9811366 Ruden Certain wide band gap semiconductor materials are quickly emerging as the materials of choice for important device applications. Among these applications are ultraviolet (UV) photodetectors, blue and UV light emitters, and high frequency, high power electronic devices. In this context, GaN and the related III-nitride materials have recently attracted great interest, and much work has been done aimed at assessing their basic properties. In a past NSF sponsored effort, the Georgia Tech and University of Minnesota groups involved in this proposal have teamed to begin the development of a novel, materials theory based device modeling technique. Application of this technique to the study of GaN bulk material has resulted in much new information: the first determination of the high field carrier drift velocities, the first determination of the hole transport properties, the first determination of the electron and hole impact ionization rates, the first determination of the breakdown electric fields in GaN, and the low and intermediate field electronic transport properties of A1N and AlGaN. Building on these achievements, a more device oriented study of the potential of GaN and the related III-N materials for several different applications can now be made. It is the principal goal of this proposed research effort to analyze the prospectus of the III-N materials and of SiC for use in high frequency power amplifiers. The program involves a theory and modeling effort in conjunction with an experimental program. The proposed theory and modeling work will be complemented by experiment in order to help refine and compare the models to real structures. By working in conjunction with an experimental team, the effect of nonidealities such as dislocations and impurities within the materials can be explored theoretically in the presence of a crucial feedback mechanism that will enhance the accuracy and relevance of the theoretical models. In addition, the theory and modeling effort w ill explore ways in which novel physical effects can be used to improve device performance. Specifically, the modeling will examine the usage of piezoelectrically induced charge densities to increase the current carrying capability of a heterojunction field effect transistor and how the p-type carrier concentration within the base of a heterostructure bipolar transistor can be increased to reduce the base resistance. In addition, the theory and modeling will be used to determine the ultimate limits of performance of SiC and GaN based high frequency, high power amplifiers by providing the first accurate determination of the parameters which influence their performance, i.e., the breakdown field, current carrying capability, "knee" voltage and heat dissipation. The study of the breakdown characteristics of different FET designs will build on our earlier work on impact ionization in bulk GaN material. However, it will not be limited solely to ideal bulk material but will include the effect of field non-uniformity in realistic device structures as well as effects associated with impurities and dislocations on the breakdown properties. For a meaningful device modeling effort close coupling to experimental work is desirable. This will be accomplished through extensive collaboration with three experimental groups active in III-N and SiC research. The principal collaborators in this effort are Superior Vacuum Technology Assoc. (SVTA) of Eden Prairie, MN, which has produced state of the art III-N materials and devices. Prof. Ruden has had close and fruitful interaction with SVTA in this area for several years. SVTA will provide device quality III-N structures for this proposed program at cost of fabrication. The second key collaborative group is at Motorola in Tempe, AZ. The Motorola group will be principally involved in high frequency device testing. Lastly, Prof. Willander's group at Chalmers University, Sweden, will provide SiC material and will be strongly involved in the fabrication and characterization of both SiC and III-N devices. Direct comparison of the calculations to experiment can thus be made, greatly aiding the refinement of the models and ensuring that they address issues confronted in state of the art devices. Additional collaborations of the principal investigators relevant to the proposed program involve the Honeywell Corp., Lockheed Martin Corp. (III-N devices), and Nortel(SiC devices). ***
