Non uniform charged defect distribution can arise in single phase, a priori uniform, mixed ionic-electronic conductors (MIECs) subject to gradients in the chemical potential and/or to applied voltages. The electronic defects may form graded n, graded p regions and pn or pin junctions. Yet this need not require the existence of a space charge. In the case of zero space charge the I-V characteristics of the pn junctions are very different from those of normal pn diodes. The electronic defect distribution for the main types of MIECs are discussed and the effect of chemical potential gradient and applied voltage in the steady state are demonstrated. Criteria for neglecting the space charge are given.