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Numerous industries require electronics to operate reliably in harsh environments, such as extreme high temperatures (HTs), low temperature (LT), radiation rich environments, multi-extreme, etc. This dissertation is focused on two harsh environments: HT and multi-extreme. The first study is on HT optoelectronics for future high-density power module applications. In the power modules design, galvanic isolation is required to pass through the gate control signal, reject the transient noise, and break the ground loops. The optocoupler, which consists of a lighting emitting diode (LED) and photodetector (PD), is commonly used as the solution of galvanic isolation at room temperatures. There is a need to develop high-temperature optoelectronic devices to meet the high-density power modules' isolation requirements with wide operating temperatures. In this study, different commercial LED epitaxy materials have been analyzed. All materials are multiple quantum-based (MQWs) with indium gallium nitride (InGaN) and aluminum gallium indium phosphide (AlGaInP). The InGaN-based are with blue for lighting and display (BL & BD) and green for display (GD) applications, while the AlGaInP with red color for display (RD) applications. All these materials are studied and compared to evaluate if they can satisfy the optocouplers' light output requirements at HTs. Temperature and power department photoluminescence (PL) spectroscopy were conducted in temperatures ranging from 10 to 800 K to estimate the spontaneous emission quantum efficiency (QE) for these materials using the ABC model. The highest peaks of QEs were obtained from GD, followed by BD, BL, and RD. After studying the materials, LEDs from the same materials have been characterized by a wide range of temperatures for their emissions and spectral responses. The study demonstrates that LEDs can serve as a light source and detection (PDs). The results demonstrate the possibility of integration of LED-PD to fabricate HT optocouplers. It is worth to mention that red-red devices showed the best performance due to high overlapping in the wavelength between LED and PD. The second study is on multi-extreme environments, such as space environments. The unique bandgap-engineered features of silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) suggest an important opportunity to survive with multi-extreme environments simultaneously. In this study, SiGe films have been grown using chemical vapor deposition (CVD) on Si (100), Si (111), and c-plane sapphire substrates. Structural and optical characterizations were conducted on the grown films. The Si composition was up to 29.37, 23. 39, and 22.4% Si for films produced on Si (100), Si (111), and c-plane sapphire substrates. X-ray diffraction characterizations show that the SiGe films are oriented in the (111) direction on the sapphire (0001) substrates. However, 60-rotated twin defects are observed as well. Transmission electron microscopy (TEM) shows the crystalline growth of the film grown on sapphire substrates. The high surface roughness observed in the TEM images and the atomic force microscopy scans of the films indicate the formation of two different orientations of SiGe on sapphire substrates.