Fabrication And Research On Gallium Nitride Rf And Microwave High Electron Mobility Transistors

The excellent characteristics of gallium nitride high electron mobility transistors in the high frequency and high power fields make gallium nitride have wide area of application in the field of RF microwave devices.Thus,it is important to improve the frequency characteristics and the output current of devices.The thesis introduces the basic working principles of gallium nitride high electron mobility transistors,with the basic fabrication process,and analyzes the influences of the fabrication process on the device performance.The fabrication process of gallium nitride high electron mobility transistors includes the fabrication of isolation region,source/drain ohmic contact,gate metal,passivation,via,and PAD.The lithography and cleaning process exist in every step of the fabrication process.The lithography process directly affects the accuracy of patterning,while the cleaning process affects the surface state density of devices,which are both important.The etching process is mainly used for the fabrication of isolation region and via on passivation layer.Rapid thermal annealing process is used for the fabrication of ohmic contact in source and drain region.During those process,the etching depth,the annealing time and temperature need to be precisely controlled.Because the gate lengths of RF devices are usually at the sub-micron,deep submicron and even nanometer level,the traditional ultraviolet lithography cannot meet such small linewidth due to the wavelength limit.Therefore,electron beam lithography is required to define the small gate pattern.There is trade-off between the thickness of electron beam resist and the linewidth of the exposure pattern,so the resolution and aspect ratio of electron beam resist directly affect the minimum gate length and the maximum metal height of the device.In order to improve device performance,electron beam resist with high aspect ratio and high resolution can be choosed to use in process.Moreover,gate can be fabricated in T shape in order to reduce the gate resistance,so as to meet the requirement on the height of gate metal more easily.Two methods are introduced in this thesis to solve the peeling-off problems of electron beam resist during the metal evaporation processes,one is rising the evaporation rates of gate metal,another is the fluorine plasma treatment before gate metal evaporation.The performance of the device is directly related to the process.This thesis introduces the effect of gate metal layer on the transfer properties of the devices,and compares the transfer properties,threshold voltage,output properties and frequency properties among devices with different gate lengths and gate widths.The thesis discusses the short channel effects of the devices taking devices with T-shape gates as an example.The threshold voltage decreases with the shrink of the gate length,and the voltage between the drain and the source.These phenomena are especially obvious in the devices with the designed gate length less than 100 nm,showing obvious short channel effect.