Research On ?-Ga₂O₃ Based High-Power Metal-Oxide-Semiconductor Field-Effect Transistors

Compared with the first,second and third generation semiconductor materials,?-Ga₂O₃,as a new type of semiconductor material,has excellent characteristics such as an ultra-wide band gap of 4.8?4.9 e V and an ultra-high criticality breakdown field strength of 8 MV/cm.Baliga's figure of merit(???E_b³)is usually used to characterize the suitability of materials for power electronic devices.The Balijia figure of merit of?-Ga₂O₃ is 4 times that of Ga N,10 times that of Si C,and 3444 times that of Si.?-Ga₂O₃ power devices have the same breakdown voltage as Ga N and Si C devices,with lower on-resistance and lower power consumption,which can greatly reduce the power loss during device operation.?-Ga₂O₃single crystal substrate can be obtained by a melting method,which is similar to the sapphire substrate,has inherent advantages in terms of price and cost.Therefore,?-Ga₂O₃ power devices have the triple advantages of high breakdown,low on-resistance and low cost,and has great application potential in terms of high voltage,high power,high efficiency and energy saving.It can meet the high demand for for critical voltage and low power consumption of power electronic devices in the power system of future.?-Ga₂O₃ is considered as the most powerful competitor of next-generation power electronic devices.?-Ga₂O₃ power devices have a wide range of application prospects in military and civilian applications.In the military field,it can be used in power control systems for high-power electromagnetic guns,tanks,fighter jets,and ships,as well as radiation-resistant and high-temperature aerospace power supplies,which can greatly reduce the losses of weapon equipment systems,the volume and weight of the heating and cooling system,and meet the requirements of military application components for miniaturization,light weight,rapidization,radiation resistance and high temperature resistance.In the civil field,it can be used for power grids,electric traction,photovoltaics,electric vehicles,household appliances,medical equipment,consumer electronics and other fields,which can achieve greater energy conservation and emission reduction.Based on the above reasons,MOSFETs based on?-Ga₂O₃ material has been systematically studied in this article,from improving the surface state of?-Ga₂O₃ and the interface state of the gate dielectric/?-Ga₂O₃,to the device structure design,process manufacturing,device characterization and performance optimization of depletion and enhanced?-Ga₂O₃MOSFETs.The specific research results are described as follows:1.ALD was used to perform the in-situ O₃ surface pretreatment on the?-Ga₂O₃ material and deposit Al₂O₃ as gate dielectric.The properties of?-Ga₂O₃ surface and Al₂O₃/?-Ga₂O₃interface before and after O₃ pretreatment were characterized by XPS test and conductivity method:Ga:O on?-Ga₂O₃ surface before and after O₃ pretreatment were 2:2.78 and 2:2.91,respectively.After O₃ pretreatment,the hypoxia state of the?-Ga₂O₃ sample surface is significantly improved.At the same time,most of the metastable Ga-O bonds on the surface of?-Ga₂O₃ are converted into stable Ga-O bonds.It shows that O₃ pretreatment fills the oxygen vacancies on the surface of the?-Ga₂O₃ sample,and its bonding has been significantly optimized and improved.For the discontinuity of the energy band at the Al₂O₃/?-Ga₂O₃ interface,due to the increase of stable Ga-O bonds,Ga 2p_3/2 core level move to the high energy direction,so that the conduction band order?E_C of the?-Ga₂O₃ and Al₂O₃interface after O₃ pretreatment increases from 1.19 e V to 1.33 e V,which is advantageous for the design and manufacture of power devices.Finally,by using the C-V hysteresis method and conductance method,the quantity of interface traps(Q_it)of the Al₂O₃/?-Ga₂O₃ interface before and after O₃ pretreatment is reduced from 7.04×10¹¹ cm^-2 to 3.26×10¹¹ cm^-2.The density of deep level interface traps has been reduced from 2.0?3.5×10¹² e V^-1cm^-2 to0.5?1.0×10¹² e V^-1cm^-2,which is at a leading level compared with similar international indicators.Therefore,pretreating the?