The electrical performance of hafnium silicate (HfSiOx) gate stacks grown by atomic layer deposition (ALD) has been evaluated in capacitors and transistors. First, scaling potential of HfSiOx layers was studied as function of composition and thickness. It is shown that the equivalent oxide thickness scales down with decreasing layer thickness and increasing Hf-content. The gate leakage (at Vfb-1V), however, is mainly determined by the physical layer thickness. For the same equivalent oxide thickness (EOT) target, the lowest leakage is observed for the layers with the highest Hf-content. Leakage values as low as 1x10-3 A/cm2 for an equivalent oxide thickness of 1.3 nm have been obtained. Second, the thermal stability against crystallization of the ALD HfSiOx has been studied and related to their electrical properties. The thermal stability of HfSiOx decreases with increasing Hf-content that necessitates the use of nitridation. The influence of various annealing conditions on the nitrogen incorporation is also studied. Finally, the effect of HfSiOx composition and postdeposition nitridation is discussed on transistor level. TaN metal gate transistor data indicate that nitridation reduces the gate leakage and that Hf-rich HfSiOx layers show the best scaling potential, i.e., highest performance for the lowest gate leakage.