Antimonide based compound semiconductors have gained considerable interest in recent years due to their superior electron and hole transport properties [1-3]. Among the various high mobility material systems (Fig. 1), arsenic-antimonide based MOS-HEMTs have great potential to enable complementary logic operation at low supply voltage. Integrating a high quality dielectric is key to demonstrating a scalable arsenic-antimonide MOS-HEMT architecture for 15 nm logic technology node and beyond. It is hypothesized that an ultra-thin GaSb surface layer is more favorable toward high-Κ integration than In 0.2Al0.8Sb barrier as it avoids Al at the interface and the associated surface oxidation. Here, we study the effects of various surface passivation approaches on the capacitance-voltage characteristics (C-V) and the surface chemistry of n-type and p-type GaSb(100) MOS capacitors made with ALD and Plasma Enhanced ALD (PEALD) Al2O3 dielectric. We demonstrate for the first time, unpinned Fermi level in GaSb MOS system with high-Κ PEALD Al2O3 dielectric using admittance spectroscopy and XPS analysis.