Fundamental Research Of GaN-based HEMT Devices

With demand of high frequency and wireless devices, high electron mobilitytransistors (HEMTs) play a more and more important role due to their low noise andhigh power. The performances of heterojunction based HEMTs are to a large degreedetermined by their sheet carrier density and distribution of two dimensional electrongas (2DEG) at an interface of semiconductor heterostructure. Currently, anAlGaN/GaN based heterostructure is one of most intensively studied structures.Therefore, the investigation on the sheet carrier density of 2DEG and distribution inthe AlGaN/GaN heterostructure is of critical importance to study their transportproperties of the carriers of the 2DEG in AlGaN/GaN and designing new structureswith improved performances. The main content and key points in the dissertation aresummarized as follows:1. The sheet carrier density and distribution of 2DEG at AlGaN/GaN interfacehave been calculated using self consistent solutions of both Poisson's andSchr?dinger's equations using a program made from Dr Greg Snider atUniversity of Notre Dame. Relevant factors have been discussed. Thecalculation indicates that the sheet density of the 2DEG at Al_0.25Ga_0.75N/GaNinterface is about 10¹²～10¹³cm² due to a strong polarization effect and alarge discontinuity of their conduction bands.2. The sheet carrier density and distribution of the 2DEG in anAl_0.25Ga_0.75N/AlN/In_0.1Ga_0.9N/GaN herterostructure have been calculated bya same method as above. The study shows that the 2DEG sheet density in theparticular herterostructure is 4 times higher than that at Al_0.25Ga_0.75N/GaNinterface. Relevant factors have been discussed. Furthermore, this partsystematically studies the dependence of indium composition in theadditional InGaN layer and aluminum composition in AlGaN barrier layer onthe sheet carrier density and distribution of the 2DEG. The thickness of AlNinsert layer has also been found to play a significant impact on the 2DEGsheet density. 3. The different models and the possible mechanisms for current collapses havebeen summarized. In addition, the mechanism for the current collapses hasbeen discussed in different heterostructures from the point view of energy.