Field‐effect devices with evaporated silicon monoxide coatings for liquid‐crystal alignment exhibit rectification under dc and ac excitation when the coatings are asymmetric. The rectification mechanism is a polarity‐ and electrode‐dependent depletion of residual ionic carriers from the bulk. Polarity reversal transients offer a novel means of studying the electrode double‐layer charging and demonstrate hydrodynamic contributions to the mobility. The threshold conditions for rectification under ac excitation are outlined.