砷化鎵型電晶體於兆赫波至遠紅外波段輻射特性之研究

Abstract

我們展示一個利用反射率頻譜萃取高阻值砷化鎵基板於兆赫波至遠紅外波段光學參數(包括折射率與吸收係數)的方法。反射率頻譜分為有干涉條紋與無干涉條紋兩部分。我們使用一個含有四個參數的單震盪子模型所計算的反射率頻譜擬合無干涉條紋處的頻譜，求出砷化鎵於兩波段的折射率頻譜。有干涉條紋處可利用其上封包曲線搭配折射率頻譜求得單聲子與雙聲子貢獻的吸收頻譜。萃取出的吸收頻譜用於分析電晶體的電激輻射機制。 我們研究以砷化鎵為基板的兩個電晶體之電激輻射頻譜與效率。第一個為含砷化銦鎵通道的假性高電子遷移率電晶體，第二個為異質接面雙載子電晶體。由於磊晶結構的不同，兩種電晶體於兆赫波至遠紅外波段的電激輻射頻譜有相當大的差異。假性高電子遷移率電晶體的輻射頻譜出現峰狀特徵，我們將其歸因於砷化鎵中雙聲子過程的貢獻。而異質接面雙載子電晶體的輻射頻譜無明顯的峰狀特徵，頻譜功率隨頻率上升大致呈現單調性增加，我們歸因於自由載子制動輻射的貢獻。高電子遷移率電晶體(異質接面雙載子電晶體)於飽和區(順向主動區)的輻射功率轉換效率較線性區(飽和區)的轉換效率為佳，此現象與元件中空乏區的強電場加速自由電子，有效提升其平均能量有關。在中紅外波段方面，兩種元件的輻射頻譜主要成分由其上氮化矽保護層的輻射譜線所貢獻。根據上述結果，我們提出一個能有效提升電晶體於兆赫波至遠紅外波段的電激輻射效率的結構。

We demonstrate a method of extracting optical constants, including the refractive index and absorption coefficient, of a semi-insulating gallium arsenide (SI-GaAs) substrate from a reflectance spectrum in the terahertz regime and the far infrared. The reflectance spectrum contains fringing and fringless parts. The fringless spectrum is fitted by a reflectance spectrum calculated based on a four-parameter single-oscillator model. The parameters are used to calculate the refractive index of SI-GaAs in the two frequency regimes. The index is then used along with the upper-envelope function of the fringing spectrum to extract the absorption spectrum of SI-GaAs associated with one-phonon and two-phonon absorption processes. The absorption spectrum is used to analyze electroluminescence (EL) mechanisms of GaAs-based transistors. We investigate EL spectra and EL efficiency of two GaAsbased transistors. The first one is a pseudomorphic high electron mobility transistor (pHEMT) and the second an heterojunction bipolar transistor (HBT). The two transistors have quite different features in their EL spectra due to the differences in their epitaxial structures. The EL spectra of the pHEMT contain peak-like features that are attributed to two-phonon processes in GaAs. The EL spectra of the HBT exhibit no such obvious features and spectral power increases monotonically with frequency, which is attributed to bremsstrahlung radiation of free electrons. Power conversion efficiency of a pHEMT (or HBT) in the saturation (or forward active) region is higher than that in the linear (or saturation) region, which is related to the increased average energy of free electrons associated with the high-strength field in the depletion region. Mid infrared EL spectra of the two transistors are dominated by features associated with emission of silicon nitride passivation layers. Based on our results, we propose a structure that can increase the EL efficiency of a transistor in the THz regime and the far infrared.