Research On Flexible Microwave Gallium Nitride Devices And Their Models

Flexible electronic technology has the advantages of bendability/stretchablity,low cost,strong portability,and light weight.With the development of flexible electronic technology,flexible microwave electronic devices for wearable wireless communication systems,conformal radar and other applications have attracted attention in recent years.Compared with silicon,gallium arsenide,and carbon nanotubes,gallium nitride(Ga N)-based devices have the advantages of high power density and high efficiency,and have become the research hotspot of flexible microwave power devices.As it is difficultto directly grow Ga N on flexible substrates,a transfer method is typically used.However,flexible Ga N power devices face bottlenecks such as insufficient bending ability and low output power,which limit their application in the field of flexible microwave electronics.The main contributions of the authers in this disserations are as follow:(1)Research on flexible microwave Ga N HEMT based on Parylene-C(PC)substrate.In order to solve the problem of insufficient bending ability of current flexible microwave Ga N HEMTs,a device structure based on PC substrate and its realization method are proposed in this dissertation.The structure is realized by evaporating the polymer plastic PC substrate after thinning the Si C-based Ga N HEMT.The stress-free and dense coating PC material can effectively reduce the influence of contact interface roughness between PC with Si C and reduce the thermal resistance of the device.Furthermore,a liquid phase low-damage transfer method is proposed.The method utilizes the surface tension of the liquid to fix the device before thinning,evaporates PC on the thinned device,and finally achieves peeling by applying a slight lateral tension,which can effectively reduce the mechanical damage caused by the external strain in transfer process.The experimental results show that the fabricated flexible microwave Ga N HEMT has a saturated output power density of 0.42 W/mm in a flat state at a frequency of 3 GHz,reaching the stateof-the-art performance.The bending test results show that the saturated output power density can still reach 0.41 W/mm when the minimum bending radius is 0.4 cm,and the RF power output of flexible Ga N HEMTs under bending is realized for the first time.(2)Research on high-power flexible microwave Ga N HEMT based on back copper technology.In order to further improve the output power of flexible Ga N HEMTs,this dissertation proposes a high-power flexible Ga N microwave transistor structure based on the back copper technology.Compared with PC materials,copper is more suitable for device heat dissipation and integrated circuit design due to its higher thermal conductivity and electrical conductivity.In addition,copper is strechable,allowing for better bending and heat dissipation after Si C thinning and copper deposition on the backside of the device.The experimental results show that the fabricated flexible microwave Ga N HEMT has a flat state output power density of up to 2.65 W/mm at 3 GHz,and a saturated output power density of 2.24 W/mm at a bending radius of 0.6 cm,which is 5 times higher than that of PC-based Ga N HEMTs,realizing a flexible microwave transistor with an output power density comparable to that of the rigid transistor before transfer.(3)Research on the large-signal model of the electromechanical coupling of the flexible microwave Ga N HEMT.In oder to improve the accuracy of the trap model,the traps influence under strain is investigated by pulse I-V measurements and drain current deep-level transient spectroscopes.Furthermore,a method for establishing the largesignal model of the electromechanical coupling is proposed.This method introduces external strain variables into the traditional carrier concentration and threshold voltage models,and then proposes a strain-related nonlinear capacitance model and nonlinear current model based on the surface potential theory and their parameter extraction methods,in which the strain-related parasitic parameters are considerred.The verification results at 6 GHz show that the accuracy of the output power,power gain and power added efficiency of the model is greater than 90% at bending radius of 0.8 cm and 0.4 cm,and compared with the traditional model without considering external strain,the accuracy of power added efficiency is improved respectively up to 10.4% and 24.5%.(4)Design and realization of flexible microwave Ga N power amplifiers.In order to verify the proposed device structure and modeling method,a study of a flexible power amplifier is carried out in this dissertation,and a fabrication method of a heterogeneous integrated Ga N power amplifier based on Parylene-N(PN)is proposed.In this method,the self-built electromechanical coupling large-signal model is used for input and output matching design.Since the dielectric constant of PN is stable and hardly changes with frequency,PN is used to be evaporated on the transistor as the substrate of passive circuit to fabricate capacitors and inductors.Furthermore,A metal partially embedded structure is provided to enhance the metal adheresion on PN.Finally,flexible heterogeneous integrated amplifiers fabrication is completed using PC substrate and low-damage transfer technology.The experimental results show that the power amplifier can achieve a saturated output power greater than 200 m W at 1.5 GHz,and the maximum power added efficiency is greater than 9%.The measured results are in good agreement with the simulation results,which verifies the correctness of the proposed fabrication method and model.The work of this dissertation is significant for the fabrication of flexible high-power microwave devices,circuit design methods and the application of Ga N devices in the field of flexible electronics.