1. A crossbar structure comprising: a first layer or layers including at least one first p-type region and at least one n-type region; a second layer or layers including at least one second p-type region and at least one second n-type region; and a resistance programmable material formed between the first layer(s) and the second layer(s), wherein the first layer(s) and the second layer(s) include first and second intersecting wiring portions forming a crossbar array.