Research On Novel Structures And Critical Process Steps Of High Voltage GaN Based Power Switching Devices

For realizing high speed and efficiency power conversion module,new requirements for semiconductor power switching devices have been put forward in the field of modern power electronics.As the third generation semiconductor material,Gallium nitride?GaN?has shown superior electronic and thermal capability,which was suitalble in the application of high speed and efficiency power conversion module.The research of this doctoral dissertation was based on the GaN power switching devices,and was aimed to solve the key scientific problems of GaN power switching devices.The solved key scientific problems in this article included:a big gap between the Baliga figure of merit?BFOM?of GaN based power switch devices and the GaN material limit,the trade-off between threshold voltage and on-state resistance of AlGaN/GaN heterostructure field effect transistor?HFET?,the trade-off between reverse leakage current and turn-on voltage of AlGaN/GaN Schottky barrier diode?SBD?,the characterization and modeling of traps at the AlGaN/GaN interface in the GaN based metal-insulator-semiconductor high electron mobility transistor?MIS-HEMT?,the optimization of critical process steps and fabrication of high voltage GaN based power switching devices.The main reseach contents and the achievement of the reseach is concluded into five parts below.?1?For achieving reliable numerical calculation results,the caliberaion of the parameters in the polarization model,the mobility model,the impact ionization model and field induced tunneling model was conducted by fitting the experimental results.At last,these caliberated models were used in the numerical analysis of the new structures and improve the reliability of the numerical simulation results.?2?A high-K passivated and high-K/low-K compound passivated AlGaN/GaN SBDs with gated edge?GET-SBD?were proposed.The leakage current could be suppressed by inserting high-K dielectric under the gated edge,and breakdown voltage?BV?could be improved by adding low-K dielectric into the high-K passivation layer.Because the electric field is discontinuous at the interface of the high-K/low-K interface,and the electric field distribution along the channel could be modulated by this discontinuousty.According to the simulation results,the leakage current could be as low as 0.5 nA/mm after inserting high-K dielectric under the gated edge,and finally a device with BFOM of 5.9 GW/cm² could be achived by adding low-K dielectric into the high-K passivation layer.The compound passivation layer could not induce any parasitic capacitances by comparison the C-V characteristics between the proposed device and conventional AlGaN/GaN GET-SBD.?3?A novel duel junction gate GaN based HFET?DJG-HFET?was proposed to improve the threshold voltage and reduce on state resistance.The device could improve the thresthold voltage by inducing a p-GaN gate under the AlGaN/GaN channel,which could raise the conduction band at the AlGaN/GaN interface.On the other hand,a new electron channel could be formed when the back junction gate biased at high voltage,thus reduce the on-state resistance,and break the trade-off between the thresthold voltage and on-state resistance.A novel GaN based HFET with junction field plates?JFP-HFET?was also proposed.The 2-dimentional electron gas?2DEG?could be longitudinally depleted due to the transverse depletion of JFP when the high blocking voltage was located at both side of the JFP.So the electric field distribution along the channel between the gate and the drain become much uniform,thus the BV and BFOM could be improved.The simulation results of C-V characteristics and swithing behavior of JFP-HFET and MFP-HFET were comparied,and JFP-HFET showed a better performance than that of MFP-HFET in power switching applications.?4?Two different passivation technologies of GaN HFET were discussed and charactered.SiN_x deposited by plasma enhanced chemical vapor deposition?PECVD?with zero residual stress showed a worse reverse blocking characteristics but a better forward characteristics than that deposited by inductive coupled PECVD?ICPCVD?.A new method to character the AlGaN/GaN interface traps in MIS-HEMT was proposed.According to the frequency dependent C-V measurement results of devices with gate length and C-V simulation results,the acceptor-like traps induced by the residual stress of passivation layer with the concentration of 3×10¹11 cm^-2 and the energy level of E_C-0.37 eV under the gate at AlGaN/GaN interface was the main reason for the degradation after the passivation.Furthurmore,the acceptor-like traps was realated to the defects of the crystal defects at the interface of the AlGaN/GaN?5?High voltage and high current GaN based power switching devices were fabricated.Firstly,the exposure parameters of AZ5214 were determined to realize 2?m line exposure.Then,the isolation technology was discussed and compared between the F^-ion implantation isolation technology and mesa isolation technology,and the F^-ion implantation isolation technology was much better than the mesa isolation technology.The optimum annalling temperature of 870?was achieved by comparing the specific contact resistivity under different annalling temperature.The best surface treatment method with 1:10 HCl solution was obtained by comparing different surface treatment methods.A high on/off ratio of 10¹⁰ could be achieved by both LPCVD depositon SiN_x gate dielectric and F^-ion implantation isolation technology.By considering both gate extention structure and field plates structure with independent electrode,the high voltage and high current GaN based power switching devices were fabricated on the AlGaN/GaN epi-layer on Si substrate by using the optimum results of each process steps above.Finally,average BV of 1130 V could be achieved and the maximum saturation output current could reach 18 A after packaging the muti-gate-finger devices.The swiching test indicated that the fabricated power swiching device achieved a turn-on time of 7.1 ns and a turn-off time of 6.5 ns.