-Ga₂O₃ surface with O₃ can improve its surface oxygen-deficient state and reduce the interface trap charge density,which provides an effective way to optimize the performance of subsequent power MOSFET devices.Furthermore,band alignment of Si O₂/?-(Al_xGa_1-x)₂O₃ and Hf O₂/?-(Al_xGa_1-x)₂O₃ interface were studied.The test results show that when the Al components of/?-(Al_xGa_1-x)₂O₃ film increase from 0 to 0.53,bandgap of the film increases from 4.8 e V to 5.4 e V.At the same time,?E_C of the Si O2/?-(Al_xGa_1-x)₂O₃ interface increases from 1.9 e V to 2.6 e V,while that of Hf O₂/?-(Al_xGa_1-x)₂O₃ interface increased from 1.1 e V to 2.0 e V,which is meet with the design expectations of power devices.2.By transferring the?-Ga₂O₃ nano-membrane which is obtained from the?-Ga₂O₃ bulk substrate to the heterogeneous sapphire substrate,the depletion-mode?-Ga₂O₃ MOSFET is fabricated and characterized.AFM and Raman spectroscopic characterization showed that there was neither surface roughness degradation nor stress during the transfer of the?-Ga₂O₃nano-membrane to the sapphire substrate.Secondly,the electrical characteristics of the manufactured device were characterized,and the device output saturation current,threshold voltage,transconductance,and ON/OFF ratio are 231.8 m A/mm,-16.5 V,13.8 m S/mm,and?10⁸,respectively.The pulse test shows that the current degradation percent of sapphire-based?-Ga₂O₃ MOSFET is 4.16%.The specific on-resistance of 7.41 m?·cm² and the breakdown voltage of 800 V was obtained,which corresponds to the DC power figure of merit(PFo M)of 86.3 MW/cm².It is the highest value of the international PFo M value in the same period.At the same time,peak field strength along the channel is obtained through Silvaco TCAD is extracted as 7.06 MV/cm,indicating that the?-Ga₂O₃ nano-membrane has the excellent properties on the withstanding of high voltage.3.An enhanced?-Ga₂O₃ MOSFET was designed,manufactured and characterized based on Hf Zr O₂ ferroelectric charge storage gate dielectric:Hf Zr O₂ film grown by ALD with rapid thermal annealing at 500 ^oC and 30 s obtained a polarization of 27?C/cm²,which corresponds to a polarization charge density of 1.69×10¹⁴ cm^-2.The output current and on-resistance of the device is not significantly degraded before and after initialization,and the threshold voltage drift reaches 4.57 V.Under the gate-stress at room temperature and 150 ^oC,the threshold voltage of the device reached the failure threshold(26.5%and 26.2%)at 10⁵ s and 10⁴ s,and the characteristic on-resistance increased by 17.8%and 17.1%,respectively.The breakdown voltage of the device reached 670 V,and the power figure of merit is the same as that of enhanced?-Ga₂O₃ MOSFET devices in the same period,and has reached the theoretical limit value of Si based devices.4.The electrical characteristics of high-voltage depletion-mode and enhanced?-Ga₂O₃MOSFETs with field-plate terminal structures are studied.Depletion-mode?-Ga₂O₃MOSFETs with conventional structure,gate,source,and gate-source field-plate structures are designed and manufactured.The three types of field-plate increase the breakdown voltage of the device from 985 V to 1665 V,1620 V,and 2430 V,and the peak electric field along the channel of the device is reduced from 12.34 MV/cm to 6.41 MV/cm and 5.99 MV/cm,4.31 MV/cm,respectively.The laminated-ferroelectric charge storage gate was designed and manufactured to realize the enhancement-mode?-Ga₂O₃ MOSFET.The positive threshold voltage drift reached 10.92 V after the initialization.At the same time,the device showed good stability in the stress test.The maximum threshold voltage drift was only 2.76%after the gate-stress of 10⁴ s was applied.With the specific on-resistance is 23.84 m?·cm²,combined with the gate field plate structure,the device breakdown voltage V_BR reaches 2142V,and the calculated power figure of merit of the device is 192.5 MW/cm²,which is the the highest among the lateral enhancement-mode?-Ga₂O₃ MOSFET.