Mechanism And Experimental Study Of Novel High-voltage And Low-loss Gallium Oxide Power Devices

Beta-phase gallium oxide（β-Ga₂O₃）has excellent material properties,such as an ultra-wide bandgap（4.5-4.9 e V）and high critical electric field strength（8 MV/cm）.Thus,the Baliga’s figure of merit ofβ-Ga₂O₃-based power devices is theoretical approximate 4times and 10 times as large as that value of Ga N and Si C,respectively,contributing to a lower loss and smaller size at the same breakdown voltage（BV）.Therefore,β-Ga₂O₃devices are suitable for high-power and low-loss applications.At present,most research onβ-Ga₂O₃ power devices focuses on high BV and low specific on-resistance(R_on,sp);meanwhile,low turn-on voltage(V_on)for diodes and high threshold voltage(V_th)for transistors are important.However,some key issues still need to be addressed as follows:（1）The BV ofβ-Ga₂O₃ devices is much lower than the theoretical value because of the absence of P-typeβ-Ga₂O₃,which makes it difficult to improve BV by conventional junction terminal technologies.（2）The thermal conductivity ofβ-Ga₂O₃ is low,and the current researches mainly focuses on small-size devices to explore methods to alleviate the electric field crowding phenomenon,but few reports focus on the large-size high-powerβ-Ga₂O₃devices and the long-term high-temperature characteristics.（3）Fin channel is introduced inβ-Ga₂O₃ power devices to suppress the leakage current and increase BV,but it inevitably leads to a large turn-on voltage(V_on)forβ-Ga₂O₃ Fin diodes and a large reverse turn-on voltage forβ-Ga₂O₃ Fin FETs as well as a high conduction loss.In order to address the key issues,a series of innovative researches on high-power and low-lossβ-Ga₂O₃ power devices are carried out.1.Mechanism and experimental study of high-powerβ-Ga₂O₃ Schottky barrier diode（SBD）with a new compound termination（CT）In this work,the ultra-fast reverse recoveryβ-Ga₂O₃ SBD with improved BV is proposed and investigated experimentally.It features the compound termination,consisting of air space field plate（ASFP）and thermal oxidation terminal.The compound termination not only reduces high-density interface states at the dielectric/β-Ga₂O₃interface and the electron concentration in the oxidation terminal,but also modulates the electric-field distribution and suppresses the peak electric-field at the bottom of anode.Therefore,the reverse leakage current is suppressed,as well as the reverse recovery and breakdown characteristics are improved effectively.The measured BV=400 V of the CT SBD with the diameter of 1000μm increases by 176%compared with the BV of the device without ASFP structure.The CT SBD obtains temperature-independent ultra-short reverse recovery time T_rr=7.5 ns and ultra-low reverse recovery charge Q_rr=1.0 n C at di/dt=50 A/μs with good rectification characteristics.The fabricated CT SBDs have great potential for high-power and high-frequency applications.2.Experimental study on mechanism and reliability of high-powerβ-Ga₂O₃ vertical diodesIn this work,high-voltage design technology of high-powerβ-Ga₂O₃ diodes is proposed,and a novel verticalβ-Ga₂O₃/Ni O heterojunction barrier Schottky diode（JBSD）as well as a novel vertical reduced surface field（RESURF）SBD with surface charges modulation capacity are fabricated.The fabricated large-size JBSD and RESURF SBD achieves high voltage,high current and low reverse leakage current.The long-term stress test results exhibit the good thermal stability,showing the great potential ofβ-Ga₂O₃power diodes to operate under high-temperature conditions.The novelβ-Ga₂O₃ JBSD covers the advantages of Schottky barrier diodes（SBDs）with low turn-on voltage and PN junction diodes with small reverse leakage current.The measured values of BV and R_on,sp for the JBSD with the Anode area of 9mm²/16mm² are550V/500V and 11mΩ·cm²/15mΩ·cm²,respectively,and the forward output current at the forward bias（V_F）of 10 V is up to 51 A/88 A.Before and after long-term high-temperature stress,the capacitance-voltage（C-V）and forward current-voltage（I-V）curves of 16mm² JBSD are almost coincide,as well as the same BV.The novel vertical RESURF SBD features the oxidation region and Si O₂ trench termination to improve the breakdown characteristics.First,the oxidation region reduces the electron concentration at the surface layer,so as to reduce the surface peak electric field(E_max)and simultaneously optimize the vertical electric field profile;second,the Si O₂ trench further decreases the surface E_max.The RESURF SBD exhibits a withstand voltage of 600 V with the reverse leakage current（I_R）of less than 10μA,as well as a forward current（I_F）of 7A at V_F=3 V.After the high temperature storage test,the forward I-V curves exhibit a less than 2%positive shift with almost constant on-resistance,and the turn-on voltage increases by only 0.05V;the withstand voltage is still up to 600V without a sharp increase in I_R.3.Operation mechanism and simulation design of low-lossβ-Ga₂O₃ devices with Fin structureIn this work,a new design concept to achieve low-lossβ-Ga₂O₃ Fin power devices are investigated by Sentaurus TCAD simulation,including aβ-Ga₂O₃ diode with a Fin channel combined with an Ohmic contact anode and Composite Field-plate（FOCF）,and a reverse conduction（RC）vertical field-effect transistor（RC-Fin FET）with an integrated Fin Diode（FD）.The FOCF diode achieves both a low turn-on voltage(V_on)and high BV by two features:one is a Fin channel combined with Ohmic contact anode（FO）to achieve Metal-Insulator-Semiconductor（MIS）-like forward/reverse characteristics and thus realize a very low V_on;the other is Composite Field-plate（CF）to improve BV,consisting of Al₂O₃and Si N_X dual-dielectric layers to form step-shaped field plate.When the forward voltage is 0 V,the Fin channel is pinched off due to the work function difference between the anode metal andβ-Ga₂O₃;thus,the pinch-off effect allows the Ohmic contact anode to take place of Schottky contact anode to significantly reduce V_on.Moreover,the pinch-off effect suppresses reverse leakage current and CF modulates the electric field distribution,both of which improve BV.The proposed FOCFβ-Ga₂O₃ diode achieves low V_on=0.45V and high BV=2204V.The V_th and a reverse turn-on voltage V_on of RC-Fin FET can be independently regulated,so as to improve reverse conduction characteristics with little effect on V_th and BV.In the reverse conduction state,the FD realizes a very low reverse turn-on voltage V_on;in the forward conduction and blocking states,the Fin channel of FD is pinched off without obvious influence on the forward conduction and blocking characteristics.Compared with the conventional Fin FETs,the RC-Fin FET reduces the V_on by 71%with almost the same V_th and BV values,achieving low V_on=0.45 V,high V_th=1.6 V and BV=2545 V.Additionally,compared to a FET externally connecting a freewheeling diode to reduce V_on,the RC-Fin FET reduces the parasitic inductance and the total chip area,enhancing its application potential for high-power and low loss power conversion systems.