High electron mobility transistors (HEMTs) and methods of manufacturing the same. A HEMT may include a source electrode, a gate electrode, a drain electrode, a channel formation layer including at least a 2-dimensional electron gas (2DEG) channel, a channel supplying layer for forming the 2DEG channel in the channel formation layer, a portion of the channel supplying layer including a first oxygen treated region. The channel supplying layer may include a second oxygen treated region that extends from the first oxygen treated region towards the drain electrode, and the depth and concentration of oxygen of the second oxygen treated region may be less than those of the first oxygen treated region.