Investigations And Optimization On The Gate Structure And Passivation Technology Of The P-GaN/AlGaN/GaN HEMTs

GaN-based materials have the advantages of large bandgap energy,strong breakdown electrical field strength,high polarization coefficient,high electron mobility,and high electron saturation drift velocity,which are the preferred materials for fabrication of new generation high-performance power electronic devices with many application prospects.In recent years,developed countries and regions such as the United States,Japan,and Europe have all listed GaN-based power electronic devices in their major strategic research plans and have made important progress.However,GaN-based HEMTs are intrinsically normally-on devices because of the high density two-dimensional electron gas(2DEG)induced by strong polarization effects at the AlGaN/GaN interface.In practical applications,the enhancement-mode(E-mode)devices with positive threshold voltage(Vth)are preferred over depletion-mode(D-mode)devices owning to their inherent fail-safe operation and simpler gate control scheme.So far,there are several approaches to realize E-mode operation,such as a recessed gate structure,p-GaN gate and fluoride-based plasma treatment.A p-doped GaN layer on top of the AlGaN barrier which is employed to lift up the electric potential of the heterojunction below the gate and to deplete the 2DEG channel at zero gate bias is a promising candidate for an E-mode device due to excellent figure-of-merits and industrialization potential.The main results of this work are as follows:1.The effect of thickness of the AlxGa1-xN barrier layer,Al molar fraction of the AlxGa1-xN barrier layer,hole concentration of p-GaN cap layer and gate metal on the performance of p-GaN gate HEMTs was studied by Silvaco ATLAS simulation software.The device with higher hole concentration of the p-GaN cap shows a higher threshold voltage,but an excessively high hole concentration p-GaN is difficult to achieve in reality.The thickness of the p-GaN cap layer also affect the performance of p-GaN gate HEMTs.The output current and gate control capability of the device are correspondingly reduced with the higher threshold voltage.In addition,devices with Ohmic contact on the p-GaN gate exhibit higher output current and smaller threshold voltagee compared with those of devices with Schottky contact on the p-GaN gate.Based on the results of device simulation and the actual conditions of epitaxial growth,the epitaxial structure of the p-GaN gate HEMTs is optimized.2.A typical E-mode GaN HEMT with p-GaN cap layer is successfully fabricated,the performance of p-GaN gate HEMTs with Schottky gate contact and Ohmic contact is compared.Devices with Schottky contact gate exhibit threshold voltage of?1.74 V and an on/off ratio up to 107.It was found although devices with Schottky contact gate presented a higher threshold voltage and lower gate leakage at saturation region,larger hysteresis between forward sweep transfer curves and reverse sweep transfer curves was observed,which is caused by hole accumulation at the p-GaN/metal interface.Besides,frequency dispersion of C-V curves further confirmed the hole accumulation effect in the devices with Schottky contact gate.3.Passivation of the remaining p-GaN layer in non-gate region is investigated.SiO2 and SiN,deposited by PECVD are evaluated to passivate the damaged surface induced by p-GaN dry etching process.To compare transient properties of devices with different passivation layers,temperature-dependent pulsed ID-VD measurements have been carried out.Self-heating effect and surface state induced current collapse are both observed.Dynamic on-resistance measurement and low-frequency noise test confirmed that SiNx passivation layer is more advantageous for p-GaN cap layer based E-mode HEMT.