An integrated circuit device may include a semiconductor substrate including an active region and a transistor in the active region. The transistor may include first and second spaced apart source/drain regions in the active region of the semiconductor substrate, and a semiconductor channel region between the first and second source/drain regions. The semiconductor channel region may include a plurality of channel trenches therein between the first and second source/drain regions. A gate insulating layer may be provided on the channel region including sidewalls of the plurality of channel trenches, and a gate electrode may be provided on the gate insulating layer so that the gate insulating layer is between the gate electrode and the semiconductor channel region including the plurality of channel trenches. Related methods are also discussed.