1. A memristive device ( 100 ) comprising: a first electrode ( 110 ); a second electrode ( 115 ); a silicon memristive matrix ( 105 ) interposed between said first electrode ( 110 ) and said second electrode ( 115 ); and a mobile dopant species ( 210 , 215 ) within said silicon memristive matrix ( 105 ); said mobile dopant species ( 210 , 215 ) moving within said silicon memristive matrix ( 105 ) in response to a first programming electrical field, said mobile dopant species ( 210 , 215 ) remaining substantially in place after removal of said first programming electrical field.