氮化鋁鎵/氮化鎵金氧半高電子遷移率場效電晶體之製作與特性研究

Abstract

本研究主要探討兩大部分：1. AlGaN/GaN MOS-HEMT以及2. 閘極掘入式AlGaN/GaN MOS-HEMT兩種元件，除了對於製作過程中所需的各項技術進行確認，更對於製程中光致電化學氧化法對於元件特性的影響提出一些觀察。 在AlGaN/GaN MOS-HEMT方面，我們發現到薄層的PEC閘極氧化層對於元件特性(電流開關比、閘極漏電流)有所提升，使電流開關比由5個數量級提升至6個數量級、閘極漏電流下降1個數量級，而閾值電壓往負偏移約2 V，可能與PEC氧化層/AlGaN介面狀態有關、或與AlGaN受到氧化層的應力有關。在PEC閘極氧化層增厚的情況下，比起薄層的PEC閘極氧化，閾值電壓再往正電壓偏移2.2 V左右。然而，元件特性卻大幅下降，在VG=0 V時，飽和電流由410 mA/mm下降至144.5 mA/mm，轉導峰值則由63 mS/mm下降至30.67 mS/mm，說明以PEC氧化法削減AlGaN層厚度並非製作增強型元件的優良手段。 閘極掘入式AlGaN/GaN MOS-HEMT證明了閘極掘入可製作出增強型元件，而PEC氧化層鈍化表面可以進一步提升元件的各種電特性，電流開關比由5個數量級提升至5.5個數量級，轉導峰值由95 mS/mm上升至120 mS/mm，調整PEC氧化時間以及原子層沉積的氧化層厚度等，可望做出特性更優良的電晶體。

This work is focused on the effects of the photoelectrochemical oxidation (PEC oxidation) method on both AlGaN/GaN Metal-Oxide-Semiconductor High Electron Mobility Transistors (MOS-HEMTs) and gate recessed AlGaN/GaN MOS-HEMTs. The measurement of AlGaN/GaN MOS-HEMTs with PEC oxide gate shows that with lightly PEC oxidation, the current on/off ratio has increased from 5 to 6 order of magnitude and the gate leakage current decreased almost 1 order of magnitude. However with short-time PEC oxidation, the threshold voltage has shifted -2 V. This phenomenon is related to the interface state of the PEC oxide and the AlGaN layer or the strain of the AlGaN layer induced by the PEC oxide. Moreover, with long-time PEC oxidation, the threshold voltage has shifted 2.2 V, but the device performance is greatly decreased. The measurement of gate recessed AlGaN/GaN MOS-HEMTs shows that the gate recess method can effectively shift threshold voltage from -5 V to -1 V. The PEC oxidized passivation layer further increases the threshold voltage to nearly zero volts. with moderate PEC oxidation, the transconductance can be increased from 95 mS/mm to 120 mS/mm, though the recessed gate structure is suffered from the poor gate leakage current. In a nutshell, we propose a useful method to fabricate an enhancement mode MOS-HEMT, but we still need some calibrations and optimizations to further increase our device performance.