室溫操作之pHEMT於兆赫波段之輻射特性

Abstract

本論文研究InGaAs/GaAs pHEMT在兆赫波段的輻射特性。我們建構兆赫波段輻射場型量測系統，用於量測pHEMT輻射場型，並且對pHEMT輻射頻譜作系統性的量測。實驗結果發現pHEMT兆赫波段輻射強度與出光角度的關係類似餘弦函數；正向輻射強度最強，隨著兩側角度的增加輻射強度也隨之遞減。輻射強度在元件從線性區進入飽和區會有大幅的提升。輻射頻譜呈現寬頻分佈在1∼18THz，並且出現數個頻譜峰值。在9 THz以及15.5 THz兩個主要峰值的強度比例隨輻射出光角度以及輸入功率變化，顯示這兩個峰值的特性以及機制並不相同。經由比較GaAs基板的吸收頻譜，我們認為輻射頻譜峰值是來自於材料聲子震盪的熱輻射。其中9 THz來自於GaAs基板optical phonon；而15.5 THz是由於多個聲子結合所產生。我們也進一步量測不同材料GaN/Si HEMT的輻射頻譜，其完全不同於InGaAs/GaAs pHEMT的輻射頻譜更加印證了我們對pHEMT兆赫波段輻射來自於材料聲子的假設。

This thesis studies radiation characteristics of InGaAs/GaAs pHEMTs in THz region. We construct a system capable of measuring both angle-resolved intensity and radiation spectrum of a pHEMT under electrical excitations. Experimental results show that the intensity of a pHEMT has an angular distribution similar to cosθ. Furthermore, the in-tensity increases substantially when a pHEMT is biased from the linear region into the saturation region. The spectrum extends widely in fre-quencies from one to 18 THz and shows several peaks. We find that two of the peaks, at f = 9 and 15.5 THz, have an intensity ratio that depends on the angle and the input power, indicating the difference in their origin. By comparing the radiation spectrum with the absorption spectrum of a GaAs substrate, we infer that the peaks originate from thermal radiation of phonons. The peak at 9 THz comes from the radiation of optical pho-nons in GaAs while the peak at 15.5 THz from a combination of two phonons that are infrared active. The mechanism is further confirmed by comparing the radiation spectrum with that of a GaN/Si HEMT.