English: Current-voltage (IV) graph illustrating current controlled negative resistance (CCNR). It shows a device with negative differential resistance, meaning the voltage decreases with increasing current in some parts of the graph, colored red It is a variety called current controlled negative resistance, which means the voltage is a single-valued function of the current. The curve is also called S-type, because it is shaped vaguely like the letter S, with a single negative resistance region along the current axis (shaded) sandwiched between two positive resistance regions.