In this thesis we have extensively investigated the deposition of various high-k dielectric films—including HfOxNy, Al2O3, and Gd2O3—onto the bulk Ge and GaAs substrates and the electrical characteristics of the devices with these developed high-k dielectric films. At first, two surface passivation methods, i.e., the NH3 plasma pretreatment and the ultrathin Si capping, were employed to improve the quality of the sputtered HfOxNy films on the Ge substrates. Not only the severe incorporation of Ge species into the overlying HfOxNy was eliminated, but also the smaller amount of GeOx remained at the dielectric interface and the less oxide trapped charge were observed after undergoing surface nitridation or Si capping processes. These improvements can be attributed to the reduced GeOx volatilization and also enhanced thermal stability of the high-k/Ge interface. We subsequently studied the material and electrical properties of atomic-layer-deposited (ALD) Al2O3 thin films on the Ge substrates. It was found that an increase in the deposition temperature did improve both the density and stoichiometry of the Al2O3 films; nevertheless, temperatures exceeding 200 C caused the intermixing of top Al2O3 and interfacial GeO2 layers, i.e., the formation of a GexAl1–xO intermediate layer. It led to the consequence of an increasing gate leakage (Jg) and a higher interface state density (Dit, >1013 cm–2eV–1) in the Pt/Al2O3/Ge capacitors. On the other hand, for improving the quality of low-temperature (100 C) Al2O3 films on the Ge, the forming gas annealing (FGA) at 300 C was conducted further; the value of the Dit close to midgap was evidently lowered to ca. 6  1011 cm–2eV–1 accompanying with the reduced hysteresis width. In addition, we combined these device experiments with MEDICI simulations and thus validated that a high value of intrinsic carrier concentration (ni) in Ge, via the bulk-trap generation/recombination as well as the diffusion from the bulk substrate, were responsible for the presence of low frequency C-V curves in the kHz regime and the gate-independent inversion conductance. The density of the bulk trap was estimated to be ca. (2–4)  1015 cm–3 in the low-doped Ge in terms of their conductance-voltage (G-V) characteristics. The plot of the Arrhenius-dependent substrate conductance (Gsub) also indicated a larger energy loss occurring in Ge than in Si by at least four orders of magnitude, reflecting the fast minority-carrier response rate. We also, afterward, presented the study of the effects of alterant wet-chemical clean and (NH4)2S treatment on the electrical characteristics of high-k/GaAs devices. Through analysis of the surface chemistry, it was evident that employing the NH4OH alkaline solution and then (NH4)2S-H2O passivation at 80 C effectively suppressed the formation of GaAs native oxides and elemental As close to dielectric interface, thereby abating the “Fermi level (Ef) pinning” effect on the electrical performance. With undergoing the optimized sulfidization processes, the fabricated Gd2O3/GaAs capacitors exhibited the Jg of ca. 1.5 × 10-5 A/cm2 @ Vg = (VFB + 1) V with the capacitance-equivalent-thickness of ca. 20 Å, which is comparable to the earlier study of high-performance HfO2/GaAs system with an ultrathin Si/Ge interfacial passivation. Also, the sulfidized ALD-Al2O3/GaAs capacitors can reveal the resultant higher oxide capacitance, the smaller frequency dispersion, the decreased hysteresis width, the reduced Dit, and the smaller Jg, respectively. Further replacing the (NH4)2S solvent from H2O to C4H9OH indicated a higher electrical improvement, in which an obvious suppression of elemental As and AsOx surface species was found with an increasing sulfur bonds on the GaAs substrates. On the other hand, the influences of post annealing ambient, O2 and N2, adopted in the Al2O3/GaAs capacitor were investigated and we clarified the characteristic differences by identifying the underlying thermochemical mechanisms. In our studies, another interesting feature was noticed that the deposition of the ALD-Al2O3 films on GaAs favored to at the higher temperature, 300 C, which is opposite to the behavior that those on Ge favored to at low temperatures (<200 C). Finally, we have successively demonstrated the device characteristics of the inversion-mode Ge p- and n-MOS field-effect-transistors (FETs) with the ALD-Al2O3 gate dielectrics, respectively. The respective peak mobility and on/off ratio was ca. 225 cm2 V–1 s–1 and >103 for Ge p-FET (W/L = 100 μm/4 μm), while these values were less than 100 cm2 V–1 s–1 and ca. 103, respectively, for Ge n-FET (W/L = 100 μm/9 μm). The inferior n-FET device performance can be mainly correlated to the larger source/drain (S/D) contact resistance, the severe Dit scattering, and the lower level of substrate doping. Subsequently performing FGA at 300 °C can enhance the driving on-current, decrease the Dit at the Al2O3–Ge interface, and improve the negative bias temperature instability (NBTI) reliability. A higher FGA temperature of 400 °C led to a dramatic increase in the off-current of the n-FET, arising from the severe out-diffusion of phosphorous dopant from S/D. Herein, the characteristics of Ge junction diodes and their reverse leakage paths were also examined in more details. The magnitudes of the rectifying ratios for the Ge p+n and n+p junctions exceeded three and four orders of magnitude (in the voltage range of ±1 V), respectively, with the values of reverse junction leakage of ca. 10-2 and 10-4 A/cm2, respectively. The site of the primary leakage path, at either the surface periphery or junction area, was determined by (i) the thermal budget during dopant activation and (ii) whether FGA was employed or not. Furthermore, the transfer and output characteristics of the depletion-mode ALD-Al2O3/GaAs n-FET with the (NH4)2S-C4H9OH chemical passivation were also presented. The maximum Id was 250 mA/mm measured at Vg–Vth = 4.8 V, Vd = 4 V. However, the extracted peak electron mobility was only 336 cm2V-1s-1, indicating that optimization of the dielectric/GaAs interface was still required.