An ion source is disclosed for use in fabrication of semiconductors. The ion source includes an electron emitter that includes a cathode mounted external to the ionization chamber for use in fabrication of semiconductors. In accordance with an important aspect of the invention, the electron emitter is employed without a corresponding anode or electron optics. As such, the distance between the cathode and the ionization chamber can be shortened to enable the ion source to be operated in an arc discharge mode or generate a plasma. Alternatively, the ion source can be operated in a dual mode with a single electron emitter by selectively varying the distance between the cathode and the ionization chamber.