100-nm-wide 2DEG wires. Direct current measurements while changing the device length and operating temperature revealed the role of ballistic transport in the nonlinear behavior and elucidated that the intervalley transfer mechanism took over the nonlinear behavior under the high electric field. Electrical response at terahertz frequencies was studied by ultrafast time-domain analysis based on an electro-optic sampling and sub-picosecond pulse generation using a photoconductive switch. The terahertz measurement validated the superior high-speed performance of this device as a consequence of small internal capacitance. This research opens up a way to realize various ballistic devices with room-temperature operation and terahertz-bandwidth performance.