Novel Structure And Mechanism Research Of Gallium Nitride Power Device For High Voltage And Low Loss

Gallium nitride（GaN）material has the advantages of wide band gap,high critical breakdown electric field and high-electron saturation velocity,which is a typical representative of the third-generation semiconductor.Moreover,the AlGaN/GaN heterostructure has high mobility and high-density two-dimensional electron gas（2DEG）.Therefore,the AlGaN/GaN heterojunction-based power device has unique advantages in high voltage,high power,low power loss and miniaturization.It has great potential for applications,such as electric vehicles,Li DAR,power supply equipment and data center.However,there are many challenges to the novel structure,including physical characteristics,fabrication process and reliability of the high-performance GaN devices.The breakdown voltage（BV）of the AlGaN/GaN device is far below the theoretical limit due to the gate failure caused by the hot electrons in high field and leakage current surge.For practical use of the transistors in topologies of power converters,reverse conduction with low-loss is also demanded.Although the conventional HEMTs（Con.HEMT）itself can realize reverse conduction,the reverse turn-on voltage(V_RT)depends on the V_TH and the gate bias.The realization and optimization of the integrated freewheeling diode（FWD）should be taken into consideration.Moreover,it is difficult to obtain enhancement-mode devices with high V_TH due to the high density of 2DEG under the gate.The lateral HEMTs suffer from severe stability and reliability issues,such as threshold voltage shift and current collapse,owing to the high-density interface states and high surface electric-field peak.Two novel devices with high voltage and low power loss are proposed based on the polarization junction theory.The relevant fabrication processes are also developed and optimized.At the same time,the vertical transistors with fin-shaped diode are proposed and investigated.1.An AlGaN/GaN Schottky barrier diode（SBD）with double-heterojunction is investigated to solve the electric field concentration and optimize the surface electric-field.At the off-state,the double-heterojunction（GaN-top/AlGaN/GaN）with fixed positive and negative polarization charges form the polarization-junction.Therefore,a more uniform electric field distribution of the drift region and a high BV is obtained.The optimized fabrication processes include the low damage ICP etching,low-resistance ohmic contact forming and high-quality passivation dielectric depositing.The fabricated SBDs achieve a low V_RTof 0.68 V,a high on/off current ratio¹0¹⁰,a low specific on-resistance(R_ON,SP)of 1.17 mΩ·cm²,a high BV of 1109 V and a high Baliga’s figure-of-merit（FOM）of 1.051 GW/cm² with the L_GD of 11μm.In addition,the process platform based on double-heterojunction epitaxy lay the groundwork for the following Reverse-Conducting HEMT（RC-HEMT）.2.A RC-HEMT with polarization junction is proposed and fabricated to reduce the power loss of the reverse conducting.It features an integrated Schottky barrier diode（SBD）to realize reverse conduction and the double-heterojunction to enhance BV.The SBD part and the HEMT part share the same drift regions,which is favor to reduce the size of the device.The gate dielectric depositing is also optimized to reduce the interface states.Experimental results show that the novel structure achieves a low V_RT of 0.68 V(@V_GS=0 V),which decreases 69.1%compared with that of the Con.HEMT.The BV(with 7.5μm L_GD)is 723 V.In addition,the leakage of the Al₂O₃ gate is～10^-10 A/mm and the interface trap density decreases to～10¹² cm^-2·e V^-1.The dynamic resistance of the device increases only 25%under the stress voltage of 300 V.3.Vertical GaN transistors featuring an integrated Fin-shaped non-junction diode are proposed to improve reverse conduction and switching characteristics.The Fin-shaped diode is based on the work function difference of the MIS structure in the Fin sides.At the blocking state of the transistor,the Fin-shaped diode exhibits low leakage current and realizes the avalanche breakdown with a high BV.The Fin-shaped diode achieves better reverse recovery characteristics because it works in the unipolar mode.The reverse recovery time of the proposed structure is decreased by 47.5%compared with that of the P-i-N diode embedded in the conventional structure.The V_RT of the proposed structure is78.9%lower than that of the conventional structure.In addition,the proposed structure exhibits a low Q_GD.The turn-on time and the turn-off time of the proposed structure are decreased by 58.2%and 62.4%respectively,compared with those of the conventional structure.The Fin FET with Fin-shaped non-junction diode is proposed due to the difficulty of the p-type realization and regrowth,in which the Fin FET takes place of the planar MOS.The V_TH is 1.11 V and the V_RT is 0.49 V for the novel structure.