This research program will investigate electron transport in heterostructure devices with the goal of improving the performance of terahertz-frequency semiconductor devices. Three specific types of two- terminal devices will be investigated; metal semiconductor junctions, heterojunction barrier diodes, and a high-electron mobility diode. Metal- semiconductor junctions have been the workhorse device in millimeter and submillimeter wavelength applications for many years, and continue to yield state-of-the-art performance at THz frequencies. The goals of our continued work on these devices will be to increase the operating frequency to the 10 THz range and determine the fundamental limit to their maximum operating frequency. The heterojunction barrier devices will use the barrier between two III-V semiconductor materials to control the current flow perpendicular to the interface. Since the composition and doping of the semiconductor materials can be varied over a wide range, the properties of the barrier can, to a degree, be tailored to yield the desired device characteristics for specific applications. The heterojunction barrier varactor which is a novel device that promises to greatly improve the maximum frequency and output power of frequency multipliers, will be extensively investigated. As time permits, the effect of a heterojunction barrier on a metal-semiconductor junction will also be investigated, by varying the design of the heterojunction, the important properties of the diode can be controlled, leading to greatly increased design flexibility and improved performance. The high electron mobility diode is a fundamentally new two-terminal device in which the electron conduction is in a two dimensional electron gas at the heterojunction interface. Our previous NSF grant led to the investigation of a prototype high- electron mobility varactor diode that exhibited strongly non-linear capacitance-voltage characteristics and large reverse breakdown voltage. Research on each device investigated in this grant will have three specific components; the investigation of fundamental device physics, the study of the feasibility of the proposed device for (2) scientific applications and the fabrication of prototype devices to evaluate the theoretical predictions.
