Study And Fabrication Of The High Breakdown Voltage Power GaAs MESFET's

GaAs MESFET is a kind of important GaAs device. They have been widely used in wireless communications, information technologies and phase array radars etc. Gate-Drain breakdown voltage(BVgd) is one of the important parameters of GaAs MESFET. Presently, the BVgd of domestic GaAs MESFET is only more than 10V, which limits the output power of GaAs MESFET. Improving the BVgd and meliorating the power characteristics have become the important field focused by researchers.In this thesis, we analyse the breakdown mechanism of GaAs MESFET's, and design the structure of high breakdown voltage GaAs MESFET with a undoped LT GaAs cap layer and a AlAs spacer layer. A series of experiments including material growth, selective wet etching and ohmic contact were carried out, and than the high breakdown voltage GaAs MESFETs were fabricated.Molecular Beam Epitaxy (MBE) LT GaAs materials were grown at different substrate temperature. The annealing experiments of LT GaAs were carried out at 600 ℃. Before and after annealing, the structure characteristics have been tested with double-crystal x-ray diffractometer, and the electric characteristics have been obtained with Hall testing system. The results show that the lower the growth temperature is, the bigger the crystal lattice constant is. The annealed GaAs layer grown at 180℃ is semi-insulating and has better electric characteristics. Theresistivity of the annealed GaAs layer is 4×107Ω cm.The volume ratio of 50% citric acid solution to 30% H2O2 was changed from 1:1 to 10:1. and the selective wet etching experiments of GaAs over AlxGa1-xAs(x=0.2,0.3,1.0) were carried out. We used a-step instrument and the atom force microscopy (AFM) to study the etching rate and the surface of the different Al mole fraction samples etched with the solution of different volume ratio of 50% citric acid solution to 30% H2O2. The results indicate that the etching speedof AlxGa1-xAs increases with the ratio increase, and the etching speed of GaAs increases first with the increase in the ratio and than decreases with the further increase in the ratio. The bigger the speed of etching is, the rougher the GaAs surface is. The larger the Al mole fraction of AlxGa1-xAs is , the smoother the AlxGa1-xAs surface is. The mechanism of the selective etching was analysed. It was confirmed that the optimum volume ratio of 50% citric acid solution to 30% H2O2 is 1.5 : 1.The LT GaAs cap layer of GaAs MESFET was etched off with the 50% citric acid/H2O2 solution, and the AlAs spacer layer with hydrochloric acid. The alloying experiments were carried out on GaAs active layer which doped concentration is 1×10¹⁷cm-3 and the contact resistances under different alloying condition were examined by use of the probe stage. A transmission line model (TLM) was presented to calculate the contact resistivity and the optimum alloying condition of 450℃, 90s was chosen.On the basis of process study, power GaAs MESFETs with a output power over 500mW, a power gain no less than 7dB at 4GHz, and a Gate-Drain breakdown voltage of 30V were fabricated.