Research On GaN-based Gate-free Prototype Transistor Controlled By Ferroelectric Polarization

GaN-based High Electron Mobility Transistor(HEMT)is a transistor that uses twodimensional electron gas conduction at the material heterojunction interface.The twodimensional electron gas is generated due to the stress polarization and spontaneous polarization in Al GaN and GaN materials.The ferroelectric material with higher polarization intensity than the Al GaN material can be heterogeneously integrated with GaN-based transistors,where the ferroelectric material is used as the gate dielectric,and the electric field generated by the metal gate electrode is used to control the polarization intensity of the ferroelectric material.In the previous work of our research group,the heterogeneous integration of ferroelectric thin films and GaN-based HEMTs through substrate transfer technology is successfully realized,and it is found that adjusting the polarization characteristics of the ferroelectric materials through the gate metal can change the twodimensional electron gas concentration at the interface,and realize the regulation of the device performance.However,the conventional HEMT preparation process is relatively complicated,the preparation of the gate metal requires engraving and stripping,and the effect of gate metal on the polarization properties of ferroelectric material is not intuitive.Based on the previous work,this thesis designs and prepares a new type of gate-free device structure,and uses PFM conductive probes to control the polarization of the ferroelectric material in the gate area,and through the non-volatile polarization characteristics of the ferroelectric material,affect the two-dimensional electron gas concentration at the Al GaN/GaN heterojunction interface,and then adjust the IV characteristics of the device,and realize the prototype device without metal gate.The main work and innovations of this paper are as follows:First,use Sentaurus simulation software to model and simulate the ferroelectric HEMT,and simulate the influence of ferroelectric thin film on the transfer characteristics of the device,and the influence of different ferroelectric material thickness on the device.In the prepolarization simulation process,combined with the transient simulation settings,the effect of pre-polarization on the transfer characteristics of the device is successfully simulated.Second,the ferroelectric gate-free GaN-based transistor structure without gate metal is designed,and the prototype device is prepared through the process flow of alignment mark preparation,field isolation,and ohmic metal evaporation.The patterning is all based on FIB equipment with electron beam lithography system.Finally,through the method of substrate transfer,the 20 nm thick PZT ferroelectric film epitaxially grown on the STO substrate is heterogeneously integrated with the above devices,and the designed device structure is successfully prepared.The film quality before and after the transfer is measured by AFM and XRD,and the metal peeling experiment on the PZT film showed that the transferred PZT film has good mechanical strength.Third,the PFM conductive probe is used to control the polarization of the ferroelectric material in the gate area of the device.The non-volatile polarization control of the ferroelectric material replaces the traditional metal gate control.The output characteristics of ferroelectric gate-free GaN-based transistors are analyzed by 1500 and 4200 semiconductor testers,and show that when the gate length is 3?m,the gate width is 30?m,and the polarization voltage is 6V,the two-dimensional electron gas concentration at the heterojunction interface increases by the original 120%,the equivalent gate voltage is 5.14V;at the-9V polarization voltage,the two-dimensional electron gas concentration is reduced to 18% of the original,and the equivalent gate voltage is 1.27 V.The polarization control ability of the two-dimensional electron gas is greater than 500%.