Semiconductors are crystalline or amorphous solids with distinct electrical characteristics. They are of high resistance - higher than typical resistance materials, but still of much lower resistance than insulators. Their resistance decreases as their temperature increases, which is behavior opposite to that of a metal. Finally, their conducting properties may be altered in useful ways by the deliberate introduction ("doping") of impurities into the crystal structure, which lowers its resistance but also permits the creation of semiconductor junctions between differently-doped regions of the crystal. The behavior of charge carriers at these junctions is the basis of diodes, transistors and all modern electronics.