A charge pump comprises a plurality of stages connected in series, an input terminal of the charge pump being connected to a voltage supply and an output terminal of the charge pump providing an output voltage higher than the voltage supply. Each stage comprises unidirectional current flow MOS transistor means connected between a stage input terminal and a stage output terminal allowing current to flow only from said stage input terminal to said stage output terminal, and a first capacitor with one plate connected to said stage output terminal and another plate driven by a respective first digital signal periodically switching between ground and said voltage supply. The unidirectional current flow MOS transistor means of the stages have independent bulk electrodes, and a bias voltage generator circuit is provided for biasing the bulk electrodes of said unidirectional current flow MOS transistor means at respective bulk potentials which become progressively higher going from the stages proximate to said input terminal to the stages proximate to said output terminal of the charge pump.