Silicon (Si) is the most commonly used n-type dopant in AlGaN, but the conductivity of Si-doped Al x Ga 1-x N was often reported to drop abruptly at high Al content (x>0.7) and the reason was often speculated to be due to either compensation by deep levels or self-compensation of the so-called DX (or negative-U) center. Understanding the electronic structure of Si and carrier compensation processes is the essential for improving the n-type doping of high-Al-content Al x Ga 1-x N. In our studies of Si-doped AlGaN layers grown by metal-organic chemical vapor deposition, Electron Paramagnetic Resonance (EPR) was used to study the electronic structure of Si in high-Al-content Al x Ga 1-x N.From the temperature dependence of the concentration of the Si donor on the neutral charge state E d determined by EPR, we showed that Si already forms a stable DX center in Al x Ga 1-x N with x ~0.77. However, with the Fermi level locating only ~3 meV below E d , Si still behaves as a shallow donor and high conductivity at room temperature could be achieved in Al 0.77 Ga 0.23 N:Si layers. In samples with the concentration of the residual oxygen (O) impurity larger than that of Si, we observed no carrier compensation by O in Al 0.77 Ga 0.23 N:Si layers, suggesting that at such Al content, O does not seem to hinder the n-type doping in the material. The result is presented in paper 1.In paper 2, we determined the dependence of the E DX level of Si on the Al content in Al x Ga 1-x N:Si layers (0.79≤x≤1) with the Si concentration of ~2×10 18 cm -3 and the concentrations of residual O and C impurities of about an order of magnitude lower (~1÷2×10 17 cm -3 ). We found the coexistence of two DX centers (stable and metastable ones) of Si in Al x Ga 1-x N for x≥0.84. For the stable DX center, abruptly deepening of E DX with increasing of the Al content for x≥0.83 was observed, explaining the drastic decrease of the conductivity as often reported in previous transport studies. For the metastable DX center, the E DX level remains close to E d for x=0.84÷1 (~11 meV for AlN).The Z 1 /Z 2 defect is the most common deep level revealed by Deep Level Transient Spectroscopy (DLTS) in 4H-SiC epitaxial layers grown by chemical vapor deposition (CVD). It has previously been shown by DLTS to be a negative-U system which is more stable with capturing two electrons. The center is also known to be the lifetime killer in asiv grown CVD material and, therefore, attracts much attention. Despite nearly two decades of intensive studies, including theoretical calculations and different experimental techniques, the origin of the Z 1 /Z 2 center remains unclear. EPR is known to be a powerful method for defect identification, but a direct correlation between EPR and DLTS is difficult due to different requirements on samples for each technique. Using high n-type 4H-SiC CVD free-standing layers irradiated with lowenergy (250 keV) electrons, which mainly displace carbon atoms creating C vacancies, C interstitials and their associated defects, it was possible...