Study On GaN-based Enhancement-mode HEMT Based On Polarization Modulations

As the third-generation semiconductor materials,gallium nitride?GaN?-based materials are going to move to central stage as a result of their unique characteristics.The research for GaN-based and related?-N compounds devices attracts more and more attention.With the great development of GaN-based high-electron-mobility transistors?HEMTs?in RF power electronics during the last decade,the integration of the ferroelectric materials and GaN-based materials has aroused the attention from scholars.Researchers begin to explore the possibility of the integration of ferroelectric materials and GaN-based devices.This study is focused on the electric characteristics of PZT/AlGaN/GaN HEMT based on the ferroelectric gate dielectrics.This study provides the following results:?1?A high performance enhancement-mode PZT/AlGaN/GaN HEMT is fabricated.In order to obtain better quality of the PZT in a thin-film level,a buffer layer of Al₂O₃ is induced to reduce the lattice mismatch between PZT and AlGaN before depositing the PZT film by PLD.The in-situ treatment method is used to reduce the interface state between the dielectric and AlGaN before depositing the Al₂O₃ buffer layer.By combining the buffer layer and in-situ treatment technique,a high performance PZT/AlGaN/GaN HEMT is realized.By poling the gate electrode,an enhancement-mode PZT/AlGaN/GaN HEMT with threshold voltage of 1V is achieved.The on-off ratio,saturation output current and on resistance of the device are 10¹⁰,335mA/mm and 11?·mm,respectively.The off-state leakage is only 20pA/mm.The subthreshold swing is 90mV/dec,which approaches its ideal value.?2?Influence of the ferroelectric polarization charge on the channel of the PZT/AlGaN/GaN HEMT.In order to study the influence of the initial polarization of PZT on the channel resistance R_S,the transmission line model?TLM?structure is utilized to extract the R_S.As for the device in this study,the R_S is 5 times higher after depositing the PZT with in-situ treatment,which demonstrates that the initial polarization of PZT has the depletion effect on the channel.Therefore,it can be concluded that the initial ferroelectric polarization of the PZT in this study is towards surface.The gate transmission line model?GTLM?structure is used to study the influence of the different poling voltage on the channel resistance under the gate.0⁹V voltage are applied on the gate electrode of the PZT/AlGaN/GaN HEMT for 30s.With the increase of the gate poling voltage,the channel resistance under the gate electrode has a nonlinear increment.When the gate is applied with8V poling voltage,the 2DEG in the channel is completely depleted and the channel resistance under the gate was at 10⁸ order.What's more,the current of the channel is almost0.When the gate is applied with 9V poling voltage,the channel resistance under the gate increased by 5 order of magnitude compared to that with no poling.Therefore,it is able to realize the enhancement-mode device by poling the gate electrode of the PZT/AlGaN/GaN HEMT.?3?Influence of drain electric field on device characteristics of PZT/AlGaN/GaN HEMT.Experimental results show that drain electric field has a great influence on the switch behaviors of the ferroelectrics under the gate electrode,which leads to the different hysteresis behavior of transfer characteristics with different drain voltage condition.The hysteresis of the transfer curves at V_D=0.1V is more obvious than that at V_D=10V,which was owing to the ferroelectric pinning effect at V_D=0.1V.As for V_D=10V,less ferroelectric polarization charge was pinned because of the negative V_GD.?4?Study of the PZT/AlGaN/GaN HEMT using PZT transfer technique.The integration of the PZT and AlGaN/GaN heterostructure by transfer technique makes the great quality of the PZT ferroelectric gate dielectric.The 2DEG in the channel can be modulated by poling the gate electrode with different voltages.The modulation capability of the device using transfer technique is greater than that of the most of the reported GaN HEMT with ferroelectric gate dielectric.After poling the gate with 5V,the variation of the threshold voltage in 10⁵s is only 0.25V.The PZT/AlGaN/GaN HEMT using transfer technique can realize a better quality of the ferroelectric gate dielectric in a thinner film,and prevent the inter-diffuse during the growth of PZT at high temperature.These advantages of the transfer technique make the better threshold stability of PZT/AlGaN/GaN HEMT,which makes PZT/AlGaN/GaN HEMT have the huge potential in enhancement-mode application.PZT/AlGaN/GaN HEMT using transfer technique paves the way for hetero-integration of the ferroelectric and the GaN-based devices,which makes it possible for us to apply the device on novel applications.In summary,an enhancement-mode PZT/AlGaN/GaN HEMT is fabricated in this study.The ferroelectric modulation mechanism of the devices were studied in depth.The integration of the PZT and AlGaN/GaN heterostructure by transfer technique was realized for the first time,which is the critical breakthrough of the integration of the ferroelectrics and GaN-based device.These approaches provide a good reference and guideline for integration of the ferroelectrics and GaN-based devices.