Deep level traps in two emerging classes of devices, InGaAs/GaAsSb multiple quantum well (MQW) photodiodes for mid-IR detection and GaAs-based MOSFETs using InAlP native oxide and Al2O3 as gate dielectrics for microwave-frequency circuit applications, are studied using several characterization techniques, with an eye towards using this information in order to further improve device performance.   For the InGaAs/GaAsSb MQW photodiodes, several device structures, including lattice-matched MQWs, conventional layer-by-layer strain-compensated MQWs, and a strained MQW, were evaluated. Three samples with unstrained and strain-compensated structures have been characterized using low frequency noise spectroscopy (LFNS), random telegraph signal (RTS) characterization, and deep level transient spectroscopy (DLTS). Several deep levels were observed in these structures; some of the defect levels were found to be common to all structures, while others are unique to specific device structures. These distinctions can provide guidance for the design of improved MQW photodiodes. In addition, detailed DLTS study was used to identify the spatial location of the identified defects; this study indicates that one of the traps is actually located in the transition layer outside the quantum well region, thereby highlighting the importance of considering the full device structure - and not just the 'active' layers - during device design. Additionally, a novel quantum well photodiode with a strained active region was also investigated. This structure is attractive since it is promising for extending the detection range to longer wavelengths. Two new deep levels were found in this novel structure, and were found to be located in the strained quantum well region by using DLTS. This study has provided the first detailed investigation of the trapping/detrapping mechanisms at work in these photodiodes.   To facilitate the study of defects in GaAs-based MOSFETs, MOS capacitors incorporating two different gate oxides were investigated. InAlP oxide was first studied for the advantage of its easy integration to current GaAs FET fabrication process. Although high-performance devices have been obtained, an interface state density (Dit) in the range of 10^12~10^13 eV^-1cm^-2 was typically extracted for these structures, imposing a practical limit on subthreshold performance of the devices. As an alternative to InAlP oxide, Al2O3 deposited by atomic layer deposition was also evaluated. A 'buried channel' approach with an InGaP layer between the dielectric and channel was used to reduce the impact of defects on device performance. A range of surface pretreatments and post-deposition annealing treatments were investigated in order to optimize the interface quality. The Dit was found to be somewhat lower than for InAlP oxide, on the order of ~10^12 eV^-1cm^-2. The lower interface state density, coupled with the larger dielectric constant of Al2O3 compared to InAlP oxides, suggests this is a promising path for improved microwave device performance.