高電流與低動態電阻RON增強型氮化鎵MIS-HEMT

Abstract

增強型(E型)氮化鎵高電子遷移率電晶體(GaN HEMT)最近已經開始取代傳統的空乏型元件，因為它為常閉型電晶體，所以應用於高功率開關時能夠提供額外的安全性。其中下凹式閘極(Gate Recess) 為實現增強型HEMT的方法之一，而製作gate recess的主要困難在於以傳統的蝕刻技術很難控制recess的深度。 本論文我們同時驗證了以乾式和濕式蝕刻所製作的E型GaN MIS-HEMT gate recess的操作特性。在蝕刻和表面鈍化之前，我們以氮氣電漿作表面預處理來填補氮的空孔。最後沉積HfO2使閘極底下產生強大的電荷累積形成大電流密度元件。 結果顯示可以獲得非常好的蝕刻輪廓與實驗目標相符，因為濕式蝕刻可以降低傳統乾式蝕刻所造成的電漿損傷。此外AFM結果顯示表面非常平坦，RMS值約0.49 nm。 電流密度高達700 mA/mm顯示表面鈍化極佳，伴隨的1.3 V臨界電壓對於E型電晶體極為重要。以HfO2當作閘極介電層顯示具有良好的介電強度，擁有11V的順向崩潰電壓。本研究發現此電晶體擁有較低的臨界電壓遲滯約80 mV，我們也得到很低的動態與靜態RON比，值約為1.11。

Enhancement mode (E-Mode) GaN HEMT has recently started replacing conventional depletion mode devices due to additional safety in high power switching applications as it is always in normally off condition. Gate recess is one of the ways to achieve enhancement mode operation. Major problem in doing gate recess process is that it is very difficult to control recess depth by conventional etching technique. In this thesis, we have demonstrated enhancement mode operation of GaN MIS-HEMT through gate recess technique by both dry and wet etch process. Prior to etching and surface passivation, we have done nitrogen plasma treatment on the surface to recover nitrogen vacancy. Finally, HfO2 was deposited by ALD to create strong accumulation under the gate which is responsible for large current density of the device. It is seen that etching profile obtained is quite good which fits the purpose of wet etching as it reduces plasma damage by conventional dry etch method. Further, AFM result indicates a smooth surface morphology with RMS value to be around 0.49 nm. Very high current density of 700 mA/ mm has been obtained which indicate good surface passivation along with threshold voltage of 1.3 V essential for E-Mode operation. HfO2 deposited as gate dielectric shows good dielectric strength with forward breakdown voltage of 11V. Lower value of threshold voltage hysteresis around 80 mV is also reported from this work. We have also obtained a low value of 1.11 for dynamic RON to static RON ratio.