Model, Key Process And Fabrication Of AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN high electron mobility transistors (HEMTs) have advantages over electronic devices based on Si, GaAs and their alloys in high-frequency,-temperature and-power applications, owing to large direct bandgap energy, high saturation drift velocity, large conduction band discontinuities, high thermal stability and strong piezoelectric and spontaneous polarization, but AlGaN/GaN HEMT in theory and experiment is not researched in detail. In the dissertation, AlGaN/GaN HEMT in theory, key process and production is researched in detail. The major research work and results are as follows:1. The electron distribution of AlGaN/GaN heterostructure materials is obtained by self-consistently solving Poisson’s and Schrodinger’s equations considering the essential polarization. The bandage profile, electron and ionized concentration of AlGaN/GaN HEMTs are calculated in three conditions (with spontaneous and piezoelectric polarizations, with spontaneous polarization and without polarizations). And the influence of polarizations on two-dimensional gas(2DEG)sheet charge density is analyzed. The effects of different factors on electron distribution and 2DEG sheet charge density of AlGaN/GaN HEMT are analyzed with different Al contents, AlGaN barrier thickness, AlGaN spacer thickness and doped concentration in AlGaN doped layer. And the effect of strain relax degree in AlGaN barrier layer on bandage, electron concentration and 2DEG sheet charge density is solved, which providing the basis to study the electrical characteristics of AlGaN/GaN HEMT ulteriorly.2. A nonlinear charge-control model is developed that describes the accurate relation of 2DEG sheet charge density and gate voltage above. The field-velocity, parasitic resistances and channel-modulation effect are included in the model. The current-voltage characteristics, small-signal parameters (transconductance and conductance) and the variation of effective channel length are obtained. The effects of three conditions (with spontaneous and piezoelectric polarizations, with spontaneous polarization and without polarizations) on current-voltage characteristics and transcoductance are calculated. The variation of direct-current, transconductance and drain conductance is analyzed with different Al contents and strain relax degrees. The model including the self-heating effect is developed to analyzing the influence of the effect on direct-current characteristics of AlGaN/GaN HEMT on sapphire underlay.3. Ti/Al and Ti/Al/Ni/Au metallizations are adopted to form ohmic contacts to AlGaN/GaN heterostructure. The effects of different rations and thicknesses on the specific contact resistance are analyzed. The variations of specific contact resistance of Ti/Al/Ni/Au to AlGaN/GaN heterostructure are researched with thermal annealing and surface dealing conditions. The influence of temperature on specific contact resistance of Ti/Al/Ni/Au to AlGaN/GaN heterostructure is studied. The effect of ohmic contacts with different Ti/Al/Ni/Au metallization on device characteristics of AlGaN/GaN HEMT is discussed.4. The AlxGa1-xN/GaN heterostructures with different Al contents (x≤35%) and two different AlGaN/GaN heterostructures with high Al content are fabricated by using Metalorganic Chemical Vapor Deposition(MOCVD) which is developed by ourselves. And the materials are measured and 2DEG is occurred in AlGaN/GaN heterostructures. The Hall test of the AlGaN/GaN heterostructures is performed.5. Finally, AlxGa1-xN/GaN HEMTs with different Al contents (x≤35%) are fabricated, and the influence of Al content on the performances of AlxGa1-xN/GaN HEMT is researched. The high-Al content AlGaN/GaN HEMTs with different structures are obtained, and the characteristics of AlGaN/GaN HEMTs are analyzed by HP4156C. The AlGaN/GaN HEMTs with different gate length are studied. The effects of temperature on AlGaN/GaN HEMTs are obtained.Altogether, in this dissertation the influence of different factors on electron distribution of AlGaN/GaN HEMT is studied theoretically by solving Poisson and Schrodinger self-consistently. The electrical characteristics of AlGaN/GaN HEMT and the effects of Al content and strain relax degree are researched by solving the nonlinear charge model which is developed based on the accurate relation of 2DEG sheet charge density and gate voltage. The influence of different metallization on ohmic contact which is the key process of AlGaN/GaN HEMT is discussed. Using the home-made MOCVD system, AlxGa1-xN/GaN heterostructures and HEMTs with different Al contents are fabricated. A lot of meaningful results are obtained. These results are the basis of optimizing the performances of AlGaN/GaN HEMT.