氮化鎵異質結構場效電晶體之研究

Abstract

在此論文中，我們設計了幾種AlxGa1-xN/GaN 異質結構來觀察極化效應對二維電子氣平板載子濃度的影響並討論電子在通道中的傳輸機制。我們藉由改變鋁成分、AlGaN厚度及是否有modulation doping來觀察此效應。結果，我們發現當鋁成分越高或AlGaN 厚度增加、極化效應越強、二維電子氣濃度越高。低溫下二維電子氣的電子傳輸特性是由interface roughness scattering機制主導。此外，在高鋁成分下(Al=0.3)，modulation doping對載子濃度貢獻僅有23%，故極化效應才是影響載子濃度主因。其二維電子氣的電子遷移率受impurity scattering影響降低。 此外，我們成功製作出undoped Al0.3Ga0.7N/GaN與 modulation doped Al0.3Ga0.7N/GaN異質結構場效電晶體，量測其直流、高頻及高溫操作特性，並進一步觀察極化效應對整個元件特性的貢獻。 對於閘極寬度為50 µm、閘極長度為1 µm的undoped Al0.3Ga0.7N/GaN 異質結構電晶體，室溫下的最大通道電流(Idmax)高達34.5 mA，單位閘極寬度之電流密度達到690 mA/mm。最大外部轉導為131 mS/mm，內部轉導為168 mS/mm。元件之崩潰電壓達到-97.5V。扣除金屬襯墊電容後的 ft與fmax分別達到9.2 GHz、16.4 GHz。 對於相同尺寸的Modulation doped Al0.3Ga0.7N/GaN 異質結構電晶體，室溫下的最大通道電流(Idmax)高達42.3 mA，單位閘極寬度之電流密度達到826 mA/mm。最大外部轉導為144 mS/mm，內部轉導為190 mS/mm。元件崩潰電壓達到-83V。扣除金屬襯墊電容後的ft與fmax分別達到7.25 GHz、13.25 GHz。 元件高溫特性方面， undoped結構的元件，在200oC時的最大通道電流與最大外部轉導分別是20.8 mA與82 mS/mm。100oC下，ft達到9.1 GHz，fmax為12.75 GHz。對於modulation-doped結構的元件，200oC時，最大通道電流與最大外部轉導分別是24.3 mA與80 mS/mm。100oC時，ft為6.8 GHz，fmax為10.75 GHz。此外，元件在高溫下仍維持良好的截止特性。 因此，不論是undoped或modulation doped結構的試片，皆可製作出特性非常好的元件，充分展現出了大電流、高崩潰電壓、高速以及高溫操作的特性，這主要歸功於極化效應的貢獻。

In this thesis, several AlxGa1-xN/GaN heterostructures with different Al composition, different AlGaN thickness and with/without modulation doping layer were designed to observe how polarization effects influence two dimensional electron gases (2DEG) sheet carrier concentration. Electron transport mechanisms in 2DEG channel were discussed at the same time. We found that both higher Al composition and thicker AlGaN layer would lead to higher polarization effect, which would cause higher 2DEG sheet carrier concentration. At low temperature, electron transport characteristics in 2DEG channel were dominated by interface roughness scattering. Furthermore, under high Al composition (Al=0.3), modulation doping layer contributes 23% of total 2DEG sheet carrier concentration. Therefore, total 2DEG sheet carrier concentration was dominated by polarization effects. In addition, modulation doping would result in 2DEG electron mobility reduction due to the impurity scattering. We have successfully fabricated undoped Al0.3Ga0.7N/GaN and modulation doped Al0.3Ga0.7N/GaN Heterostructure Field Effect Transistors (HFETs). Their DC, RF and high temperature characteristics were measured to observe the contribution of polarization effects. For undoped Al0.3Ga0.7N/GaN HFETs with gate length 1 µm and gate width 50 µm, their characteristics are as follows. Maximum channel current is 34.5mA, with unit gate width current density reaching 690 mA/mm. Maximum extrinsic transconductance is 131 mA/mm, and the intrinsic transconductance is 167mA/mm. The breakdown voltage is as high as -97.5 V. After deembeding the pad capacitance, the ft and fmax reach 9.2 GHz and 16.4 Ghz respectively. For modulation doped Al0.3Ga0.7N/GaN HFETs with same device dimension, their characteristics are as follows. Maximun channel current is as high as 42.3 mA, with unit gate width current density reaching 826 mA/mm. Maximum extrinsic transconductance is 144 mA/mm, and the intrinsic transconductance achieves 190 mA/mm. The breakdown voltage is -83 V. The ft and fmax after deembeding the pad capacitance are 7.25 GHz and 13.25 GHz respectively. The device high temperature characteristics are as follows. For undoped structure, maximum channel current and maximum extrinsic transconductance at 200oC are relatively 20.8 mA and 82 mS/mm. The ft and fmax at 100oC are 9.1 GHz and 12.75 GHz respectively. For modulation-doped structure, maximum channel current and maximum extrinsic transconductance at 200oC are relatively 24.3 mA and 80 mS/mm. The ft and fmax at 100oC are 6.8 GHz and 10.75 GHz respectively. Furthermore, both devices show good pinch-off characteristics under high temperature operation. Therefore, either undoped or modulation doped structures are suitable for fabricating high performance devices. These devices fully demonstrate high current, high breakdown voltage, high speed and high temperature operation characteristics, which are mainly due to the contribution of polarization effects.