Studies of terahertz and far-infrared radiation from GaAs based transistors
|關鍵字:||兆赫輻射;遠紅外線輻射;砷化鎵型電晶體;聲子;自由載子;terahertz radiation;far-infrared radiation;GaAs-based transistor;phonon;free carrier|
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 GaAsbased 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.