A semiconductor device including a threshold voltage detector and a boosted voltage generating unit. The threshold voltage detector detects a threshold voltage level of cell transistors and outputs a detected threshold voltage level. The boosted voltage generating unit changes a target level of a boosted voltage in response to the detected threshold voltage level. The threshold voltage detector includes a detected current generating unit and a detected voltage generating unit. The detected current generating unit has a plurality of cell transistors in a cell array and generates a detected current whose amplitude varies corresponding to an average level of the threshold voltages of the cell transistors. The detected voltage generating unit generates the detected threshold voltage level whose level is determined corresponding to the amplitude of the detected current.